Upper ocean thermal and flow fields at 0°, 28°W (Atlantic) and 0°, 140°W (Pacific) during 1983–1985

Deep-Sea Research, VoL 36, No. 3, pp. 407-418, 1989. Printed in Great Britain.

0198-0149/89 $3.00 + 0.00 ~ 1989 Pergamon Prem plc.

U p p e r o c e a n t h e r m a l a n d flow fields at 0 °, 28°W (Atlantic) a n d 0 °, 140°W (Pacific) d u r i n g 1983-1985 DAVID HALPERN* and ROBERT H. WEISBERG'~ (Received 16 March 1988; in revisedform 13 September 1988; accepted 26 September 1988) Abstract--Moored current and temperature measurements were recorded simultaneously for 2 years (August 1983-July 1985) at six or seven depths between 10 and 250 m on the equator at 28°W in the Atlantic and at 140°W in the Pacific. The mean depth of the 20°C isotherm, which was representative of thermocline displacements, was identical at both sites. Substantially different annual cycles of the thermal and flow field at 28%V and 140°W represent an enigma. The annual variation of the 20°C isotherm was much less at 140°W than at 28°W. The annual cycle of the Equatorial Undercurrent core speed was much more pronounced at 140°W than at 28%V. The annually modulated 20- to 30-day meridional current oscillation had a larger amplitude and occurred for a longer duration at 140°W than at 28°W.

INTRODUCTION

EQUATORIALoceanography has received much attention in recent years. Long period, large-scale sea surface temperature fluctuations in the equatorial oceans are associated with atmospheric circulation variations in extratropical regions. Wind-driven equatorial upwelling alters the exchange of fixed carbon between the surface layer and the deep sea, which influences the concentration of atmospheric carbon dioxide. Physical principles governing equatorial ocean circulation dynamics are the same for the Atlantic and Pacific oceans. However, distinct geometries and longitudinal distributions of the surface zonal wind component help produce different characteristics of the ocean thermal and flow fields. Along the equator, the 80°W-120°E wide Pacific is 2.7 times greater than the 10°E-50°W Atlantic. The longitudinal distribution of the east-west surface wind component, which is of utmost importance in the development and modification of equatorial currents, is considerably different along the Atlantic equator than in the Pacific. Along the Pacific equator, the westward wind component is maximum near the center of the basin and near-zero near the eastern and western boundaries. The wind pattern in the Atlantic resembles the eastern half of the Pacific regime, i.e. the westward wind component is maximum at the western boundary and near-zero near the eastern boundary. Variations of the surface zonal wind component (Fig. 1) are believed to be the primary generation mechanism of the observed annual cycles of upper ocean current and temperature along the equator. The August 1984-July 1985 annual mean westward wind * Earth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, U.S.A. t Department of Marine Science, University of South Florida, St. Petersburg, FL 33731, U.S.A. 407

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speed at 140°W was nearly 2 m s -1 greater than at St. Peter and St. Paul Rocks (I°N, 29°W). At 29°W the westward component was minimum in April-May and maximum in October-November: this difference was about 5 m s-1. At 140°W the times of minimum and maximum westward wind speed were April and December-January: this 4 m s-1 difference was slightly smaller than at 29°W. For linear local ocean response to wind forcing, the amplitudes of the annual cycles of upper ocean current and temperature fields would be expected to be slightly larger in the Atlantic than in the Pacific. PHILAndER (1979) and CANE and SARACmK (1981) suggest the same situation because of the travel time of wind-forced equatorial waves to cross the ocean basin to effect equilibrium adjustment. This paper describes annual and mesoscale variations of upper ocean current and temperature measurements recorded concurrently at a single site each in the Atlantic and Pacific oceans. Because of longitudinal variations of the annual and mesoscale current and temperature fluctuations along the equator ( P H ~ D E r et al., 1986; WETSBErO and COLIN, 1986; HAWERNet al., 1988), caution is advised in extending the results described herein to the entire equatorial basin. METHODS

Surface winds and upper ocean currents and temperatures were recorded for long periods at several sites along the equator in the Atlantic (WEISBERO et al., 1987) and Pacific (HALP~RNet al., 1988). Data from the westernmost moored buoy locations, which were situated near the center of the ocean basin, were chosen for this comparative study bemuse ocean dynamics in the eastern regime are greatly influenced by coastline geometry, which differs greatly between the Atlantic and Pacific. Also, the concurrent 1 August 1983 to 31 July 1985 record-length at 0°, 28°W and 0°, 140°W was more complete than elsewhere. The 140°W and 28°W sites were II,000 and 2400 km from the western boundary, respectively, and they were 6600 and 4200 km from the eastern boundary, respectively. The Pacific and Atlantic moored buoy sites were located 60 and 45%, respectively, of the basin width measured from the western boundary. Mooring design, materials, instruments, equipment and shipboard methods for mooring deployment and recovery were virtually identical at both sites. Current and tempera-

Upper ocean thermal and flow fields

409

ture measurements were made with vector-averaging current meters (VACM) suspended beneath a surface-following toroidal buoy tautly moored in water depths of 4.8 km at 140*W and 4.1 km at 28°W. Each mooring contained six or seven VACMs between 10 and 250 m. VACM depths were 10, 50, 75,100, 150 and 200 m at 28°W and 10, 25, 45, 80, 120, 160 and 250 m at 140*W. HALPERN(1987a) reported that the quality of the equatorial VACM data was virtually the same as that from a vector-measuring current meter. Surface wind measurements made at 140°W with a vector-averaging wind recorder mounted on the surface buoy of the current meter mooring were virtually uncontaminated by mooring noise (HALPERN, 1987b). Wind observations at St. Peter and St. Paul Rocks were recorded by GARZOLI and KATZ (1984). Each VACM measured time-averaged temperature and east-west and north--south current components at 15-min intervals. Data recorded simultaneously for a few months earlier than August 1983 are omitted from our analyses because of the unusual oceanographic conditions prevailing in the Pacific due to E1 Nifio which, according to HALPERN (1987C), concluded by July 1983. Data recorded concurrently at 28°W and 140°W for an additional few months beyond the selected 2-year interval were not considered because our search for the annual cycle dictated the need for an integral number of 12-month periods. Record gaps at about 6-month intervals of about 1-day duration caused from mooring deployment and recovery operations were filled by linear interpolation between data recorded before and after the mooring was changed. Record gaps at 10 m were filled by linear extrapolation of VACM data measured at 25 and 45 m or at 25 and 80 m. The meridional and zonal current components at 10, 25 and.45 m were correlated with 95% statistical significance and with zero phase difference (HALPERNet al., 1988). Other gaps were filled by linear interpolation using data measured above and below the depth where data were missing. RESULTS

Upper ocean current and temperature measurements recorded along the Pacific and Atlantic equator contain four dominant temporal scales: record-length mean, annual, 20-30 days, and tidal (semidiurnal and diurnal). The small amplitude (1-2 cm s-I) tidal current oscillations will be described elsewhere. Mean conditions

Statistics of the 2-year mean conditions are listed in Table 1 for quick reference. The 1 August 1983-31 July 1985 mean temperature and current profiles (Fig. 2) portrayed a 100-m thick thermocline, which is defined as the interval where the vertical gradient of temperature was approximately 0.1°C m -I, between 50 and 150 m at both sites. The near-surface temperature was nearly 2°C higher at 28°W, which is a consequence of the much larger longitudinal extension of the cold tongue in the Pacific and the relative positions of 28°W and 140°W with respect to these cold tongues. We chose the 20°C isotherm as an index of thermocline behavior because the position of this isotherm along the Pacific equator is now routinely estimated by LE~TMAA (1987); also, the 20°C isotherm was used by HALrE~ (1987C) to designate the middle of the thermocline along the Pacific equator. The 2-year averaged mean depth of the 20°C isotherm at 140°W was

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Table 1. Mean and standard deviation values of 15-day average moored current and temperature data recorded during 1 August 198331 July 1985 28°W

140°W

88.6 85.2 82.6 45.2 -5.6 1.2 0.6 70.1 22.5 9.7 11.3

88.0 102.9 116.5 77.4 -14.7 1.7 0.7 98.6 40.2 20.8 29.7

Mean depth, 20°C isotherm (m) Mean depth, EUC core speed (m) Mean EUC core speed (cm s-t) Mean range, EUC core speed (cm s-t) Mean zonal current, 10 m (cm s-1) Mean shear, 25-75 m (s-t) Mean shear, 125-200 m (s-t) Mean 10-150 m fu dz, u > 0 (m e s-t) S.D. zonal current, 10 m (cm s-t) S.D. EUC core speed (cm s-t) S.D. 10-150 m fu dz, u > 0 (m 2 s-1)

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88 m, which was only 0.6 m less than at 28°W; this was a coincidence of the moored buoy positions. The 2-year mean zonal current distributions at both sites contained a thin (15--20 m), near-surface, westward South Equatorial Current (SEC) above an intense, eastwardflowing Equatorial Undercurrent (EUC). The linearly extrapolated surface SEC strength was about 8 cm s-1 (or 50%) greater at 140°W than at 28°W. As will be described, the near-surface annual mean westward flow was not representative of monthly mean conditions and the SEC was not a permanent feature. The E U C strength, inferred from the core speed, was 34 cm s-1 (or 41%) greater at 140°W. The mean depth of the E U C core speed at 140°W was 103 m, which was 18 m (or 20.4%) deeper than at 28°W. At both sites the depths of the E U C core speed were approximately coincidental with the center of the thermocline. Extrapolation beneath the E U C core depth to the depth of zero zonal current indicated that the 260 m mean thickness of the E U C at 140°W was 27% greater than at 28°W. Above the E U C core speed the shear at 140°W was 42% larger than at 28°W, in contrast to the equivalent shears below the E U C core. Annual cycle Depth-time sections of 15-day averaged temperature data were computed from arithmetic means of non-overlapping 15-day segments (Fig. 3A,B). Time variations of the 20"C isotherm depth were quite dissimilar at the two sites (Fig. 3C). The standard deviation of the 20°C isotherm depth was 5 m smaller at 280W than 140*W. At 140*W the depth of the thermoeline was less than normal, which was defined as the 2-year average, during the first year and deeper than normal in the second year; no annual cycle was apparent. At 28°W the 20"C isotherm showed an annual cycle, being deeper during August to December and shallower than normal from January to July. WEISBERG and TANG (1987) attributed the thermocline uplift at 28"W to the annual relaxation of easterly wind stress as the Intertropical Convergence Zone reaches it southernmost position during February-March. The 50 m displacement in 1984, which was 15 m greater than that in 1985, appeared to have been associated with unusual meteorological conditions over the tropical Atlantic ( ~ E R , 1986). At 28"W the least-squares linear trend of the 20"C isotherm depths throughout the 2year period indicated that the thermociine rose about 2.3 m per month, in contrast to the situation at 140*W where the depth of the 20"C isotherm progressively deepened at the rate of 3.1 m per month. The opposite trends of the thermocline displacements are not understood, and may reflect an out-of-phase relationship between the large-scale atmospheric forcing over the Atlantic and Pacific oceans. It is tempting to speculate that this slow sinking of the central Pacific thermoeline was foretelling the subsequent E1 Nifio episode which, according to AnraN et al. (1987), began in July 1986. The depth distributions of the VACMs were sufficient to resolve the vertical structure of the westward-flowing SEC and the eastward E U C (Fig. 4A,B). The 2-year average SEC strength at 10 m at 140*W w a s - 9 cm s-1 (or 160%) greater than at 28"W. However, the SEC was not a permanent feature at either location, in contrast to the EUC. At 28"W the SEC occurred during May-July 1984, October 1984--January 1985 and June--July 1985. At 140*W the SEC was measured regularly from September to March, indicating an annual reversal in direction of the near-surface zonal current component. The depth of the E U C core speed (Fig. 4C) could only be estimated with the coarse resolution of the spacing between VACMs. Smaller vertical displacements (CrmRESKm et al., 1986)

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D. HALPERNand R. H. WELSBERO

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were aliased. The standard deviation of the EUC core speed depth was 50% greater at 140°W than at 28°W. During our selected 2-year interval, the mean EUC core speed was 40% greater at 140°W than 28°W (Table 1). The EUC core speed range was 70% greater at 140°W than at 28°W. The standard deviation of the 15-day average EUC core speed variations was twice as large at 140°W than at 28°W. Evidence for an annual cycle was greater at 140°W, where the core speed was above averge during April to July and less than normal at other times (Fig. hA). The fluctuations of the depths of the EUC core speed and 20°C isotherm (Figs 3C and 4C) were correlated with 95% statistical significance. At 28W~ where the correlation coefficient was 0.75, an upward displacement of the core speed was associated with a 3.5% smaller upward movement of the 20°C isotherm. At 140°W where the correlation coefficient was 0.72, an upward displacement of the core depth corresponded to an 18% increase in the upward movement of the 20°C isotherm. The eastward transport per unit meridional width, which KNox and HALI'ERI~(1982) showed was linearly related to the total EUC transport at 150°W, was computed between 10 and 150 m using the trapazoidal integration method. At 140°W the mean eastward transport per unit width was 40% greater than at 28°W (Table 1). Time variations of the transport were nearly 3 times larger at 140°W than 28°W At 14(Y'W a well-defined annual cycle was visible in the time variations of the eastward transport per unit width: the U~XmUUF.AS~W~OCURRENT

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transport was greater than normal from March to July and below average throughout the remainder of the year (Fig. 5B). No such periodicity was observed at 28°W.

Twenty- to 30-day period oscillatkms Satellite infra-red sea surface temperature measurements first revealed the existence of seasonally modulated, westward-moving, long equatorial waves in the Pacific (LEGECKIS, 1977), and subsequently in the Atlantic (LEGE¢~S, 1986). These waves have been described from Lagrangian current measurements (I'IANsEN and PAUL, 1984), Eulerian current measurements (WEISeERG, 1984; I-IALP~.RNet al., 1988), and sea surface height data ( M u s ~ , 1986). PHILANDER(1978) and Cox (1980) suggested that these waves were due to barotropic and baroclinic instabilities of the zonal current. The amplitude of the 20- to 30-day period oscillation decreased rapidly with depth. The amplitude of the meridional component was larger than the zonal component and the envelope of these annually modulated waves was of longer duration and larger amplitude at 140°W than at 28°W (Fig. 6). These waves occurred during the interval of sustained westward flow at 10 m. The wave packet at 140°W contained about 50-75% more individual waves than at 28°W. In the Pacific the features of the annual wave packet were fairly constant from year to year, except during El Nifio of 1982-1983 (I'IALPERN, 1987C). At 28°W the wave season in 1983 was confined to June and July when only 2-3 waves were observed; these wave signatures had amplitudes similar to those found in the Pacific and were the largest recorded at 28°W (WEISBERGand WEINGART[qER,1988).

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Table 2. Root-mean-square amplitudes (cms -1) of 20-day (h~quency bandwidth: 0.045825--0.055405 cpd for 2-year da_!aduration; 0.045085-0.056005 cIxl for 1-year data duration) and 30-day (frequency bandwidth: 0.028045-0.037625 cpd for 2-year data duration; 0.025955--0.036885for 1-year data duration) period meridional current fluctuations recorded at 28"W and 140*W 1 Aug. 83-31 Jul. 85 20 day 30 day 28"W 140*W

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Spectral estimates of the north-south component contained 95% statistically significant peaks at 20- and 30-day periods at 140*W and 28"W, respectively. When the 280W data are analysed over smaller segments of the record, the central wave period was 25 days and the amplitude was more similar to that in the Pacific (WEISBERG, 1984; W~ISBERG and WEI~GARTNER, 1988). ThUS, whether the Atlantic wave period was 30 or 25 days was not as significant as the longer period of individual waves and shorter duration of the annual wave packet compared to the Pacific. The 2-year average root-mean-square (r.m.s.) amplitude of the 20-day period meridiohal current fluctuation was nearly twice as large at 140*W than at 28"W (Table 2). During each of the two consecutive 1-year intervals the r.m.s, amplitudes of the 20-day period meridional current oscillation differed by 5% at 140*W and 50% at 280W, indicating the much greater year to year constancy of wave amplitude at 140*W. The 2-year average r.m.s, amplitude of the 30-day period motion was 25% greater at 140*W than at 28°W (Table 2). The year to year amplitude variations of the 30-day period fluctuation were about 15% at both sites. DISCUSSION

Along the equator, isotherms in the upper ocean tilt upward toward the east in response to the westward component of the trade wind, and the isothermal surface layer thickness increases from east to west. The zonal slope of the thermoeline was relatively linear from 0° to 40°W in the Atlantic (I-hsARO and HEN~, 1987) and from 100°W to 150*W in the Pacific (HALPERN, 1987C). TWO processes causing variations of thermocline depth at a specific location on the equator are vertical translation of the entire zonally sloping thermoeline and changes in the zonal slope of the thermocline along the equator. The latter process is related to the zonal pressure gradient and is expected to produce greater variations in the EUC. An example occurred during El Nifio of 1982-1983 when the longitudinal slope of the thermocline decreased to such an extent that the EUC disappeared (FIPOr~Get al., 1983; I-IAU'ERN, 1987C). The annual cycles of the thermocline and EUC at 30*W has been simulated by PHILANDER and PACANOWSgJ (1986) with a climatological-mean wind-driven ocean general circulation model. Using the same model, PI-IIt.Am)~Ret al. (1987) also simulated the EUC annual cycle at 154*W in the central Pacific. The existence of the annual cycle in the upper ocean current and temperature fields is not in doubt. The purpose of our paper is to contrast the magnitudes of the annual cycle observed at centrally located sites in the Atlantic and Pacific. The zonal wind component contained an annual cycle of similar magnitude at both locations. During 1983-1985 the annual cycles of the EUC core speed and the 10-150 m eastward transport per unit width were greater at 140*W than at 28°W.

Upper ocean thermal and flow fields

417

However, the annual variation of the thennocline was less pronounced at 140°W than at 28°W, indicating substantial nonlinear contribution to the dynamics of EUC annual variations (CASE, 1980). Our observations suggest an enigma. Why did the zonal pressure gradient but not the EUC at 28°W adjust to the annual wind variations, and vice versa at 140°W? The longitudinal integrated effect of the wind may be different in the two oceans. Just how the pressure gradient evolves at the depth of the EUC and how it relates to the EUC core speed and transport remain to be clarified. Acknowledgements--The 28°W and 140°W measurements were obtained during the SEQUAL and TROPIC H E A T Expeditions, respectively, and we are indebted to numerous colleagues and friends who participated at sea and on shore in data acquisition and data processing. Support for analyses of these data from the National Science Foundation (RHW: OCE-8740380) and National Aeronautics and Space Administration (DH: UPN161-80-42-40) are gratefully appreciated. Dr S. Garzoli kindly supplied the St. Peter and St. Paul Rocks wind data used in Fig. 1. Stimulating and informative comments by Dr M. Cane, Dr G. Philander and Dr E. Sarachik were gratefully appreciated. Graphical and tabular data displays were expertly prepared by J. Newman. The research described in this paper was performed, in part, by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. REFERENCES ARraN P. A., C. F. ROPELESWKIand V. E. KOUSKY (1987) Meteorological anomalies associated with the 1986-87 ENSO event. Transactions of the American Geophysical Union, 68, 1317. CANE M. (1980) On the dynamics of equatorial currents, with applications to the Indian Ocean. Deep-Sea Research, 27, 525-544. CANE M. A. and E. S. SARACHIK(1981) The response of a linear equatorial ocean to periodic forcing. Journal of Marine Research, 39, 651-693. CHERESKIN T. K., J. N. MOUM, P. J. STABENO,D. R. CALDWELL,C. A. PAULSON,L. A. REGIER and D. HALP~a,~ (1986) Fine-scale variability at 140°W in the equatorial Pacific. Journal of Geophysical Research, 91, 12887-12897. Cox M. D. (1980) Generation and propagation of 30-day waves in a numerical model on the Pacific. Journal of Physical Oceanography, 10, 1168-1186. F I R I ~ E., R. LUKAS,J. SADLERand K. WYRTKI(1983) Equatorial undercurrent disappears during 1982-83 El Nifio. Science, 222, 1121-1122. GARZOU S. L. and E. J. KATZ (1984) Winds at St. Peter and St. Paul Rocks during the first SEQUAL year. Geophysical Research Letters, 11, 715-718. HALPERN D. (1987a) Comparison of upper ocean VACM and VMCM observations in the equatorial Pacific. Journal of Atmospheric and Oceanic Technology, 4, 84-93. HALPERND. (1987b) Comparison of moored wind measurements from a spar and toroidal buoy in the eastern equatorial Pacific during February-March 1981. Journal of Geophysical Research, 92, 8303-8306. HALPERND. (1987c) Observations of annual and El Nifio thermal and flow variations at 0% ll0°W and 0% 95°W during 1980-1985. Journal of Geophysical Research, 92, 8197-8212. HALPERN D., R. A. KNOX and D. S. LUTHER (1988) Observations of 20-day period meridional current oscillations in the upper ocean along the Pacific equator. Journal of Physical Oceanography, lg, 1514-1534. HANSEN D. V. and C. A. PAUL (1984) Genesis and effects of long waves in the equatorial Pacific. Journal of Geophysical Research, 89, 10431-10440. HISARD Ph. and C. ~ (1987) Response of the equatorial Atlantic Ocean to the 1983-84 wind from the Programme Fran~als Ocean et Climat dans i'Atlantique Equatorial cruise data set. Journal of Geophysical Research, 92, 3759-3768. KNox R. A. and D. HALPERN (1982) Long range Kelvin wave propagation of transport variations in Pacific Ocean equatorial currents. Journal of Marine Research, 40, Suppl., 329-339. LEETMAA A. (1987) Progress towards an operational ocean model of the tropical Pacific at NMC/CAC. In: Further progress in equatorial oceanography, E. J. KAT'Z and J. M. Wrrr~, editors, Nova University Press, Fort Lauderdale, pp. 439--450. LEGECKts R. (1977) Long waves in the eastern equatorial Pacific Ocean: a view from a geostationary satellite. Science, 197, 1179-1181. LEG~KIS R. (1986) Long waves in the equatorial Pacific and Atlantic Oceans during 1983. Ocean-Air Interactions, 1, 1-10.

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