The upper-ocean response to monsoonal forcing in the Arabian Sea: seasonal and spatial variability

Deep-Sea Research II 47 (2000) 1177}1226

The upper-ocean response to monsoonal forcing in the Arabian Sea: seasonal and spatial variability Craig M. Lee!,*, Burton H. Jones", Kenneth H. Brink#, Albert S. Fischer# !Applied Physics Laboratory, University of Washington, 1013 NE 40th St, Seattle, WA 98105-6698, USA "Department of Biological Sciences, University of Southern California, Los Angeles, CA 90089-0371, USA #Department of Physical Oceanography, MS-21, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 5 December 1998; received in revised form 13 September 1999; accepted 13 September 1999

Abstract Observations from four towed pro"ler surveys undertaken between December 1994 and October 1995 examine the seasonal and spatial variability of the upper ocean response to the Monsoon cycle in the Arabian Sea. Although observed atmospheric forcing agrees well with modern climatologies, cross-basin patterns of mixed-layer depth and water properties observed in 1994}1995 are not entirely consistent with an upper-ocean response dominated by Ekman pumping. During the winter monsoon, the mixed-layer deepens dramatically with distance o!shore. Surface cooling intensi"es with o!shore distance, and a one-dimensional response dominated by convective overturning could explain observed wintertime mixed-layer depths. Except for waters associated with a "lament extending o!shore from the Omani coast, mixed-layer depths and water properties show only modest cross-basin contrasts during the Southwest Monsoon. Filament waters di!er from surrounding mid-basin waters, having shallow mixed-layers and water properties similar to those of waters upwelled near the Omani coast. In September, following the Southwest Monsoon, waters within 1000 km of the Omani coast have cooled and freshened, with marked changes in strati"cation extending well into the pycnocline. Estimates of Ekman pumping and wind-driven entrainment made using the Southampton Oceanographic Center 1980}1995 surface #ux and the Levitus mixed-layer climatologies indicate that during the Southwest Monsoon wind-driven entrainment is considerably stronger than Ekman pumping. Inshore of the windstress maximum, Ekman pumping partially counters wind-driven entrainment, while o!shore the two processes act together to deepen the mixed-layer. As Ekman pumping is too weak to counter wind-driven mixed-layer deepening inshore of the windstress maximum, another mechanism must act to maintain the

* Corresponding author. Fax: 001-206-543-6785. E-mail address: [email protected] (C.M. Lee). 0967-0645/00/$ - see front matter ( 2000 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 1 4 1 - 1

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shallow mixed-layers seen in our observations and in climatologies. O!shore advection of coastally upwelled water o!ers a mechanism for maintaining upper ocean strati"cation that is consistent with observed changes in upper ocean water properties. Ekman upwelling will modulate wind-driven entrainment, but these results indicate that the primary mechanisms acting inshore of the windstress maximum are wind-driven mixing and horizontal advection. ( 2000 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction The Monsoon cycle produces highly repeatable patterns of strong atmospheric forcing over the Arabian Sea (e.g. Findlater, 1969). During the Southwest (summer) Monsoon, warm, moist air prevails, and a strong southwesterly wind jet, often referred to as the Findlater Jet, runs diagonally across the Arabian Sea (Fig. 1). Winds during this period remain remarkably unidirectional, though magnitudes vary somewhat with time and space (Weller et al., 1998; Cayula et al., 1998). Temporal averaging de"nes a jet-like structure with a core of maximum windstress over the central Arabian Sea, though at any given instant the jet can be di$cult to discern (Cayula et al., 1998). Reputedly, the Southwest Monsoon produces the strongest sustained oceanic winds outside the Southern Ocean (Knox, 1987). The pattern reverses during the Northeast (winter) Monsoon (Fig. 1), typi"ed by relatively cool, dry air and sustained, but weaker, winds blowing to the southwest. The strong, sustained atmospheric forcing over the Arabian Sea o!ers an attractive natural laboratory for studying forced upper-ocean dynamics. The spatial structure and reversing nature of the surface winds drive mixing through both mechanical stirring and convective overturning in addition to both coastal and open-ocean upwelling and downwelling. This facilitates investigations of mixed-layer evolution governed by a variety of processes. Entrainment and upwelling also produce substantial nutrient #uxes into the euphotic zone (e.g. McCreary et al., 1996). This input makes the Arabian Sea one of the world's most productive ocean basins, and motivated the recent Joint Global Ocean Flux Study (JGOFS) e!ort to understand production and its fate in this region (Smith et al., 1998). Bauer et al. (1991) provide an explanation of the upper ocean response to monsoonal forcing in the Arabian Sea, which has in#uenced many subsequent studies of the region. During the Southwest Monsoon, lateral variations in windstress to either side of the Findlater Jet drive Ekman pumping, producing open ocean upwelling to the north of the windstress maximum and downwelling to the south (Fig. 1). Climatologies (Fig. 1 and Rao et al., 1989) reveal shallow mixed-layers north of the Findlater Jet and much deeper mixed-layers to the south. Bauer et al. (1991) argue that Ekman pumping is the dominant mechanism governing the mixed-layer response to the Southwest Monsoon. Thus, open-ocean upwelling to the north of the Findlater Jet and downwelling to the south are used to explain the observed cross-basin contrasts in mixed-layer depth. In contrast, the southwestward winds of the Northeast Monsoon form a broad #ow without strong lateral gradients (Fig. 1). Climatological

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Fig. 1. Southampton Oceanographic Center 1995 windstress (0.02 N m~2 contour interval), Levitus climatological mixed-layer depth and schematic representations of the various physical processes that may act during the Southwest and Northeast Monsoons. A black line extending o!shore from the Omani coast marks the US JGOFS Southern Line, while a red dot marks the location of the moored array. The large hollow arrow marks the Findlater Jet in the Southwest Monsoon schematic. The three smaller vectors represent weaker, less concentrated winds during the Northeast Monsoon. Extremely shallow mixed-layers in the Levitus climatology o! the west coast of India during the Southwest Monsoon and mid-basin during the Northeast Monsoon are artifacts of data sparsity.

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mixed-layer depths show only weak cross-basin contrasts, with slightly deeper mixedlayers in the northern half of the basin (Fig. 1). Bauer et al. (1991) "nd Ekman pumping to be less important during this period, and identify mixing as the dominant mechanism. Several aspects of Bauer et al.'s (1991) explanation of the upper-ocean response to the Southwest Monsoon warrant further investigation. The Ekman pumping formulation used by Bauer et al. (1991) neglects latitudinal variations in Coriolis frequency, and thus does not correctly estimate the temporal and spatial patterns of the resulting vertical motions. An additional term, proportional to the zonal windstress, results from allowing the Coriolis frequency to vary with latitude. During the Southwest Monsoon, this term can signi"cantly change the patterns of Ekman pumping from those derived by considering only the windstress curl term. The strong, steady winds of the Southwest Monsoon also may drive signi"cant mixed-layer deepening through turbulent mixing. Wind-driven turbulent entrainment may overwhelm the e!ects of Ekman pumping in setting the mixed-layer depth, though Ekman pumping might serve to modulate the response to wind-mixing by altering the strati"cation at the mixed-layer base. Alongshore winds drive strong coastal upwelling/downwelling at the boundary. These vertical motions are typically far stronger than those generated in the open ocean through windstress curl. Models (Young and Kindle, 1994; Keen et al., 1997) suggest that lateral advection might then extend the in#uence of coastal upwelling hundreds of kilometers o!shore, contributing to primary production and phytoplankton biomass near mid-basin. This mechanism, acting with wind-driven mixing and Ekman pumping, could produce the shallow mixed-layers seen in the climatologies. McCreary et al. (1989) discuss the idealized generation of a substantial downwind, upper-ocean jet, an additional consequence of strong, unidirectional wind-forcing. Instabilities of this wind-driven jet may produce eddies. Recent observations (Flagg and Kim, 1998) suggest that eddies dominate upper-ocean current variability during the Southwest Monsoon. These may drive both lateral and vertical advection and can be expected to complicate isolation of the response forced directly by the atmosphere. Motivated by the biological importance of the Arabian Sea (Smith et al., 1998) and the attractiveness of the region as a laboratory for studying oceanic response to strong atmospheric forcing, we undertook an investigation of upper-ocean evolution through an entire monsoonal cycle. Four cruises (December 1994, February, June and September 1995) targeted the Northeast and Southwest Monsoons and the Intermonsoon periods. Each cruise sampled variability across the Arabian Sea in an e!ort to observe cross-basin contrasts in the physical and biological responses. Our sampling made extensive use of a heavily instrumented, undulating, towed pro"ler known as the SeaSoar. In addition to a variety of physical and biological observations made by SeaSoar (described below), we collected extensive shipboard current (Acoustic Doppler Current Pro"ler), meteorological and surface nutrient measurements. We devoted the second half of each cruise to a series of hydrographic stations resampling parts of the SeaSoar survey path, with additional coverage near the Omani coast. These stations included extensive nutrient and phytoplankton measurements, which will be touched on brie#y here.

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This study uses our observations and data from the Southampton Oceanographic Center (SOC) climatology (Josey et al., 1998) to examine the ocean response to the 1994}1995 Monsoon Cycle and to argue that mechanisms other than Ekman pumping play important roles during the Southwest Monsoon. We begin by contrasting the seasonal and spatial variability of current and hydrographic patterns observed through the 1994}1995 Monsoon Cycle. Calculations using the SOC data explore the relative importance of Ekman pumping and wind-driven mixing. Following this, we examine the spatial scales of physical and biological variability and their implications for other components of the Arabian Sea program. We discuss how the SOC-derived Ekman pumping and wind-driven entrainment patterns, combined with our observations, suggest a response that di!ers from one dominated by Ekman pumping. The paper concludes by investigating whether the observed response provides evidence that o!shore advection of coastally upwelled waters plays a signi"cant role.

2. Data Four cruises executed over the 1994}1995 Monsoon Cycle observed upper ocean variability throughout the northern Arabian Sea. Each cruise sampled an established survey pattern (Fig. 2) using a towed, undulating pro"ler (SeaSoar). At typical tow speeds of 8 knots, SeaSoar provided along-track horizontal resolutions of 3 km and a vertical pro"ling range of about 1}300 m. The survey pattern was designed to observe conditions on both sides of the climatological windstress maximum and to

Fig. 2. SeaSoar survey ship tracks for (a) the Northeast Monsoon, Spring Intermonsoon and Fall Intermonsoon and (b) the Southwest Monsoon. Grid patterns indicate intensive surveys. A black square marks the WHOI mooring and the waypoint letters provide reference points used in subsequent "gures.

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contrast variability along the coast with open-ocean conditions. Intensive surveys augmented a section running o!shore to the southeast from approximately 183N, 583E to 14.53N, 653E, coincident with the southern hydrographic line occupied by the US JGOFS Arabian Sea Process Cruises (Smith et al., 1998; Morrison et al., 1998). The intensive surveys had 25 km cross-track separation and focused on resolving the three-dimensional structure of the upper ocean to either side of the windstress maximum, with an additional survey undertaken during the Southwest Monsoon (June) targeting a cool "lament identi"ed in Advanced Very High Resolution Radiometer (AVHRR) imagery. A typical intensive survey required approximately 2.5 days to complete, and motions with similar or shorter time-scales, such as nearinertial oscillations and internal tides, will be aliased by this sampling. To obtain biological and chemical measurements requiring in situ water sampling, a hydrographic survey along the JGOFS Southern Line followed SeaSoar operations on each cruise. SeaSoar carried a wide range of physical and biological sensors for sampling upper-ocean variability in the Arabian Sea. Two Seabird conductivity sensors were mounted on top of the pro"ler, oriented to maximize #ow through the cells. A pair of Seabird temperature sensors sat in-line directly behind these, o!set slightly upward to escape the wake created by the forward pair. A SeaTech chlorophyll #uorometer occupied a #ow-through region inside the body of the pro"ler. An instrument cage mounted beneath the SeaSoar body carried a SeaTech Dissolved Organic Matter (yellow) #uorometer (Coble et al., 1998), a SeaTech transmissometer, a fast response oxygen sensor and, on two cruises, a Tracor acoustic zooplankton sensor. Ducting problems degraded data quality from the oxygen sensor on all but the last (September}October) cruise. A sensor measuring Photosynthetically Available Radiation (PAR) was mounted on top of the pro"ler's tail. Both sensor placement and di!erences in inherent response times may cause lags that require correction. Di!erences in response between temperature and conductivity sensors produce the most obvious problems. Temperature/conductivity pairs were lag-corrected to minimize salinity spiking. We supplemented pre- and post-cruise laboratory calibrations with comparisons between sensor and bottle salinities from data obtained using SeaSoar sensors on the numerous hydrographic casts that occupied the second half of each cruise. Similar to Rudnick and Luyten (1996), we found no evidence of errors induced by the thermal mass of the conductivity cell (Lueck and Picklo, 1990) and thus applied no corrections of this type. Fluorometer and transmissometer data exhibited clear hysteresis associated with instrument response times. We corrected using lags determined by minimizing the rms di!erence between successive up- and down-pro"les. To further reduce errors and facilitate calculations, we binned the data at 4 m in the vertical and temporally averaged over 15 min, approximately one full cycle of SeaSoar's pro"ling path. Converting SeaSoar's chlorophyll a #uorometer measurements into absolute concentrations requires calibration against chlorophyll a concentrations derived from water samples. Both for calibration and for constructing near-surface chlorophyll a maps, we drew batch samples from the ship's underway pumping system at 1}2 h intervals. To minimize the e!ects of spatial variability on the calibration procedure,

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we timed water sampling to coincide with the near-surface portion of the SeaSoar pro"ling cycle. Samples were "ltered through GFF-type glass "ber "lters (0.7 lm nominal pore size) and extracted in acetone for 24 h. Chlorophyll a concentration was then measured #uorometrically and calculated according to Holm-Hansen et al. (1965). Correlating one-minute averages of SeaSoar #uorescence centered on the vehicle's surface arc with the extracted chlorophyll a concentrations produced calibration coe$cients. Attempts to evaluate day}night di!erences and regional variations in the chlorophyll a #uorescence/chlorophyll a concentration relationship revealed no statistically signi"cant patterns. Thus, a single chlorophyll a/#uorescence relationship was applied over the entirety of each cruise. Shipboard measurements of meteorological variables and currents complemented SeaSoar observations. An Improved Meteorology (IMET, Hosom et al., 1995) sensor package mounted on the R/V Thomas G. Thompson's jacksta! made observations of wind velocity, incoming shortwave radiation, longwave radiation, relative humidity, barometric pressure and air temperature, while the R/V Thompson's pumped underway systems provided observations of sea-surface temperature and salinity. Shipboard meteorological sensors were calibrated against measurements made from the Woods Hole mooring in the central Arabian Sea (Weller et al., 1998) during periods when the ship was nearby, and also were compared with a portable sensor system used during three separate mooring cruises (Baumgartner et al., 1997). The Woods Hole mooring (Weller et al., 1998) also provides a year-long meteorological record near the mid-basin climatological windstress maximum. Fluxes were calculated following Weller and Anderson (1996) and Weller et al. (1998). Positive heat #uxes indicate heat gain by the ocean. The quality of absolute upper ocean current estimates derived from underway shipboard Acoustic Doppler Current Pro"ler (ADCP) measurements depends strongly on how accurately ship speed and heading can be determined. Fortunately, all but the initial SeaSoar cruise employed highly accurate P-code GPS navigation, improving rms position accuracy by a factor of "ve over the 30 m accuracy provided by the more readily available C/A code GPS. Spectra of 1-s GPS "xes revealed an energetic peak at approximately 0.1 cps associated with the ship's roll. To avoid aliasing this high-frequency energy into the 5-min navigation needed for referencing ensembleaveraged ADCP data, we low-pass "ltered the navigational "xes at 0.017 cps prior to use in computing absolute currents. The February cruise also bene"ted from an Ashtech GPS heading device, which served to reduce the typical 1}33 heading-dependent gyrocompass errors (Gri$ths, 1994) by approximately an order of magnitude. We attempted to extend the bene"ts of Ashtech-derived heading corrections to other cruises by developing a heading-dependent gyrocompass error model from an analysis of the February results. Unfortunately, application of this model to the other cruises failed to improve the quality of the resulting ADCP velocities, as might be anticipated given the possibility of temporal drift in gyrocompass-heading errors (Gri$ths, 1994). Acoustic Doppler Current Pro"ler water track calibration (Joyce, 1989) using the Ashtech-enhanced data from the February cruise con"rmed the alignment angle and gain coe$cients found by Flagg and Shi (1995), and we used their values. Data from

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the ADCP were collected in 5-min ensembles and initial processing was performed as described in Flagg and Shi (1995). We merged the resulting ensembles with the low-pass "ltered navigation to make estimates of ship velocity and velocity pro"les over the upper 400 m at 5-min intervals. Diel migration of larger zooplankton and mesopelagic "shes produced large variations in the ADCP's depth range, which extended from 350 to below 400 m during the day, but shoaled to near 250 m at night (Flagg and Kim, 1998). To reduce errors further, these estimates were averaged to form 15-min pro"les with times corresponding to those of the 15-min average SeaSoar pro"les. Flagg and Kim (1998) estimate the errors in the resulting velocity estimates to be approximately $3 cm s~1.

3. SeaSoar and hydrographic observations of the 1994+1995 monsoon cycle Seasonal and spatial contrasts of atmospheric and oceanic variability observed in 1994}1995 match many expectations from previous observations, but also reveal evidence supporting alternative explanations of the upper ocean's response to monsoonal forcing. A discussion of temporal and spatial variability of wind stress and surface heat #ux provides a framework for examining the observed upper-ocean response. We then examine oceanic variability along two sections. The "rst runs parallel to the coastal boundary, and the second extends o!shore from the Omani coast near Ra's Madrakah to mid-basin. Each discussion proceeds sequentially through our four observation periods, beginning with the Northeast Monsoon (December, 1994). Vertical sections are produced using one-dimensional objective mapping along each depth level (Bretherton et al., 1976). We use a Gaussian covariance function with 5 km e-folding scale and set fractional observational errors to 0.01. Only regions where the ratio of map error energy to signal energy falls below 0.2 are plotted. Although e-folding scales estimated from the observations (Section 5) are all longer than the 5 km chosen here, our goal is to retain the high along-track horizontal resolution attained by SeaSoar. 3.1. Meteorological contrasts The annual cycle of net surface heat #ux and wind stress (Fig. 3, after Weller et al., 1998) observed by the Woods Hole surface mooring in the central Arabian Sea (Fig. 2) provides temporal context for the four SeaSoar cruises. Our "rst cruise sampled conditions during the "rst half of the Northeast Monsoon (Fig. 3, NEM), when the moored measurements show moderate winds from the northeast and net surface cooling driven by evaporative heat #ux. The cruise started early in the monsoon and ended prior to the period of maximum surface cooling at the mid-basin mooring site. The Spring Intermonsoon (Fig. 3, SIM) cruise began approximately 10 days after the mid-basin net surface heat #ux reversed, with moderate warming under light winds. Strong surface warming and weak winds marked the interval between the February and June/July (Southwest Monsoon) cruises, but winds strengthened and net surface heat #ux fell with the onset of the Southwest Monsoon in early June. The Southwest

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Fig. 3. (after Weller et al., 1998) A year-long record of surface #uxes estimated from meteorological measurements taken at the WHOI mooring in the central Arabian Sea (Fig. 2). Green boxes mark the periods of the four SeaSoar cruises.

Monsoon (Fig. 3, SWM) cruise occurred when mid-basin winds were strong and the net surface heat #ux provided weak surface warming. Except for two brief periods in mid-June and mid-July, net surface heat #ux at the mooring site remained positive throughout the Southwest Monsoon (Weller et al., 1998). In the latter half of the Southwest Monsoon, net surface heating attained the highest values seen in the moored record, accompanied by moderately strong winds from the southwest. Winds subsided by the time of the Fall Intermonsoon (Fig. 3, FIM) cruise, with surface heat #ux remaining strong and positive. Surface #uxes estimated from shipboard meteorological measurements complement the moored observations by o!ering &snapshots' of spatial variability across the basin. From previous studies (Bauer et al., 1991; McCreary et al., 1993; Young and Kindle, 1994; Cayula et al., 1998) we anticipate that cross-basin variability in atmospheric forcing will strongly in#uence how the upper ocean evolves. Along-track meteorological records re#ect both spatial and temporal variability, which complicates interpretation. We expect atmospheric systems should have horizontal scales of

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hundreds of kilometers, while moored observations reveal meteorological decorrelation time-scales longer than 10 days. Thus, given the 6}10 days required to complete a SeaSoar section along the Southern Line, we attribute patterns that persist over both in- and outbound legs to spatial location, but caution against placing much weight on small-scale features. Early in the Northeast Monsoon we found strong winds accompanied by net surface cooling in the mid-basin region (Fig. 4). Windstress increased with distance from the coast, with peaks '0.15 N m~2 at mid-basin locations southeast of the moored array. Mid-basin winds maintained steady direction, while those nearer the coast showed more variability. The 24-h-averaged net surface heat #ux hovered near zero between the Omani coast and the WHOI mooring (waypoints D}E) early in the cruise, with evaporatively driven cooling of up to !200 W m~2 at and southeast of the mooring late in the cruise (8}12 December) in association with the strongest winds. The pattern of strong winds and surface cooling at mid-basin, weakening near the Omani coast, agrees with expectations from climatologies (Weller et al., 1998). During the Spring Intermonsoon, the entire survey saw weak winds blowing generally from the northeast, though directionality varied more than during the preceding Northeast Monsoon (Fig. 4). Weak surface cooling, associated with the strongest winds seen during the cruise, dominated the outbound section (11}16 February), after which net surface heat #ux changed sign and provided approximately 100 W m~2 of warming throughout the inbound leg (16}21 February). Meteorological observations during the Southwest Monsoon revealed strong contrasts between near-boundary and mid-basin conditions. Windstress generally increased with distance o!shore, though bursts such as the one occurring around 22 June suggest that winds along the coast also could be quite strong (Fig. 4). Windstress peaked at 0.4 N m~2 southeast of the moored array, though it remained strong across much of the Southern Line. The Monsoon winds blew consistently from the southwest, the only exception being at the start and end of the record, when the ship was north of Ra's al Hadd and sheltered by the Arabian land mass. Net surface heat #ux approached 250 W m~2 in the near-boundary region, weakening to 25}75 W m~2 mid-basin but remaining positive along the entire Southern Line. The patterns of windstress generally increasing and net surface heat #ux weakening with distance o!shore qualitatively agree with the spatial distributions indicated by the Southampton climatology (Weller et al., 1998). Observed windstress during the Fall Intermonsoon peaked at 0.15 N m~2 (Fig. 4, 20}25 September and 5}10 October), blowing north-northeast near the Omani coast. Moving o!shore (28 September}2 October), winds weakened and veered, blowing eastward at the mooring site and southeastward at the o!shore end of the southern survey line. Net surface heat #ux remained uniformly positive across the survey, ranging from 100}200 W m~2 with short-lived lows at the beginning of October and again at the end of the record, as the ship approached Muscat. Observed patterns of meteorological variability agree qualitatively with the recent Southampton climatology (Josey et al., 1998; Weller et al., 1998) and, with a few exceptions, with older climatologies and observations. Net surface cooling dominated during the Northeast Monsoon, while the Southwest Monsoon began with weak

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Fig. 4. Time-series of net surface heat #ux (black), 24-hour averaged net surface heat #ux (red) and surface windstress magnitude (black) and direction (red) estimated from shipboard meteorological observations for each of the four cruises. Vertical green lines mark waypoints (Fig. 2) and reference the time-series to alongtrack location.

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warming, which strengthened dramatically just following the peak winds. During both Northeast and Southwest Monsoon SeaSoar cruises, observed winds appeared more intense mid-basin than closer to the coastal boundary, though temporal variability may color the observed patterns. Wintertime (NEM) cooling also appeared strongest mid-basin, while surface warming during the Southwest Monsoon was strongest near the boundary, in the region of weakest winds. Thus, we anticipate strong cross-basin contrasts in convective overturning (NEM) and wind-driven turbulence (SWM), with intensity growing with distance o!shore. 3.2. Along-shelf contrasts The two monsoons impose dramatically di!erent forcing regimes along the Omani Coast, with downwelling-favorable winds during the Northeast Monsoon and upwelling-favorable winds during the Southwest Monsoon. This should drive reversing coastal currents and in#uence the relative roles played by coastal upwelling, southward advection of water from the Gulf of Oman and local, one-dimensional processes in setting variability along the shelf. Sections extending southward o!shore of the shelfbreak from the eastern edge of the Gulf of Oman to Ra's Madrakah (Fig. 2, waypoints A}D) characterized variability through the 1994}1995 monsoonal cycle. 3.2.1. Northeast Monsoon (November}December) During the Northeast Monsoon, the along-shelf section revealed high mixed-layer salinities, southward currents and mesoscale eddy activity (Fig. 5). In December we observed strong, surface intensi"ed southward #ow with peak speeds '0.5 m s~1, consistent with the sense of alongshore #ow expected over the shelf in response to northeasterly winds. Typical mixed-layer depth was 40 m (as de"ned by a 0.2 kg m~3 density change from surface values), with temperatures exceeding 273C (Fig. 5). Salinities '36.8 were the highest seen along the coast in any of the four cruises, though the surface layer freshened sharply near the southern end of the section. Elevated salinities may be the result of southward advection of highly saline surface waters from the Gulf of Oman (Warren et al., 1966; Qasim, 1982; Premchand et al., 1986) or may be driven by local forcing through strong evaporation associated with the Northeast Monsoon. Climatologies (Weller et al., 1998) suggest that evaporative #uxes should be largest mid-basin, but the latter mechanism also may play a role near the coast. In contrast to the one-dimensional upper-ocean response to the Northeast Monsoon observed at mid-basin (Fischer, 1998; Weller et al., 1998), strong southward currents and high salinities along the coast suggest that horizontal advection plays a signi"cant role in this region. Thus, along the shelf the response to the Northeast Monsoon cannot be described by a locally forced one-dimensional model. The along-shelf section also revealed two eddy-like structures with anomalously saline pycnocline waters (p '25 kg m~3) characteristic of the Gulf of Oman (Qasim, h 1982). Just south of waypoint B (km 200}300), weaker than ambient strati"cation between 50}200 m and anomalously warm, saline waters mark an apparent anticylonic eddy (Fig. 5). Southeastward (northern side) to westward (southern side) currents associated with the feature suggest that its core lay inshore of the section.

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Fig. 5. Objectively mapped alongshore sections of ADCP velocity vectors (plotted at selected depths), h (0.53C contour interval) with salinity (color) and p (0.1 kg m~3 contour interval) with log [chlorophyll h 10 a] (color) during the Northeast Monsoon (December). The horizontal axis represents distance from waypoint A (running north to south), with vertical red lines marking waypoints (Fig. 2) and small inverted triangles indicating pro"le spacing. Only regions where the relative error energy falls below 0.2 are plotted. Note the generally southward alongshore #ow and the prominent, eddy-like features centered at km 250 and km 525.

Highly saline Persian Gulf Water (Qasim, 1982; Premchand et al., 1986) appeared deeper, between 26.5 kg m~3'p '26 kg m~3 and extended southward as far as km h 550. Temperature-salinity characteristics and the presence of Persian Gulf Water suggest the feature originated within the Gulf of Oman, perhaps as a warm-core eddy pinched o! in a meander of the Ra's al Hadd front. The Ra's al Hadd front appears in our Southwest Monsoon alongshore section (Fig. 7, km 90) and in remotely sensed sea surface temperature images (Arnone et al., 2000) during and immediately following the Southwest Monsoon. The second feature occupied the region just south of waypoint

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C. Beneath the mixed-layer this feature formed sharp contrasts with ambient waters, but it had virtually no surface-layer temperature or salinity signature. Similar to the "rst feature, strati"cation within the lens was weaker than outside. Anticyclonic currents clearly accompanied this eddy, with southeastward #ow on the northern side and northwestward #ow on the southern end. The #ow pattern suggests that the section passed through the eddy core, providing an 80-km length scale estimate, which is slightly shorter than those determined from ADCP observations (Flagg and Kim, 1998). Chlorophyll #uorescence in this along-shelf section was uniformly low (Fig. 5). Beam attenuation at 660 nm (C660) mirrored the chlorophyll a pattern, and is therefore not shown here. Coincident chlorophyll a #uorescence and C peaks 660 suggest a subsurface maximum in biomass rather than a #uorescence maximum due to enhanced pigment concentrations through photoadaptation. The subsurface maximum was more evident north of waypoint C, while south of C the chlorophyll a maximum tended to be near-surface. This suggests that there were di!erences in nutrient distributions, with near-surface depletion where the subsurface maxima were observed and measurable nutrients where elevated near-surface concentrations were observed. We observed this situation when we performed a series of coastal hydrographic stations 12}14 days later. South of waypoint C, nitrate concentrations ranged from (0.5 to '2.5 lM, while north of C, nitrate concentrations were often (0.1 lM. Chlorophyll a #uorescence exhibited patch distributions with along-track length scales of O(10 km) (Section 5.1). 3.2.2. Spring Intermonsoon (February) The coherent southward #ow observed during the Northeast Monsoon vanished by February, when upper ocean currents weakened to speeds below 0.25 m s~1 with the relaxation of the monsoonal winds (Fig. 6). Currents strengthened to the south but were less surface-intensi"ed and no longer showed a preferred orientation relative to the coastline. The surface layer deepened and, relative to December, cooled to 24}253C and freshened to (36.6 (Fig. 6). Near the northern end of the section surface layers reached depths of 70 m, probably established by evaporatively driven convective overturning during the Northeast Monsoon. Weak vertical density gradients evident within this layer re#ect the onset of restrati"cation under light winds and net surface warming during the Spring Intermonsoon. A simple calculation shows that entrainment, as the mixed-layer deepens from late December to February, combined with surface cooling can roughly account for the changes in mixed-layer ¹}S properties. Though apparently important during the Northeast Monsoon, advection of high salinity water from the Gulf of Oman should play a decreasing role as the strong southward currents observed along the coast in December weaken. The surface layer shoaled and freshened to the south, with saline surface waters from the northern mixed-layer extending southward in a subsurface tongue near the mixed-layer base between km 250 and 350. At greater depth, the section also shows numerous temperature-salinity interleaving features with short horizontal scales of O(10 km) and vertical scales of O(20 m). The large eddy-like structures seen in the December section were absent in February, replaced by a weak front starting at km 250 and a small cold-core feature just

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Fig. 6. Alongshore sections during the Spring Intermonsoon (February), plotted as in Fig. 5. The southward #ow observed in December (Fig. 5) has vanished. Small-scale temperature-salinity interleaving features dominate variability within the pycnocline.

south of it (Fig. 6). The front separated colder, saltier waters to the north from warmer, fresher waters to the south, with weak temperature and salinity contrasts acting in concert to produce a noticeable density front. In contrast to the eddies observed during the Northeast Monsoon (Fig. 5), the front occupied only the upper 100 m and marked a change in strati"cation and water properties dividing the section into two distinct regions. Just south of the front, doming isopycnals and cyclonic currents suggest the presence of a cold-core eddy. Although its horizontal scale appears smaller than that of the warm-core features observed during the Northeast Monsoon, it is impossible to tell how we sectioned the feature, and the observed length scale should be taken as a lower bound. Chlorophyll a #uorescence was lower at the northern end of the section (near the Gulf of Oman and before waypoint B) than in the strati"ed surface layer south of

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Ra's al Hadd. Much of the section exhibited a slight subsurface peak in chlorophyll a below 25 m. A narrow region of enhancement occurred at km 300, where shoaling of the pycnocline, perhaps due to a cyclonic eddy, may have brought nutrients closer to the surface. Mixed-layer deepening during the period of convective overturning produced elevated nutrients in the surface layer (Morrison et al., 1998). During January 1995, nitrate concentrations '3 lM were observed within the surface layer of the Arabian Sea. Subsequent restrati"cation retains phytoplankton within the newly enriched euphotic zone, driving a post-Northeast Monsoon phytoplankton bloom. Elevated chlorophyll a #uorescence and beam attenuation extended only to 40 m, suggesting that su$cient light for photosynthesis did not penetrate much deeper than this depth during our sampling period. The subsurface chlorophyll a maximum suggests that this bloom had already progressed from its initial period, when higher concentrations extending to the surface would be expected. As during the Northeast Monsoon, chlorophyll a #uorescence and beam attenuation were patchily distributed with O(10 km) length scales. Near-surface samples from the shipboard underway pumping system indicate that cyanobacteria were a signi"cant component of the upper-layer phytoplankton population during this period and that they exhibited strong diurnal rhythms (Sherry, 1995). 3.2.3. Southwest Monsoon (June}July) During the Southwest Monsoon, along-shelf currents were generally oriented northeastward (downwind) (Fig. 7). North of waypoint B, where the Arabian landmass shields the Gulf of Oman from the monsoonal winds, currents #owed southeastward along the Omani coast. Mixed-layers were vanishingly thin in this sheltered region, with strong strati"cation in the upper 50 m. Elevated surface layer temperatures ('263C) and salinities '36.5 mark water advected out of the Gulf of Oman (Fig. 7). A strong near-surface temperature}salinity front sat slightly north of waypoint B, accompanied by currents #owing o!shore from the Omani coast. This feature, which we call the Ra's al Hadd front, persists in observations during and following the Southwest Monsoon. Warm, salty water from the Gulf of Oman formed a sharp interface with colder, fresher Arabian Sea water. Temperature-salinity characteristics suggest that the southern surface water mass originates through coastal upwelling. The Ra's al Hadd front may simply be the con#uence of waters being carried southward out of the Gulf of Oman and coastally upwelled waters being advected northward in the alongshore jet that is presumably established by the winds of the Southwest Monsoon. High-salinity water extended 50 km south of the front in a 50 m thick subsurface tongue. Beneath this, again between 26.5 kg m~3'p ' h 26 kg m~3, high salinity Persian Gulf water extended southward along the Omani coast. Isopycnals along the open coast (south of B) show an average 10}20 m upward displacement relative to the Spring Intermonsoon. Coastal "laments may act as conduits for moving nutrient rich, coastally upwelled waters rapidly into the central basin, thus exerting a strong in#uence on the basinwide response to the Southwest Monsoon (Young and Kindle, 1994; Keen et al., 1997). Doming isopycnals, swift northeastward currents (1 m s~1) and the largest values of chlorophyll a #uorescence observed in any of the four cruises (8 mg m~3) mark a cool

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Fig. 7. Alongshore sections from the Southwest Monsoon (June-July), plotted as in Fig. 5. Cooler waters and elevated chlorophyll a concentrations mark a cool "lament at the southern end of the section. The section samples numerous temperature-salinity interleaving features within the pycnocline, as well as two high salinity features between km 0}100 (near p "26.5 kg m~3) which appear to have originated in the h Persian Gulf.

"lament at km 400}500 (Fig. 7). Satellite observations (Brink et al., 1998) and objective maps of SeaSoar surveys (Brink et al., in preparation) indicate that we sampled the "lament in a region where it bifurcated into strong eastward and weaker northeastward #owing branches. Filament waters were colder and fresher than their surroundings, with extremely thin mixed-layers (Brink et al., 1998). Within the "lament, patches of enhanced chlorophyll a #uorescence were con"ned to the upper 50 m and had horizontal scales of O(20 km). The highest observed values of

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Fig. 8. Alongshore section from the Fall Intermonsoon (September), plotted as in Fig. 5. The upper layer has cooled and freshened since June. Note the elevated surface layer chlorophyll a concentrations and the two high-salinity Persian Gulf water features near p "26.5 kg m~3. h

chlorophyll a #uorescence were associated with the "lament, though the section revealed other patches between km 200 and 400 with apparently smaller horizontal scales. These may have been associated with nutrient rich waters brought to the surface by coastal upwelling. 3.2.4. Fall Intermonsoon (September}October) By mid-September, both monsoonal winds and northeastward currents have relaxed, leaving generally weak, unorganized #ows along much of the Omani coast (Fig. 8). Instrument di$culties limited pro"les to the upper 100 m in the northern part of the section, obscuring the structure of the Ra's al Hadd front near waypoint B. As during the Southwest Monsoon, the front separated warmer, saltier Gulf of Oman

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waters from fresher waters with coastal ¹}S characteristics to the south. During the Fall Intermonsoon, frontal currents were oriented northward with peak speeds greater than 0.5 m s~1. Surface waters in the southern part of the section cooled and freshened over the course of the Southwest Monsoon, with strati"cation stretching nearly to the surface and isopycnal depths showing mean upward displacements '20 m relative to June. Persian Gulf water extended as far south as waypoint C, appearing in two small features with horizontal scales of O(20 km) near p "26.5 kg m~3. Small patches of elevated chlorophyll a #uorescence appeared h between the surface and 20 m depth, though the intensity of the post-Southwest Monsoon bloom is strongest in mid-basin (Fig. 13). The highest #uorescence in the coastal section occurred between waypoints B and C within the upper 20}30 m, where the previous cruise observed relatively low chlorophyll a concentrations. 3.3. Cross-basin contrasts: the US JGOFS southern line SeaSoar observations through the 1994}1995 Monsoon cycle revealed strong spatial contrasts between coastal and mid-basin conditions. SeaSoar sections extend o!shore from the Omani coast, following the Southern Hydrographic Line occupied by a series of US JGOFS Arabian Sea process cruises (Fig. 2; Smith et al., 1998; Morrison et al., 1998). Dropouts in the sections mark intensive surveys performed on either side of the climatological windstress maximum (between km 300}400 and km 775}875) and near the mid-basin moored array (km 550). Due to the intensive surveys, the Southern Line sections required approximately 10 days to complete. Though we delay a discussion of temporal scales until Section 5.1, our limited results indicate that 10-day sections cannot be considered entirely synoptic, particularly in seasons exhibiting energetic mesoscale activity. 3.3.1. Northeast Monsoon (November}December) Early in the Northeast Monsoon, mixed-layers deepened with distance o!shore (Fig. 9), consistent with cross-basin contrasts in surface cooling seen in models and climatologies (Cayula et al., 1998; Weller et al., 1998). These studies indicate that the strongest cooling, thus the most active convective mixed-layer deepening, occurs mid-basin. Intense cooling starts in the southern and central Arabian Sea and shifts northward as the season progresses. In contrast to southward currents observed along the Omani coast (Fig. 5), mid-basin currents #owed generally northward, peaking 550 km o!shore at 0.5 m s~1 (Fig. 10). Isopycnals sloped downward to the east between 500}800 km o!shore, supporting northward shears. This pattern is consistent with the observed strong, surface-intensi"ed, northward mid-basin #ow. Near the end of the section, currents turned to the southwest, consistent with ship drift observations and model results (McCreary et al., 1993). Mixed-layers deepened from 40 m near the coast to 70 m over the last third of the section (Fig. 9). Strati"cation at the base of deep mid-basin mixed-layers (Fig. 10) was weaker than that beneath shallower mixedlayers found nearer the coast (Fig. 5). Surface layer temperatures increased slightly with distance o!shore, while mixed-layer salinities peaked at 36.5 both near the coast and mid-basin, with fresher waters near the moored array (km 600, Fig. 9).

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Fig. 9. Surface layer temperature, surface layer salinity and mixed-layer depth along the southern survey line, where the horizontal axis represents distance from the Omani coast. Averaging between 0}10 m produces surface layer estimates, while mixed-layer depths are calculated by "nding the depth of the 0.2 kg m~3 change from surface layer p . Dropouts occur when pro"les do not reach shallow enough to h form surface layer estimates and when the ship turned o! the Southern Line to make intensive surveys. Di!erent colors represent the four cruises (NE Monsoon: light blue, Spring Intermonsoon: red, SW Monsoon: green, Fall Intermonsoon: dark blue). Note the lack of cross- basin mixed-layer depth contrasts during the Southwest Monsoon and the general cooling and freshening between this period and the subsequent Fall Intermonsoon.

If conditions during the following (September}October, 1995) Fall Intermonsoon (Figs. 9 and 13) can be considered representative, our observations indicate that the Northeast Monsoon drives mid-basin cooling along with coastal and mid-basin salinity enhancement. SeaSoar sections also suggest that mixed-layer deepening intensi"es with distance o!shore. The Northeast Monsoon is associated with strong mid-basin latent heat loss and net surface cooling (Fig. 4, waypoint F), and we expect convectively driven mixed-layer deepening and evaporatively driven salinity enhancement in this region. Nearer the coast, both observations (Fig. 4, waypoints A}D) and climatologies (Weller et al., 1998) show weaker winds, resulting in less latent heat loss and weaker mixed-layer deepening. Elevated salinities near the shelf may be the result

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Fig. 10. Southern Line section during the Northeast Monsoon, plotted as in Fig. 5, except the horizontal axis now represents distance from the Omani coast. Vertical red lines mark waypoints (Fig. 2). Note the surface intensi"ed northward #ow between km 500}800. The mixed-layer deepens with distance o!shore. Numerous small-scale temperature-salinity interleaving features occupy the pycnocline.

of lateral advection of more saline waters from the north. The pattern of mixed-layer deepening with distance o!shore runs contrary to the mixed-layer depths expected from simple Ekman pumping arguments, which indicate that open-ocean downwelling near the coast and upwelling o!shore of the windstress maximum should produce mixed-layers that shoal with o!shore distance. The observations suggest that Ekman pumping is too weak to compete with convectively driven mixed-layer deepening during the Northeast Monsoon. Pycnocline waters within 400 km of the coast were more saline than those farther o!shore (Fig. 10), with ¹}S characteristics similar to those observed at the southern end of the along-shelf section (Fig. 5). Southward advection and spreading of water from the Gulf of Oman may account for these elevated salinities. Mid-basin waters were cooler and fresher, similar to conditions observed in this region during the 1995 Fall Intermonsoon (Fig. 13). Farther o!shore, elevated salinities appeared in the strati"ed waters below the mixed-layer base, though deeper waters remained fresh.

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SeaSoar measurements revealed elevated chlorophyll a #uorescence throughout the surface mixed-layer, with the region of shallow mixed-layers inshore of waypoint E having the highest concentrations. Hydrographic measurements taken along the section during the inbound leg revealed upper layer nitrate concentrations of 2.0}3.4 lM near the coastal boundary (Fig. 14) associated with chlorophyll a #uorescence values similar to those observed by SeaSoar. Waters immediately beneath the pycnocline had low oxygen concentrations, with the lowest values (0.05 ml l~1) occurring nearshore at a depth of only 63 m. 3.3.2. Spring Intermonsoon (February) Upper-ocean variability observed in February re#ects patterns we might expect from convective mixed-layer deepening and cross-basin variations in windstress and surface cooling from the previous Northeast Monsoon. Currents within 600 km of the coast #owed generally southward with weaker, variable currents farther into the basin (Fig. 11). The prominent downward/eastward isopycnal tilt and resulting northward mid-basin #ow observed in December have vanished, replaced by a weakly strati"ed surface layer extending 100 m to a strongly strati"ed base (Fig. 11). Time series from the moored array (Fischer, 1997) show that convective overturning during the Northeast Monsoon produced deep mid-basin mixed-layers, onto which the strong surface heating and light winds of the subsequent intermonsoon introduced weak strati"cation. Surface waters warmed with distance o!shore, with peaks above 263C, but were cooler overall than those observed in the Northeast Monsoon (Fig. 9). Similar to conditions during the Northeast Monsoon surface-layer salinities peaked near the coast and o!shore, ranging from 36.25 near the moored array to nearly 36.5 at the start and end of the section. Southward advection of high-salinity water from the Gulf of Oman apparently produces elevated coastal salinities, while heightened mid-basin surface-layer salinities are consistent with strong evaporation during the Northeast Monsoon. High-salinity water occupied the region between km 200 and 300 km. Elevated salinities extended below the maximum pro"le depth, and were associated with cooler (warmer) surface-layer (pycnocline) temperatures and weakened strati"cation (Fig. 11). Subsurface salinities peaked between km 200 and 350 at 26(p ( h 26.5 kg m~3, consistent in location with Persian Gulf Water, but generally fresher and cooler than similar waters observed in coastal sections from other seasons (Figs. 5, 7 and 8). Currents and coastal conditions (Fig. 6) during the Spring Intermonsoon suggest decreased southward transport out of the Gulf of Oman, and this salty feature may be a remnant advected o!shore during the intermonsoon. Observed temperatures and salinities between the 26}26.5 kg m~3 isopycnals are consistent with mixing between highly saline Gulf of Oman out#ow and cooler, fresher ambient waters along the coast. Shoaling mixed-layers during the Northeast Monsoon force phytoplankton to spend a greater proportion of time in the euphotic zone. The resulting increase in light, combined with high nutrient levels produced by deep mixing during the Northeast Monsoon, initiates a post-monsoon bloom. This bloom was characterized by elevated chlorophyll a concentrations across most of the section, often extending to 40 m. The

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Fig. 11. Southern Line section during the Spring Intermonsoon, plotted as in Fig. 10. Patchy regions of elevated chlorophyll a concentration appear in the upper 50 m. The surface layer has deepened and cooled since December and is marked by weak strati"cation extending as deep as 100 m.

vertical extent of the bloom was most likely limited by light penetration, which is dependent on the concentration of phytoplankton in the water column. Where the chlorophyll a concentrations were lower, this layer extended to 60}70 m, as observed near km 650}675. Peak chlorophyll a concentrations exceeded 1.5 mg m~3 with horizontal scales of O(10 km). Southern Line near-surface nitrate concentrations were less than 1 lM at all but one station, and less than 0.5 lM at 5 of the 11 stations (Fig. 14). Silicate also dropped below 1 lM at several stations between km 400 and 800, especially where upper-layer chlorophyll concentrations were highest (Fig. 14). From this we infer that diatoms were at least a component of the post-Northeast Monsoon bloom, since both nitrate and silicate were present in similar concentrations in the depth range of 60}100 m. 3.3.3. Southwest Monsoon (June}July) Currents and mixed-layer depths along the Southern Line section di!ered both from the traditionally anticipated response to the strong, steady forcing of the

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Fig. 12. Southern Line section during the Southwest Monsoon, plotted as in Fig. 10. The cool "lament seen in Fig. 7 appears here between km 300}400 m. Cooler waters, doming isopycnals and elevated chlorophyll a concentrations mark the "lament. Mixed-layers are 30}40 m deep within the "lament and 50}60 deep outside. Contrary to traditional expectations, the mixed-layer does not deepen with distance o!shore. Surface layer salinities are largest at the section's o!shore end, with numerous interleaving features in the pycnocline.

Southwest Monsoon (Rao et al., 1989; Bauer et al., 1991) and from the Levitus climatology (Fig. 16). Within 700 km of the coast, currents displayed moderate speeds and a range of directions rather than forming a coherent, northeastward #ow (Fig. 12). Farther o!shore, the observed, generally southeastward, #ow was as anticipated from ship drift observations and model results (e.g. Rao et al., 1989; McCreary et al., 1993). Nearer the inshore intensive survey (waypoint D2), steeply sloped isopycnals associated with the cool "lament supported southward currents with peak speeds over 1 m s~1. Mixed-layer depths have shoaled considerably since the Spring Intermonsoon (Fig. 9) and remain near 60 m across most of the section. The "lament provides the sole exception to nearly uniform mixed-layer depth, where 40-m surface layers show moderate strati"cation extending nearly to the surface. Filament waters were colder and fresher than their surroundings, characteristics that suggest coastally

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upwelled origins. The pattern of mixed-layer depths depends little on position relative to the windstress maximum. Instead it re#ects the presence of "lament waters. This suggests that wind-driven mixing and advective processes play larger roles than Ekman pumping-driven upwelling and downwelling in setting open-ocean mixedlayer depths. East of the moored array (km 550), the mixed-layer warmed and became more saline following the Spring Intermonsoon, consistent with an extended period of restrati"cation followed by strong evaporative salinity enhancement driven by the strong winds of the Southwest Monsoon. Watermass properties beneath the mixed-layer remained similar to those observed during the Spring Intermonsoon. Salinities in the upper 100 m were higher than those below (Fig. 12), with horizontal variability similar to that observed during the Spring Intermonsoon (Fig. 11). This vertical structure, with elevated salinities extending below the mixed-layer base, could be obtained by simply heating the February pro"les from the surface under light winds to create a new, shallower mixed-layer with strati"ed waters below. Isopycnals of greater than 25.0 kg m~3 were displaced slightly downward from their February depths (Fig. 19), but retained similar ¹}S characteristics (Figs. 11 and 12). Unlike conditions during the Northeast Monsoon and Spring Intermonsoon, highly saline waters from the Gulf of Oman did not extend this far south, perhaps because #ow out of the Gulf appeared to turn o!shore farther to the north, where it met the northward #owing Omani Coastal Current (Fig. 7). Wind-driven mixing during the Southwest Monsoon entrained water from beneath the mixed-layer base and produced measurable near-surface nutrient levels well into the mid-basin. Elevated levels of chlorophyll a #uorescence were found at the top of the seasonal pycnocline, near the 23.5}24.0 kg m~3 isopycnal (Fig. 12). Mixed-layer nitrate concentrations fell below 1 lM in this region (Fig. 14), beneath the level required to support a large standing stock of phytoplankton. Below the mixed-layer, there was probably insu$cient light for photosynthesis. Within the "lament, cool, fresh waters had elevated nutrient levels and chlorophyll concentrations. Lateral advection may carry some of this material from near the coast, while the rest could be upwelled along the "lament or introduced through entrainment at the mixed-layer base. Phytoplankton growth continued within the "lament as it #owed o!shore (Brink et al., 1998). 3.3.4. Fall Intermonsoon (September}October) Mixed-layer and upper-pycnocline waters across the entire Southern Line have cooled and freshened (Fig. 13) from conditions observed during the Southwest Monsoon (Fig. 12). Surface-layer temperatures dropped by 13C, accompanied by freshening of 0.25 or more (Fig. 9). Both surface-layer temperature and salinity increased with distance o!shore, though salinities in the upper 150 m (Fig. 13) were everywhere fresher than those observed in summertime (Fig. 12). Despite only minor variations in mixed-layer depth across the section (Fig. 9), strati"cation changed dramatically with distance o!shore (Fig. 13). Shallow mixed-layers were found inshore of the moored array (waypoint E), while farther o!shore a similar surface layer rested atop a deeper, weakly strati"ed remnant mixed-layer. Weak, variable currents occupied

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Fig. 13. Southern Line section during the Fall Intermonsoon, plotted as in Fig. 10. Waters along the entire section have cooled and freshened since June. The section exhibits uniformly deep (20 m) mixed-layers, though o!shore of km 600 remnant mixed-layers extend to 100 m. A subsurface chlorophyll a maximum extends across the section between 20}30 m, with the highest concentrations o!shore of km 500.

the region within 600 km of the coast, while isopycnals sloping downward to the east supported a surface intensi"ed, northward #ow between km 600 and 800 (Fig. 13). High chlorophyll #uorescence, with peaks exceeding 3 mg m~3, followed the base of the mixed-layer across the entire section. Patch sizes were O(10 km) with the highest concentrations found o!shore of km 500, especially in the region overlying the deep, remnant mixed-layers. Deep, fresh, cool remnant mixed-layers indicate that the monsoon has driven signi"cant mid-basin entrainment with associated nutrient c Fig. 14. NO and SiO concentrations from hydrographic casts performed along the Southern Line 3 4 following each SeaSoar survey. To resolve low near-surface concentrations relative to the larger values found in and below the nutricline, the depth range is restricted to the upper 100 m and values *6 lM are plotted are plotted at 6 lM.

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#ux into the euphotic zone. In this o!shore region, nitrate concentration was less than 0.5 lM in the upper 25 m and increased to 5}8 lM at 75 m, compared with the Southwest Monsoon when nitrate was (0.5 lM from the surface to at least 75 m. This pycnocline shoaling and/or entrainment of the cooler, fresher, nutrient-rich water resulted in an increase of the phytoplankton biomass as nutrients were moved shallower within the euphotic zone.

4. The role of Ekman pumping The atmospheric jets associated with the monsoons produce large lateral shears in surface windstress and may thus drive vertical motions in the upper ocean through both coastal upwelling/downwelling and Ekman pumping. Previous modeling studies (Luther and O'Brien, 1985; Bauer et al., 1991; McCreary et al., 1993) suggest that Ekman pumping-driven upwelling/downwelling on either side of the seasonal windstress maximum plays a dominant role in setting mixed-layer and pycnocline depths. In contrast to the direct relationship between Ekman downwelling and pycnocline displacement downwards, Ekman downwelling is an upper bound on the mixed-layer deepening, due to possible shoaling processes at work on the mixed-layer. Other studies (McCreary et al., 1996; Keen et al., 1997) emphasize the roles played by o!shore advection of coastally upwelled waters and by local mixing processes. We explore the role that open-ocean Ekman pumping might play in the oceanic response to the 1994}1995 monsoonal cycle using gridded SOC wind and surface-#ux "elds (Josey et al., 1998; Weller et al., 1998; Josey et al., 1999). The SOC "elds are based on an analysis of the COADS data reports from 1980 to 1995 and provide monthly mean data for each year spanning the entire basin with one-degree resolution. Weller et al. (1998) "nd that the SOC data agree well with mid-basin moored meteorological observations made through the 1994}1995 Monsoon cycle. The vertical velocity driven by Ekman pumping may be expressed as

A

B

1 b w " +]q6 # qx , EP fo 0 f 0 0

(1)

where q "(qx , qy ) is the surface windstress, o the density, f the Coriolis frequency 0 0 0 0 and b a measure of the its variation with latitude. Thus, w is the sum of the EP windstress curl term and a second term, arising from latitudinal variations in the Coriolis frequency (b), proportional to the zonal windstress. As Fischer (1997) notes, the second term can play an important role at low latitudes, where b is large, during extended periods of strong zonal winds. Arguments for Ekman upwelling/downwelling on either side of the Southwest Monsoon windstress maximum (e.g. Bauer et al., 1991) typically consider only the windstress curl term, though under the strong, steady northeastward winds of the Southwest Monsoon the second term may play a signi"cant role. We calculate time series of w along the Southern Line using SOC 1995 winds EP and estimating windstress curl from centered "rst di!erences of the gridded data.

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Fig. 15. Vertical displacement driven by Ekman pumping at the inshore and o!shore intensive survey sites. SOC winds provide the curl "elds used to estimate Ekman pumping vertical velocities. Solid lines indicate the displacement resulting from +](q /f ), which is the sum of a windstress curl term +]q (dashed lines) 0 0 and a b term bqx /f (dotted line). The b term becomes important during the Southwest Monsoon, when zonal 0 winds are strong. The region inshore of the windstress maximum experiences upwelling during the Southwest Monsoon, while the o!shore region sees both weak upwelling and downwelling. Both regions experience weak downwelling during the remainder of the year.

Time-integrating the resulting vertical velocities yields vertical displacements associated with the windstress curl term, b term and total Ekman pumping over both the in- and o!shore intensive SeaSoar surveys (Fig. 15). At approximately 350 and 800 km o! the Omani coast, these survey areas were chosen to lie on either side of the climatological windstress maximum. Considering only the windstress curl term (dashed line, Fig. 15), we "nd downwelling in both regions during the Northeast Monsoon and upwelling (downwelling) inshore (o!shore) of the windstress maximum during the Southwest Monsoon. The b term (dotted line, Fig. 15) becomes important with the onset of the Southwest Monsoon (June), driving upwelling in both regions. This strengthens inshore upwelling and o!sets the windstress curl-driven downwelling in the o!shore region. Inshore, the net vertical displacement (windstress curl#b term, solid line, Fig. 15) indicates strong upwelling in June and July, followed by weak

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vertical motions in August and September. The o!shore region experiences weak downwelling in June and July, which reverses to weak upwelling in August and September. Ekman pumping in 1995 di!ers slightly from the 1980}1995 SOC climatology (Fig. 17). Inshore, 1995 upwelling is 0.5}1.5 standard deviations larger than climatology. O!shore Ekman pumping is within 0.5 standard deviation of climatology, which indicates weak downwelling through all four months of the Southwest Monsoon. Ekman pumping drives strong upwelling inshore of the windstress maximum but only weak downwelling o!shore. This di!ers sharply from the strong downwelling o!shore of the windstress maximum indicated when considering only the windstress curl. A comparison of Ekman pumping with an entrainment velocity parameterizing the e!ects of wind-driven mixing provides a measure of the relative importance of these two processes during the Southwest Monsoon over the entire basin. Kraus and Turner (1967) propose a simple parameterization for mixed-layer deepening, which balances the net rate of production of turbulent kinetic energy (TKE) from surface wind and buoyancy forcing with the gain in potential energy due to entrainment of denser water into the mixed-layer. We use the formulation of Kraus and Businger (1994) for the entrainment velocity at the mixed-layer base,

C

w "!2 KT

D

m u3 !1h(ag/o c )Q 1 H 2 0 p 0 , ag *¹h

(2)

where a is the thermal expansion coe$cient, g the acceleration due to gravity, o the 0 reference density, c the speci"c heat of seawater, Q the net surface heat #ux, p 0 h the mixed-layer depth and *¹ the temperature jump across the mixed-layer base. Friction velocity is calculated from the surface windstress, u3 "(q /o)3@2 and m is an H 0 1 &e$ciency' parameter that adjusts the emphasis between wind-mixing (m large) and 1 surface heating (m small). Kraus and Turner (1967) used m "0.5, while typical 1 1 values for m range from 0.5 to 1.0 in modeling studies of the Arabian Sea (e.g. 1 McCreary and Kundu, 1989; McCreary et al., 1993). Eq. (2) is valid only when the denominator * the rate of production of TKE * is positive, yielding entrainment (w (0). When the mixed-layer is detraining it is modeled to shoal instantaneously KT to the Monin}Obukhov length, which balances the two terms of the rate of TKE production. In our calculation this change in mixed-layer depth is divided by the month-long timestep of the climatology, yielding a velocity. We use SOC 1980}1995 monthly mean windstress and net surface heat #ux climatologies for June}September 1995 and Levitus monthly mixed-layer depth climatology (Boyer and Levitus, 1994; Levitus and Boyer, 1994) to make basin-wide estimates of Ekman pumping and entrainment velocities. Ekman pumping (Eq. (1)) is estimated using SOC climatological monthly mean windstress, with gradients calculated from centered "rst di!erences of the gridded data. Climatological monthly mean windstress and net surface heat #ux are used to estimate entrainment velocity from Eq. (2). Based on the June 1995 Southern Line section (Fig. 12) we use a temperature jump (*¹) of 23C across the mixed-layer base. We choose an &e$ciency' of m "0.5 to avoid unduly enhancing the role wind-driven mixing. Mixed-layer 1

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depths for each month are taken from the Levitus climatology. Under steady surface forcing, entrainment velocity decreases as mixed-layer depth increases. Thus, during periods of mixed-layer deepening we expect wind-driven entrainment to play a smaller role than indicated by these monthly mean estimates. We do not expect this to be an issue northwest of the windstress maximum (Fig. 1), where the sequence of Levitus mixed-layer depths (Fig. 16) shows only modest changes over the course of the Southwest Monsoon. The southeastern half of the basin exhibits rapid deepening between June and July. The June entrainment estimates for this part of the basin may thus overestimate the e!ects of wind-mixing. Northwest of the windstress maximum between June}August, estimates of winddriven entrainment velocity greatly exceed those of Ekman pumping (Fig. 16). During the Southwest Monsoon, Ekman pumping works to displace the seasonal pycnocline, and thus the mixed-layer base, upwards while entrainment acts in the opposite sense to deepen the mixed-layer. Ekman upwelling also may modulate wind-driven mixedlayer deepening by increasing the strati"cation at the mixed-layer base and by altering mixed-layer depths. While the balance of entrainment and upwelling suggests mixedlayer deepening, Levitus climatology indicates that mixed-layer depths in this region deepen slightly from June to July and then undergo little change until they shoal dramatically in September. Ekman pumping acts to partially counter wind-driven deepening, but is not strong enough to account for the consistently shallow mixedlayers seen throughout this period. Likewise, neither Ekman pumping or entrainment can account for the rapid shoaling of mixed-layer depth seen between August and September. We suggest that horizontal advection of coastally upwelled waters must also contribute to maintaining shallow mixed-layers and strati"cation in the presence of strong wind-mixing. Both AVHRR imagery (Arnone et al., 2000) and the strong net heat #ux into the ocean seen in the SOC data between the Omani coast and the windstress maximum indicate that cold surface waters extend o!shore well beyond the coastal upwelling zone. Southeast of the windstress maximum, Ekman pumping and wind-driven mixing (Fig. 16) act in concert to deepen the mixed-layer. The largest change in climatological mixed-layer depth occurs between June and July (Fig. 16), when the southeastern half of the basin undergoes rapid deepening. During this period, both wind-driven entrainment and Ekman pumping act to deepen the mixed-layer, though Ekman pumping is weak and by itself would be unable to account for the observed deepening. Both processes contribute to additional deepening between July and August. In August, weakening winds and stronger surface heating produce detrainment (Fig. 16), consistent with shoaling mixed-layers seen in the climatology between August and September. Ekman pumping indicates weak downwelling throughout this period. The magnitudes and timing of the entrainment and Ekman pumping suggest that winddriven mixing plays a dominant role in establishing deep mixed-layers to the southeast of the windstress maximum early in the Southwest Monsoon. A comparison of SOC 1980}1995 climatological with SOC 1995 Ekman pumping provides a measure of whether the 1995 Southwest Monsoon could be described in a similar fashion. The di!erence between July 1995 and July 1980}1995 climatological Ekman pumping (normalized by the pointwise standard deviation of the 1980}1995

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Fig. 16. Maps of SOC 1980}1995 climatological monthly mean Ekman pumping (w , 0.05]10~4 ms~1 EP contour interval), wind-driven entrainment (w , 0.05]10~4 ms~1 contour interval) and Levitus mixedKT layer depth during the Southwest Monsoon (June}September). We use the convention of a negative velocity representing deepening or entrainment into the mixed-layer. The locations of the Southern Line and moored array are marked as in Fig. 1. Shallow mixed-layers and large w o! the west coast of India, KT outside of our study domain, are artifacts of the Levitus climatology.

climatology) indicates that Ekman upwelling (downwelling) inshore (o!shore) of the windstress maximum was stronger than climatology (Fig. 17). Ekman pumping was usually within one standard deviation climatology, and only very rarely more than two standard deviations away. The pattern of the di!erences varies somewhat from month to month, but 1995 Ekman pumping stays within one standard deviation of climatology over most of the basin through the 1995 Southwest Monsoon.

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Fig. 16 (continued)

Errors in the SOC climatology will a!ect our estimates of Ekman pumping and wind-driven entrainment. We calculate both bias and random errors in the SOC "elds by comparing windstress and net surface heat #ux at the moored array with SOC data from the same mid-basin location. The biases were estimated using a least-squares match between monthly means of the mooring data and the SOC "elds. Random errors were then estimated by calculating the standard deviation of the di!erence between the SOC and biased mooring values for the Southwest Monsoon. Combining the random error with error estimates for the moored measurements (Weller et al., 1998) yields an upper bound for the random errors in windstress and net surface heat #ux (Table 1). Propagating the random errors through Eqs. (1) and (2) yields errors

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Fig. 17. The di!erence between SOC July 1995 Ekman pumping (not shown) and SOC July 1980}1995 climatological Ekman pumping (Fig. 16), normalized by the standard deviation of the 1980}1995 data (0.5 contour interval). Ekman pumping in 1995 was typically within one standard deviation of the mean, and rarely farther than two standard deviations away. During the 1995 Southwest Monsoon Ekman upwelling (downwelling) inshore (o!shore) of the windstress maximum was slightly stronger than the climatological mean.

Table 1 Estimates of the error in climatological values Error

Wind stress, q

Bias (SOC over buoy) Random, total Due to SOC Uncertainty in buoy

Factor of 1.32 0.021 N m~2 0.018 0.010

0

Net heat #ux, Q

0

5 W m~2 32 28 15

that are roughly an order of magnitude smaller than estimated Ekman pumping and wind-driven entrainment values (Fig. 18). At low latitudes, outside of our study domain, Ekman pumping errors can become large enough to be signi"cant. The bias errors suggest the SOC climatology overstates both wind-driven mixing and Ekman pumping. The windstress bias error appears as a scaling factor, which reduces the magnitude of Ekman pumping by approximately 24%. Although entrainment is a function of both windstress and net surface heat-#ux, the heat-#ux bias error is small, and we consider only the e!ect of the windstress bias. This indicates a 34% decrease in the magnitude of wind-driven entrainment. Random errors in Ekman

Fig. 18. Errors in Ekman pumping and wind-driven entrainment estimates resulting from random errors in SOC windstress and net surfae heat #ux. The color scheme and the contour interval (0.01]10~4 ms~1) are di!erent than in Fig. 16. Ekman pumping errors depend on latitude, while layer depth modulates the size of entrainment errors.

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pumping and wind-driven entrainment are too small to alter the results presented here. Although bias errors a!ect entrainment more strongly than Ekman pumping, the di!erence is not large enough to alter the relative sizes of the two terms, and their spatial patterns and timing remain unchanged. Entrainment estimates rely strongly on the mixed-layer depth, on the assumed value of &e$ciency' (m ) and on the temperature jump across the mixed-layer base 1 (*¹). Mixed-layer depth e!ects are discussed above, though it is important to note that using a mixed-layer climatology with shallower (deeper) mixed-layers would result in larger (smaller) entrainment estimates. We chose a modest value for m . 1 Larger &e$ciencies' have been used in numerical models of this region (McCreary and Kundu, 1989; McCreary et al., 1993), and these would simply amplify the e!ects of wind-driven entrainment linearly. We estimated *¹"23C from the steep temperature jump at the mixed-layer base. A *¹ of 43C would be more appropriate if we wanted to capture the temperature jump between the mixed-layer and the upper part of the pycnocline. Substituting *¹"43C simply halves the size of entrainment. In regions of strong windstress, such as the area between the windstress maximum and the Omani coast, entrainment remains larger than Ekman pumping even after this reduction. Assuming *¹"43C makes Ekman pumping and entrainment of similar size in low windstress areas and in the southern part of the basin. Ekman pumping cannot explain mixed-layer depth and water property contrasts observed along the Southern line during and following the Southwest Monsoon. Wind-driven entrainment overwhelms the e!ects of Ekman upwelling inshore of the windstress maximum. As Ekman upwelling is too weak to counter wind-driven mixed-layer deepening, another mechanism must act to maintain the shallow mixedlayers seen in climatologies (Fig. 16 and Rao et al., 1989) and observations (Figs. 9 and 13). Although not speci"cally addressed here, large isopycnal displacements and water property contrasts associated with the southward-#owing coastal "lament (Figs. 7 and 12) suggest that horizontal advection plays a strong role in both the mixed-layer and pycnocline response to the summer monsoon. Freshening and cooling observed between June and September both along the Southern Line sections (Figs. 12 and 13) and at the mid-basin mooring (Weller et al., 1999) cannot be explained by surface #uxes, mixing and vertical advection. O!shore advection of coastally upwelled waters may play a signi"cant role. Isopycnal displacements within the pycnocline exhibit some patterns consistent with those attributed to Ekman pumping in the previous model results (McCreary et al., 1993). Large seasonal variations in pycnocline isopycnal depths occur within 700 km of the Omani coast with smaller displacements farther o!shore (Fig. 19), consistent with expectations that both Ekman pumping (Fig. 15) and horizontal advection of coastal waters (McCreary et al., 1996; Keen et al., 1997; Arnone et al., 2000) might play important roles near the boundary. Horizontal advection dominates observed variability during the Southwest Monsoon, where the prominent upward slope between km 300 and 450 (Fig. 19, green line) is associated with a southward #owing coastal "lament (Fig. 12 and Brink et al., 1998). Subsequent Fall Intermonsoon and early Northeast Monsoon sections (dark blue and light blue lines) reveal

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Fig. 19. Southern Line isopycnal depths for p "25 kg m~3 and p "26 kg m~3 (both within the pycnoch h line), plotted as a function of distance from the Omani coast. Di!erent colors represent the four cruises (NE Monsoon: light blue, Spring Intermonsoon: red, SW Monsoon: green, Fall Intermonsoon: dark blue). Inshore of km 700 these isopycnals experience large displacements between seasons, while farther o!shore the range lessens considerably.

upward displacements of 30 m or more, decreasing with distance o!shore. The resulting isopycnal slopes support the northward currents seen in both observations and models and the upward shift within the pycnocline may modulate wind-driven mixing by modifying the strati"cation at the mixed-layer base. Upward shifts within the pycnocline may bring nutrient-rich waters closer to the mixed-layer base and enhance entrainment driven nutrient #ux. Both Ekman upwelling and the o!shore advection of coastal waters could contribute to the observed changes, and discerning their relative roles is the subject of ongoing investigations.

5. Length scales and sampling concerns 5.1. Spatial and temporal scales Our observations reveal numerous small-scale temperature-salinity interleaving features and patchy chlorophyll a #uorescence distribution. Seasonal and spatial contrasts in the observed scales can yield clues regarding active processes and provide valuable context for other investigators interpreting extensive biological and chemical observations sampled at more coarsely spaced intervals during the US JGOFS hydrographic program. A detailed characterization of temperature}salinity interleaving features and the mechanisms that produce them is the subject of ongoing investigation. Here, we estimate isotropic autocovariance functions using highly resolved

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SeaSoar sections to examine variance at horizontal scales between approximately 6}250 km. To distinguish between regions that we anticipate to have di!erent dynamics, we de"ne near-boundary (sections running parallel to the Omani coast, including the "lament survey during the Southwest Monsoon), inshore (Southern Line inshore of the moored array) and o!shore (Southern Line o!shore of the moored array) domains. Although some features, such as coastal "laments, show clear anisotropy, the 3/25 km (along/across track) resolution of our intensive survey patterns did not yield statistically signi"cant anisotropic statistics. We begin by calculating a structure function, S(r)"S(h(x#r)!h(x))2T, which may also be expressed in terms of autocovariance function F(r) S(r)"2F(0)!2F(r)#2e, where h is a scalar function of position x, e is noise and S T indicates an average over all pairs of observations separated by distance r. Following D'Asaro and Perkins (1984), we can express the autocovariance function as a series of orthogonal Bessel functions F(r)"+ E J (i i r), i 0 H i where the E form a discreet energy spectrum. We impose orthogonality by requiring i wavenumbers i i to satisfy J (i i ¸)"0, where ¸ is the maximum separation and H 1 H estimate energy spectra and autocovariance functions by minimizing

K G + j

C

S(r ) j ! + E (1!J (i r ))#e i 0 Hi j 2 i

DHKK

with respect to E and e. This produces wavenumber spectra which describe the i variance distribution over a range of length scales as well as autocovariance function estimates which do not require assuming F(0)"S(r)/2 at the largest sample separations r. E-folding length scales of autocovariance functions for ¹, S, p and chlorophyll h a #uorescence vary dramatically by season and with distance from the Omani coast. Temperature and p share statistically identical e-folding scales, typically 30}40 km h during the two Monsoons and 60 km during the Intermonsoon periods (Fig. 20a). Temperature and p scales vary little with depth and exhibit no spatial patterns h consistent between seasons. Within the mixed-layer, p and S exhibit similar scales. h But in the region of strong strati"cation just below the mixed-layer base, salinity frequently has shorter scales than density (Fig. 20b). Short salinity scales everywhere during the Spring Intermonsoon and away from coastal boundary during the Fall Intermonsoon re#ect numerous small scale salinity features observed below the mixed-layer (Figs. 6, 11 and 13). Contrasts between scales of density and salt weaken

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Fig. 20. E-folding scales of estimated autocovariance functions for (a) mixed-layer p and salinity, (b) upper h pycnocline (from the mixed-layer base to 175 m) p and salinity and (c) mixed-layer chlorophyll a. h Temperature scales are statistically indistinguishable from those of p . Trios of estimates are grouped by h cruise along the horizontal axis. Each trio contains near-boundary, inshore and o!shore length scale estimates for a given season. Gray bars indicate 95% con"dence intervals. Chlorophyll a #uorescence has much smaller scales than salinity or density. Within the upper pycnocline, salinity often exhibits shorter length scales than density.

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where strati"cation weakens (typically between 175 and 300 m, not shown), with salinity scales lengthening to resemble those of density. Chlorophyll a #uorescence varies at signi"cantly smaller scales than ¹, S and p , with 10 km e-folding h scales during the Northeast Monsoon and Spring Intermonsoon, and 15}30 km scales during the Southwest Monsoon and Fall Intermonsoon (Fig. 20c). Especially during the latter half of the year, the inshore region exhibits the longest scales, with smaller scales dominating to either side in the near-boundary and o!shore regions. Sections repeated 11 days apart in February and 0.8 day apart in September provide a limited, qualitative measure of temporal variability over two radically di!erent separations during two seasons. Inshore survey sections separated by 11 days in February show dramatic di!erences, primarily associated with numerous smallscale interleaving features within the pycnocline, but including some variations in mixed-layer ¹}S structure. As might be anticipated given the intense small-scale spatial variability, the sections show little coherence over the 11-day span. Repeat sections from the same region taken in September approximately a half inertial period apart show a high degree of correlation, with small-scale interleaving features virtually unchanged between passes. Horizontal velocities remain similar, rather than being 1803 out of phase as might be expected if near-inertial motions dominated current variability. 5.2. Sampling concerns Length scale statistics derived from highly resolved SeaSoar sections suggest caution in interpreting more coarsely sampled hydrographic measurements. Patchy distributions of chlorophyll a #uorescence with lateral length scales between 10}30 km imply biological variability on scales much shorter than the typical &100 km hydrographic station spacing (e.g. Morrison et al., 1998). Coarse sampling may alias biological variability and make interpolation between stations highly suspect. Assuming 100 km station spacing and a true "eld with a Gaussian covariance function and 20 km e-folding scale, objective analysis (Bretherton et al., 1976, Le Traon, 1990) provides error estimates associated with mapping hydrographic sections (Fig. 21). The Gaussian covariance function is intended to re#ect the distribution of the log of chlorophyll a #uorescence, rather than the measurement itself. As might be anticipated, such undersampling produces a poor representation of the true "eld, with large mapping errors between station locations. More interestingly, we also may consider how well the sampling supports mapping low-passed versions of the true "elds. Mapping follows as before, but using the original covariance function (20 km e-folding scale) low-pass "ltered at 100, 200 and 300 km. Note that this is di!erent from simply mapping using covariance functions with longer (e.g. 100, 200 and 300 km) e-folding scales, which would produce misleading results. Errors drop as the passband moves to longer length scales, but the fraction of the true variance captured in the map falls. Low-pass "ltering also results in a loss of variance, and the 200 km low-pass "lter, which reduces peak errors to less than 25%, also reduces variance by half.

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Fig. 21. Errors for one-dimensional objective maps of the Southern Line section. We assume that the true "eld has a Gaussian covariance function with 20 km e-folding scale. Black dots mark sample locations with 100 km resolution meant to re#ect typical US JGOFS hydrographic station spacing. The solid line marks the errors associated with objectively mapping the undersampled "eld. The various dashed and dotted lines indicate the errors associated with forming objective maps of spatially low-passed (smoothed) versions of the true "eld. Errors decrease with increasing smoothing but, as noted in the legend, the fraction of the true variance captured in the map decreases as well.

We use highly resolved SeaSoar measurements to illustrate some of the problems involved in using coarse sampling (sample interval much larger than the scales of variability) to infer large scale spatial structure. For this example, we use Southern Line observations near the depth of the Fall Intermonsoon chlorophyll a maximum. The original, highly resolved "eld (Fig. 22, thin solid line) exhibits variability on scales of O(20 km). Applying a 200 km low-pass "lter (Fig. 22, dashed line) eliminates small-scale peaks but preserves the large-scale features. We then subsample the "eld at the locations of the standard US JGOFS Southern Line hydrographic stations (Fig. 22, gray triangles). The subsampled data are objectively mapped using a Gaussian covariance function with 20 km e-folding scale, low-passed at 200 km. Although the mapped, subsampled "eld (Fig. 22, dotted line) re#ects some of the large-scale variability seen in the low-passed, full resolution observations (Fig. 22, dashed line), the subsampled "eld aliases small-scale variability. This produces regions with large di!erences between the low-passed original data and the mapped, subsampled "eld (Fig. 22, km 200}350, km 450}600, km 800}900). To obtain a statistically signi"cant comparison, we would need to average over many realizations of the

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Fig. 22. Southern Line chlorophyll a at 30 m during the Fall Intermonsoon. The thin, solid line marks the full-resolution (*x"3 km) SeaSoar measurements. The dashed line represents the 200 km low-pass "ltered full resolution measurements. Gray triangles indicate the locations of the US JGOFS Southern Line hydrographic stations. We subsample the full-resolution data at these locations and form an objective map (dotted line) using this subset and a 200 km low-passed Gaussian covariance function. Coarse subsampling clearly aliases small scale variability, and there are large di!erences between the 200 km low-pass "ltered full resolution data and the objective map of the subsampled measurements.

100 km subsampled "eld, but this example provides a simple illustration of the potential errors involved in inferring even the large-scale variability from spatially aliased observations. We examine the sensitivity of the spatial average of chlorophyll #uorescence to sampling resolution by calculating the maximum likelihood estimator (MLE) and standard deviation from SeaSoar pro"les subsampled at intervals ranging from 3 to 200 km. Potential lognormal distribution of chlorophyll #uorescence motivates the choice of the MLE rather than a simple arithmetic mean to represent the observations. Estimates of the MLE and 95% con"dence intervals follow Ruddick et al. (1997), assuming one degree of freedom for a distance of twice the e-folding length scale (Fig. 23) and counting each measurement as an independent realization when the subsample interval is greater than this distance. Reassuringly, the MLE does not vary signi"cantly over the range of sampling intervals (Fig. 23), though both standard deviations and the con"dence intervals of the MLE grow with increasing sample separation. Although the MLE does not change with sampling resolution, the statistical range varies dramatically.

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Fig. 23. The maximum likelihood estimator (MLE) and the standard deviation of chlorophyll a #uorescence for SeaSoar pro"les subsampled at increasingly coarse resolution. The horizontal axis indicates sampling interval, ranging from full 3 km resolution to 200 km subsamples. Gray bars mark 95% con"dence intervals. The MLE shows no signi"cant variation with coarse subsampling, though the uncertainty in the estimates grows with increasing sample separation.

6. Discussion and conclusions Along-shelf and cross-basin sections of physical and biological variables taken through the 1994}1995 Monsoon cycle often agree with climatologies, but also reveal patterns of atmospheric and upper-ocean variability that di!er from what would be expected if the Monsoon response were dominated by Ekman pumping (e.g. Bauer

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et al., 1991). Observed cross-basin meteorological variability along the Southern Line (Fig. 4) re#ects contrasts expected from the recent SOC climatology (Weller et al., 1998). During the Northeast Monsoon the strongest winds and surface cooling occur mid-basin, while closer to the coast we observe weak warming. Similarly, we observe the strongest Southwest Monsoon winds, accompanied by weak warming, mid-basin, with weaker winds and stronger warming nearer shore. The presence of warming across the basin, even in the region of strongest winds, di!ers from some older climatologies (Weller et al., 1998) and indicates that, at best, surface heat #ux plays only a minor role in driving upper ocean cooling during the Southwest Monsoon. Observed mixed-layer depths during and following the Northeast Monsoon (Fig. 9, light blue and red lines) show cross-basin contrasts that are absent in climatologies (Levitus, not shown; Rao et al., 1989). Observed mixed-layers deepen with distance o!shore and are deepest 400#km o! the coast during the Spring Intermonsoon. If the upper ocean response to the Northeast Monsoon were essentially one-dimensional, cross-basin contrasts in windstress and surface cooling would produce this pattern. Fischer (1997) "nds that convective overturning is the primary local mechanism driving mixed-layer deepening at the mid-basin mooring site during the Northeast Monsoon. Nearer the coastal boundary (Fig. 5), strong eddy activity and ¹}S contrasts suggest that horizontal advection plays an important role. During the Southwest Monsoon, the along-shelf section reveals cool, fresh, shallow mixed-layers, a result of strong coastal upwelling driven by southwesterly winds o! the Omani coast. Mixed-layer depth along the Southern Line varies little with distance o!shore (Fig. 9, green line), though climatologies show deepening away from the coast. Filament waters provide the only exception, with mixed-layers and water properties similar to those nearer shore. Mixed-layer depth climatologies (Fig. 16 and Rao et al., 1989) average unevenly distributed observations over long periods of time. Persistent mesoscale variability would be smeared in space and time by such averaging, and the cool "laments might appear in the climatologies as generally shallow mixed-layers in the region northwest of the windstress maximum. During the Fall Intermonsoon, deep mid-basin remnant mixed-layers and shallow near-boundary mixed-layers reveal cross-basin contrasts closer to those expected during the Southwest Monsoon, though Ekman pumping does not o!er an adequate explanation for the observed variability. Observations and calculations using the SOC climatology and Levitus mixed-layer depths indicate that wind-driven entrainment, coastal upwelling and horizontal advection likely play stronger roles in the upper ocean response to the Southwest Monsoon than does Ekman pumping. However, the processes governing the response inshore of the moored array appear to be di!erent from those that govern the response farther o!shore. Comparing estimates of wind-driven entrainment and Ekman pumping with climatological mixed-layer depths (Section 4 and Fig. 16) suggests that wind-driven mixing produces most of the o!shore mixed-layer deepening seen between June and July. Ekman pumping is too weak to produce this rapid deepening, though it drives weak downwelling throughout the Southwest Monsoon and should act to modulate wind-driven mixing. Comparing Southwest Monsoon and Fall Intermonsoon Southern Line sections (Figs. 9, 12 and 13), we "nd broadscale

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cooling and freshening that extends through the top of the pycnocline. Turbulent mixed-layer deepening followed by restrati"cation during the intermonsoon could explain the deep remnant mixed-layers and general cooling and freshening of the upper 100 m observed in the o!shore region following the Southwest Monsoon, but Ekman pumping alone cannot. Inshore of the windstress maximum, our calculations (Section 4 and Fig. 16) indicate that wind-driven entrainment is considerably stronger than Ekman pumping during the Southwest Monsoon. If no other processes act to maintain upper ocean strati"cation, this would drive mixed-layer deepening inshore of the windstress maximum. However, both climatologies (Fig. 16 and Rao et al., 1989) and our observations show shallow mixed-layers in this region during (Fig. 16) and following (Figs. 9, 13 and 16) the Southwest Monsoon. Although Ekman pumping acts in the correct sense to maintain shallow mixed-layers, it is not strong enough counter winddriven entrainment. Southern Line sections (Figs. 12 and 13) show that mixed-layers shoal, freshen and cool, a combination that cannot be achieved through mixing and Ekman upwelling given the atmospheric forcing and upper layer ¹}S structure during the Southwest Monsoon. O!shore advection of cold, fresh coastally upwelled water could help explain both changes in water properties and observed contrasts in strati"cation between nearshore and mid-basin regions. Although concentrated along the boundary, vertical velocities associated with coastal upwelling are far larger than those driven by Ekman pumping. Coastal upwelling and subsequent o!shore advection would provide a source of cold, fresh water while maintaining shallow strati"cation and countering wind-driven entrainment near the coast. Ekman pumping might also act to modulate wind-driven entrainment. Farther o!shore, advective in#uences weaken and winds strengthen, allowing more e!ective mixed-layer deepening. This would produce shallow, cold, fresh mixed-layers near the coast with deeper mixed-layers away from the in#uence of coastal upwelling. Both numerical (Young and Kindle, 1994; McCreary et al., 1996; Keen et al., 1997) and observational evidence point to the importance of coastal upwelling and lateral advection. The "lament observed during the summer monsoon (Figs. 7 and 12; Brink et al., 1998; Brink et al., in preparation) clearly carries cold, fresh, nutrient rich coastal waters away from the coastal upwelling region. Preliminary calculations indicate that within the "lament, horizontal advection overwhelms the e!ects of surface heating and Ekman pumping in a mixed-layer heat balance. Advanced Very High Resolution Radiometer imagery (Arnone et al., 2000) suggests that these are common, persistent features that can be traced hundreds of kilometers o!shore. Overall cooling and freshening inshore of the moored array between the Southwest Monsoon and the Fall Intermonsoon is consistent with an in#ux of coastally upwelled water, though in the absence of wind-driven entrainment Ekman pumping driven upwelling could produce a similar signature. Fischer (1997) also notes that horizontal advection appears to modulate mixing at the mid-basin mooring site by altering strati"cation near the mixed-layer base. Our ability to generalize from the observed response to the 1994}1995 Monsoon to a canonical Arabian Sea response depends in part on whether we experienced

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a typical cycle of atmospheric forcing. Weller et al. (1998) indicate that 1994}1995 windstress and surface heat #uxes were typical when compared with the 1980}1995 mean using the newer SOC climatology. Comparisons between SOC monthly mean maps of 1995 +](q /f ) and the corresponding SOC monthly mean 1980}1995 0 climatology show that the 1995 Ekman pumping was typically within one standard deviation of climatology, and very rarely over two standard deviations away (e.g. July 1995, Fig. 17). A comparison of 1995 and climatological entrainment velocity yielded similar results. We observed prominent mesoscale current variability, particularly during the Southwest Monsoon and near the coastal boundary during the Northeast Monsoon. Flagg and Kim (1998) report eddy-like current variability from a series of ADCP measurements taken during the US JGOFS Arabian Sea cruises. During the Southwest Monsoon, we attribute near-boundary variability to the numerous "laments seen in both our observations and remotely sensed imagery (Arnone et al., 2000). More interesting is the variability observed farther o!shore during both SeaSoar and US JGOFS hydrographic cruises (Flagg and Kim, 1998). The observed downwind #ow was con"ned near the boundary, extending not much farther o!shore than the region of the intensive "lament survey (Fig. 2, between waypoint D1 and the gap in the line). Currents between this and the start of the southeastward #ow at km 700 show considerable small-scale variability, though only the cyclonic feature near km 500 is clearly associated with a consistent density structure. Near-inertial oscillations could produce strong, surface-trapped #ows that might show similar patterns, but the observed variability occurs at smaller scales than expected given the large scale forcing of the Southwest Monsoon. Additionally, although not shown, velocities sampled approximately 24 h apart near waypoint E (where the survey detours from the Southern Line to sample near the moored array) do not show the degree of rotation expected if near-inertial motions dominated upper ocean currents. The generation of eddies by instabilities in the strongly forced, rapidly accelerating downwind jet o!ers an alternative explanation for the energetic mesoscale velocity "eld. Such features would act to enhance lateral advection and might generate considerable vertical transport through associated secondary circulations. Observed chlorophyll a #uorescence varies at smaller scales than those of density and salinity (Fig. 20). In a physically controlled system where similar processes determine the scales of temperature, salinity and nutrient concentrations, we might expect phytoplankton patch size to re#ect the scales of the physical variables. Alternatively, biological control, perhaps interactions between phytoplankton growth and zooplankton grazing, may limit phytoplankton patch scale. The shortest e-folding scales occur during the Northeast Monsoon and the following Intermonsoon, when acoustically sampled biomass was largest in the Arabian Sea (Pieper et al., 1999). A statistical characterization of the acoustical zooplankton data has not yet been made, and thus no de"nite conclusion can be drawn about the possible role grazing might play in the setting patch scales. However, in contrast to earlier acoustical observations, net tow data obtained during the Southwest Monsoon (late July and August, TN050) showed the highest zooplankton abundance in the region inshore of the windstress maximum, suggesting that zooplankton grazing may impact

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signi"cantly phytoplankton distributions (Smith et al., 1998a). Physical processes such as horizontal advection may be more important during the Southwest Monsoon, when length scales of chlorophyll a #uorescence are the longest observed. During this period, laterally sheared #ows may stretch phytoplankton patches along the direction of travel, producing longer e-folding scales. Other processes, such as diel variability, also may be important in determining phytoplankton length scales. SeaSoar traverses observed e-folding distances of 10}20 km in time periods (1.5 h, so we do not alias diel variability in the along-track sampling. However, cross-track temporal separations may be signi"cantly larger, and these estimates may well alias diel variability. In summary, four SeaSoar cruises examine oceanic response through an entire monsoonal cycle, documenting both winter and summer periods of mixed-layer deepening and cooling in the northern Arabian Sea. The observations suggest that the mechanisms governing the response to surface forcing vary both seasonally and by cross-basin location, though Ekman pumping is likely a weak component relative to mixing, coastal upwelling and lateral advection. During the winter monsoon, strong mid-basin latent heat loss drives convective overturning and produces deep midbasin mixed-layers. During the summer monsoon, wind-driven turbulent mixing entrains cool, fresh water into the surface layer and produces deep, mid-basin mixed-layers. Closer to the coast, wind-driven mixing acts to deepen the mixed-layer. Ekman upwelling opposes this deepening, but is much weaker and cannot explain the shallow mixed-layers observed in the latter half of the Southwest Monsoon. O!shore advection of coastally upwelled waters could maintain shallow mixed-layers while cooling and freshening the near-boundary region. Ekman upwelling will act to modulate wind-driven mixing here, but is too weak to counter the expected deepening. Wind-driven entrainment and coastal upwelling combined with o!shore advection in the region inshore of the windstress maximum likely produce the observed summertime cooling and freshening of surface waters in the northern Arabian Sea.

Acknowledgements We thank the members of the Woods Hole SeaSoar group, Frank Bahr, Jerry Dean, Paul Fucile, Al Gordon, Ellen Levy, Craig Marquette and Julie Pallant, whose hard work made this study possible. Charles Flagg and Hyun-Sook Kim maintained the ADCP and collected data throughout the experiment. Mark Baumgartner processed the shipboard meteorological measurements and helped greatly with the surface wind products. Jerry Wiggert performed the SeaSoar #uorometer calibration. David Phinney and Douglas Phinney generously provided their SeaSoar cruise chlorophyll measurements for use in this study. Mr. Zhihong Zheng performed nutrient measurements using equipment generously made available by the Scripps Institution of Oceanography Ocean Data Facility. Simon Josey generously provided the Southampton Oceanographic Center climatological data used in the study. This study bene"ted greatly from numerous discussions with other Arabian Sea investigators, including Robert Arnone, Karl Banse, Richard Barber, Charles Eriksen, Charles Flagg, Daniel Rudnick, Robert Weller and Jerry Wiggert. Three anonymous

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reviewers and Sharon Smith made suggestions which helped improve the manuscript. Paul Zibton drew the introductory cartoon. We thank Captain Glen Gomes and the crew of the R/V Thomas Thompson, who consistently provided professional, enthusiastic assistance above and beyond the call of duty. We gratefully acknowledge the support of the O$ce of Naval Research under grants N00014-94-1-0226 (C.M.L. and K.H.B), N00014-94-1-0362 (B.H.J.) and N00014-94-1-0161/N00014-99-1-0090 (A.S.F.). References Arnone, R.A., Martinolich, P., Kindle, J., Brink, K.H., Lee, C.M., 2000. Characteristics of the coastal "laments along the Oman coast. Deep-Sea Research, in preparation. Bauer, S., Hitchcock, G.L., Olson, D.B., 1991. In#uence of monsoonally-forced ekman dynamics upon surface layer depth and plankton biomass distribution in the Arabian sea. Deep-Sea Research 38 (5), 531}553. Baumgartner, M.F., Brink, N.J., Ostrom, W.M., Trask, R.P., Weller, R.A., 1997. Arabian Sea Mixed-layer Dynamics Experiment Data Report. Upper Ocean Processes Group Technical Report 97-3, WHOI-9708, Woods Hole Oceanographic Institution, Woods Hole, MA, USA 02543, 169 pp. Boyer, T.P., Levitus, S., 1994. Quality control and processing of historical temperature, salinity and oxygen data. NOAA Technical Report NESDIS 81. U.S. Department of Commerce, Washington, DC, USA, 65 pp. Bretherton, F.P., Davis, R.E., Fandry, C.B., 1976. A Technique for objective analysis and design of oceanographic experiments applied to MODE-73. Deep-Sea Research 23, 559}582. Brink, K., Arnone, R., Coble, P., Flagg, C., Jones, B., Kindle, J., Lee, C., Phinney, D., Wood, M., Yentsch, C., Young, D., 1998. Monsoons boost biological productivity in the Arabian sea. EOS, Transactions, American Geophysical Union 79 (13), 168}169. Cayula, S.C., Kindle, J.C., Rochford, P.A., de Rada, S., 1998. Sensitivity of numerical simulations of the 1995 SW monsoon response to wind stress products. EOS, Transactions, American Geophysical Union 79 (1). Coble, P.G., Del Castillo, C., Avril, B., 1998. Distribution and optical properties of CDOM in the Arabian Sea during the 1995 summer monsoon. Deep-Sea Research 45 (10}11), 2195}2223. D'Asaro, E.A., Perkins, H., 1984. A near-inertial internal wave spectrum for the Sargasso sea in late summer. Journal of Physical Oceanography 14 (3), 489}505. Findlater, J., 1969. A major low-level air current near the Indian ocean during the northern summer. Quarterly Journal of the Royal Meteorological Society (95), 362}380. Fischer, A.S., 1997. Arabian Sea mixed-layer deepening during the monsoon: observations and dynamics. S.M. Thesis, Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 130 pp. Flagg, C.N., Kim, H.-S., 1998. Upper ocean currents in the northern Arabian sea from ADCP measurements during the 1994}1996 JGOFS program. Deep- Sea Research 45 (10}11), 1917}1959. Flagg, C.N., Shi, Y., 1995. Acoustic Doppler Current Pro"ling from the JGOFS Arabian Sea Cruises Aboard the RV T. G. Thompson. Oceanographic and Atmospheric Sciences Division, Department of Applied Sciences, Brookhaven National Laboratory, Upton, NY, USA 11973, 30 pp. Gri$ths, G., 1994. Using 3DF GPS Heading for Improving Underway ADCP Data. Journal of Atmospheric and Oceanic Technology 11 (4), 1135}1143. Holm-Hansen, O., Lorenzen, C.J., Holmes, R.W., Strickland, J.D.H., 1965. Fluorometric Determination of Chlorophyll. Journal du Conseil 30, 3}15. Hosom, D.S., Weller, R.A., Payne, R.E., Prada, K.E., 1995. The IMET (Improved Meteorology) Ship and Buoy Systems. Journal of Atmospheric and Oceanic Technology 12 (3), 527}540. Josey, S.A., Kent, E.C., Taylor, P.K., 1998. The Southampton Oceanography Centre (SOC) OceanAtmosphere Heat, Momentum and Freshwater Flux Atlas. Southampton Oceanography Centre Report No. 6, 30 pp.

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