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Lagrangian circulation and forbidden zone on the West Florida Shelf
Continental Shelf Research 19 (1999) 1221}1245
Lagrangian circulation and forbidden zone on the West Florida Shelf Huijun Yang *, Robert H. Weisberg , P.P. Niiler, W. Sturges, W. Johnson Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016, USA Scripps Oceanographic Institute, La Jolla, CA, USA Department of Oceanography, Florida State University, Tallahassee, FL, USA Minerals Management Service, Herndon, VA, USA Received 13 August 1998; received in revised form 24 November 1998; accepted 2 February 1999
Abstract This paper presents some recent results of drifters released on the West Florida Shelf during 1996}1997 and compares with the numerical model results of the wind-driven circulation. Using satellite tracked surface drifters during the one year period from February 1996 to February 1997, a drifter free region, called the `forbidden zonea, is found over the southern portion of the West Florida Shelf. This "nding is consistent with historical drift bottle data and with a recent numerical model study of the West Florida Shelf circulation response to climatological wind forcing. Direct drifter simulations by numerical model during March 1996 show a good agreement with both the in situ ADCP current observation and drifter observation. Three mechanisms are proposed for the observed Lagrangian features. The primarily dynamic mechanism is the along-shore wind forcing, which induces a coastal jet that tends to leave the coast and the bottom onshore and near surface o!shore transports. The second one is the convergent coastal geometry and bottom topography for the southward #ow in central shelf near Tampa Bay that enforces the coastal jet and the bottom and near surface transport. The last is a kinematic one, simply due to the short along-shore Lagrangian excursion, driven by the typical synoptic weather systems. Thus near surface shelf waters over the north may not reach the southern coast of the West Florida. Implication is that surface hazard such as oil spill that may occur outside of the southern West Florida shelf may not greatly impact the southern coastal region except Florida Keys. However, the biological and chemical patches over the north that may occur in the water column such as red tides still can easily reach the southern coastal region through the subsurface and bottom waters. 1999 Elsevier Science Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected]. http://chaos.marine.usf.edu/yang (H. Yang) 0278-4343/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 9 9 ) 0 0 0 2 1 - 7
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1. Introduction The West Florida Shelf is a broad, gently sloping region extending approximately 650 km between the Florida Panhandle in the north and the Florida Keys in the south (Fig. 1). Its isobaths generally tend to parallel the coastline that is oriented approximately along 333T over the middle portion of the region, and the 100 m isobath lies approximately 100}240 km o!shore. Shelf circulation is mainly driven by winds, tides, and buoyancy #ux. The shelf circulation is bounded in the southwest by the Loop Current, which enters the Gulf of Mexico through the Yucatan Straits and exits as the Gulf Stream through the Florida Strait. By virtue of transporting and mixing shelf and adjacent ocean waters, the coastal ocean circulation largely determines the biological and geochemical properties of the water. Important biological phenomena that occur on the West Florida Shelf include episodic, seasonal blooms of (red tide) toxic dino#agellates thought to originate at mid-shelf (Steidinger, 1983; Vargo et al., 1987) and the seasonal formation of high concentration pigment plumes near the shelf break (Gilbes et al., 1996). Relatively, little is known about the role of the shelf circulation in determining their onset, movement, growth, and ecosystem impacts, however. Observations show that the variations in the shelf circulation and sea level are highly correlated with wind stress (e.g., Niiler, 1976; Balaha and Sturges, 1981; Mitchum and Sturges, 1982; Cragg et al., 1983; Marmorino, 1982; Mitchum and Clarke, 1986). A reverse of the midshelf currents was also founded in a recent study (Weisberg et al., 1996). Until recently, drift bottle releases provided the only Lagrangian view of the surface circulation over the region (Tolbert and Salsman, 1964; Gaul, 1967 and the Hourglass report by Williams et al., 1977). However, these data only provided the launch sites and recovery locations of the drifters, not the drifter trajectories. Thus, value of the information is very limited. Starting in February 1996, the Mineral Management Service (MMS) launched a satellite tracked surface drifter program in the northeastern Gulf of Mexico. The program intended for one year yielded about 1.5 years of data. These data provided actual drifter trajectories during the course of the program, and thus are much more valuable than the drift bottle data. The purposes of this paper are (1) to describe some results from the program, (2) to compare them with the numerical modeling results of the wind-driven circulation, the historical data and other observational data, (3) to propose several possible mechanisms for the Lagrangian circulation, and (4) to shed some light on possible ecosystem impacts of the naturally or man-made disasters such as red tides and oil spill over the West Florida Shelf.
2. Surface drifter data 2.1. Recent drifter observations The SCULP-drifters designed to follow the upper meter of the water column, were released every two weeks at pre-determined locations, by airplane at 15 sites between
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Fig. 1. The bathymetric map of the West Florida Shelf region and the SCULP-drifters release area, Hourglass release area, and release sites of Tolbert and Gaul experiments.
the 20 and 60 m isobaths in the northeastern Gulf of Mexico, as shown Fig. 1. A 12-month long composite of the resulting surface drifter trajectories from February 1996 to February 1997 are shown in Fig. 2 with a total of 341 drifters released. The most evident is where these drifters did not go. Intriguing is the lack of any trajectories over a large portion of the shelf, south of Tampa Bay. This drifter free area, we called it the `forbidden zonea, ranges between the coast to about 50 km o!shore in the central region to 240 km o!shore in the southern region. The near coastal region of the Big Bend is also void of trajectories. The entire drifter free area occupies about 74,000 km, or about half of the West Florida Shelf. Furthermore, no drifters made landfall on the West Florida coast during the whole course of the program. Some drifters were entrained by the Loop Current, but most drifters remained in the
Fig. 2. Observed drifter trajectories from February 1996 to February 1997 with a total of 194 drifters.
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northern portion of the shelf. During the later half year many drifters went into the western Gulf of Mexico, which was particularly noticeable during tropical storm Josephine in early October 1996 when all drifters on the shelf went westward. The surface wind and wave e!ects may impact the drifters. However, we believe that these impacts are very limited and the drifter basically follows the near surface water. More supporting evidence was provided later in Section 3. 2.2. Historical bottle drifter data Three historical data sources exist for surface Lagrangian circulation information from drift bottle programs in 1960s (Tolbert and Salsman, 1965; Gaul, 1967; and Williams et al., 1977). By employing drift bottles, these studies only provide release location and points of landfall along the coast (Figs. 1 and 3), but not actual trajectories. The resulting data may also be biased by wind and surface wave e!ects. Both the Tolbert and Salsman (1965) and the Gaul (1967) drift bottles were released o! the Florida Panhandle coast, the former near shore south of Panama City and the latter o!shore beyond the 200 m isobath. The Williams et al. (1977) Hourglass drifter data are from a series of cruises on the continental shelf between Tampa Bay and Ft. Myers from August 1965 to November 1967. Each of the 16 permanent stations was sampled monthly to investigate water properties to a depth of about 73 m, and drift bottles were released at each station to study the surface circulation. A total of 4460 drift bottles were released and 1347 were retrieved along the coast. These data are summarized in Table 1, where the following de"nitions apply: WFS is for the coast between Key West and Apalachicola, North is for the coast between Apalachicola and
Fig. 3. Historical drifter bottle launch sites and recover zones.
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Table 1 Historical drifter data: percentage of recovered bottles in the West Florida Shelf (WFS), the North and East and West
Tolbert Gaul Hourglass
No. of bottles Released
No. of bottles Recovered
WFS(%)
North(%)
East(%)
West(%)
951 6919 4460
276 1452 1347
0 2 30
68 9 1
20 63 62
12 26 7
the Mississippi River Delta, East encompasses the cost to the east of Key West and West includes the coast west of the Mississippi River Delta (Fig. 3). The major drift bottle "ndings are as follows. A majority of the drift bottle landings were on the east coast of Florida during fall and winter seasons, and a substantial number also landed on the east coast during the remainder of the year. Furthermore, along with the Hourglass releases, many of these were from northern o!shore releases (Gaul, 1967). Virtually no landings were reported along the West Florida coast south of Apalachicola Bay at any time, except from release sites in the Hourglass region within 20 km of shore. The last observation is most remarkable, considering the fact that weather systems regularly sweep over the region each year and that the West Florida beaches are populated by tourists year-round. These historical data are consistent with the recent "nding of a large drifter free region over the southern portion of the West Florida Shelf (Fig. 2).
3. Numerical model results 3.1. Response to climatological wind forcing The response of the West Florida continental shelf circulation to monthly mean climatological wind forcing was recently investigated by Yang and Weisberg (1999), using the three-dimensional, primitive equation Princeton Ocean Model (Blumberg and Mellor, 1983,1987). The model domain included realistic coastal geometry and bottom topography over a rectangular region from 87 to 813W and 24 to 30.33N (see Fig. 1). An evenly spaced 4.6 km rectangular grid was used in the horizontal plane, with 21 sigma levels in the vertical arranged to provide higher resolution within the surface and bottom boundary layers. The model was forced solely by the monthly mean wind stress climatology of Hellerman and Rosenstein (1983), linearly interpolated to "t the model grid. The horizontal boundary conditions over the land in the north and the east were implemented by a land mask, which ensures that the velocity over the land and the normal velocity along the coastline are zero. The model domain had open boundaries in the south and west over water. The depth-averaged transports normal to these boundaries were speci"ed to be zero. Thus, there were no net horizontal transports into or out of the domain, so the total water mass within the
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domain was always conserved. However, depth-dependent #ows could enter into or leave out of the domain across the open boundaries according to a Sommer"eld radiation condition (see Yang and Weisberg, 1999 for details). Twelve model runs were made for the 12 calendar months. The results were obtained after the model reached the equilibrium with the wind forcing and was in quasi-steady state for each run. Two basic seasonal patterns of circulation and sea surface elevation were found: a winter pattern from October to March and a summer pattern from April to September. The winter pattern includes an anticyclonic gyre, called the Big Bend Gyre, over the northeastern Florida Big Bend region that merges with a northwestward #ow from the south. The winter circulation also includes o!shore surface transport, coastal upwelling, and relatively low coastal sea level. The summer pattern features a continuous northwestward directed #ow, onshore surface transport, coastal downwelling, and relatively high coastal sea level. Transitions between these two seasonal patterns show either a development or relaxation of the Big Bend Gyre. In all cases the circulation patterns are fully three dimensional, and they result from wind-driven Ekman transports and surface-slope-induced geostrophic #ows. The model surface circulation patterns for each month are shown in Fig. 4, where the latitude is 24 to 30.3N from bottom to top and longitude is 87 to 81W from west to east. From April to September, the surface shelf circulation is predominantly alongshore toward the northwest. The model results are therefore consistent with the Hourglass drift bottle retrievals found to the north. In contrast, from October to March, the surface shelf circulation has an o!shore component over most of the shelf. The surface circulation is consistent with the existence of a large drifter free region in the southern portion of the West Florida Shelf. 3.2. Drifter simulation during March 1996 To further explore the surface Lagrangian circulation, we performed numerical model experiment for comparison of model drifters with observed drifters. The three dimensional, primitive equation POM is again applied to the West Florida Shelf during March 1996 with the model domain being the entire northeastern Gulf of Mexico region from 89 to 813W and 24 to 30.33N (see Fig. 3). The horizontal resolution is 6 km; there are 18 layers in the vertical, realistic coastal geometry and bottom topography are included; and the shelf strati"cation is speci"ed by a climatological monthly mean density "eld for March. The distribution of the salinity in March shows little di!erence from the north to south with contour lines parallel to the coastlines over the shelf while there is some temperature di!erence. Horizontal boundaries are open on the west and south for both the vertically integrated #ow and depth-dependent #ow, according to the radiation condition. Examining AVHRR images during March 1996 shows that the sea surface temperature (SST) is always relatively low near the coast compared with o!shore (not shown). This SST pattern favors a southeastward #ow and the SST seems to change only passively in response to the surface shelf circulation. Therefore, the baroclinicity
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during this time played a minor role, tending to enforce the southeastward #ow and weaken the northwestward #ow. The model thus, was driven solely by the low passed (36-h) hourly wind forcing (wind data were derived from NOAA Buoy station shown in top panel of Fig. 5). March 1996 starts with weak northerly winds that gradually change to southerly from March 4 and reach a maximum speed of 9 m/s on March 7. On the next day, the wind suddenly changes to northwesterly with maximum speed of 15 m/s, to northerly, and to northeasterly, as a front traverses the domain from northwest to southeast. On March 14, the wind becomes southerly. During March 19 and early March 20 the wind again becomes strongly northwesterly. Thus, two periods exist when wind is primarily northerly. However, due to the nature of the weather systems on the West Florida Shelf, the duration of continuous northerly wind is generally less than four to "ve days. This limits the time for which the shelf surface circulation can #ow continuously southward in response to winds. The shelf circulation responds rapidly to the wind changes, particularly during the two strong northwesterly wind events (Fig. 5). For example, on March 5 the surface #ow is onshore and northeastward in the south and northeastward in the north in response to the southerly wind. The surface #ow then rapidly changes direction in response to strong northwesterly wind after March 8. The surface current again moves southeastward after March 19 and moves back toward northwest on March 25. Fig. 6 shows the sample model surface circulation over the West Florida Shelf. The vertical structure of these time varying shelf circulation is also rather interesting (Fig. 7). On March 7, surface transports are onshore over most shelf. However, on the mid level and bottom onshore transports only occur in the inner shelf and the rest are basically along shore. On the mid-level on March 7, there is a counter current at the mid shelf moving toward the southeast to south of Tampa Bay. This may be explained by a pressure "led adjustment time before the wind e!ect can be felt at depth. But this changes on March 8 when #ows at all depth move toward the south (on the left half panel of Fig. 7). Thus, in response to northerly winds, the surface and mid-level currents near Tampa Bay are o!shore while the bottom currents are onshore. The strong coastal jet is evident in the shelf circulation, in particular near Tampa Bay. The same is true on March 20 (on right half panel of Fig. 7). The DeSoto Canyon is also an interesting region. The mid-level and bottom currents are all strongly onshore over the DeSoto Canyon and near coast. This strong bottom onshore transport may provide a mechanism for the observed coastal upwelling over the Florida Pandhandle coast during summer 1998 in the sea surface temperature (not shown). The strong onshore transport, in particular in the subsurface south of the DeSoto Canyon may explain the warm eddy intrusion over the DeSoto Canyon (Huh et al., 1981). Therefore, the shelf circulation is fully three dimensional and rather complex. 䉳&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Fig. 4. The model surface shelf circulation patterns (Yang and Weisberg, 1999), showing an o!shore surface transport during the period from October to March and a northwestward and an onshore surface transport during the period from April to September over the central and southern West Florida Shelf. The longitudes are from 87 (left) to 813W (right) and latitudes are from 24 (bottom) to 30.33N (top).
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Fig. 5. Wind observation at NOAA Buoy located at (28.783N, 86.043W)(top panel); in situ ADCP current observation at 3 m depth (middle panel) and model surface current located at (28.833N,84.853W)(bottom panel) during March of 1996, after applied 36 h "lter.
Fig. 6. Model surface circulation of the West Florida Shelf during March 1996. Colors denote the water depth.
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Fig. 7. Model three-dimensional structure of the West Florida Shelf circulation. Top, middle and bottom panels are for the surface, mid-level and near bottom circulation. Colors denote the water depth.
Using the hourly model Eulerian velocity "eld, we can calculate Lagrangian drifter trajectories by forward time integration. The integration is carried out by a modi"ed fourth-order RK method. Simple linearly temporal and bilinearly spatial interpolation was used. Tests with di!erent interpolation schemes show no noticeable e!ect on the Lagrangian calculation.
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Model drifters were launched on March 8 for the comparison because there were only few SCULP-drifters in water before that time. A "ve-day model and in situ drifter trajectory comparison is shown in Fig. 8. Two major events are noted. As the wind direction turns toward the southeast, the drifters also move rapidly toward the southeast primarily in the along-shore direction from March 9 until March 13. Then they reverse direction, moving back north until March 19. On the way back they retrace the paths on which they came south. Thus, on March 19, the "ve-day drifter trajectories are all very short and are located north of the original launch sites. A second southward drive occurs immediately after the wind turns toward the southeast on March 19, and the drifters continue this drive until March 23. By this time some of them are positioned o!shore to south of Tampa Bay. The repetition process results in the model drifters that are in qualitative and quantitative agreement with the observations. For example, Fig. 9 zooms in the comparison for one typical drifter. This drifter was released in the Florida Big Bend at (84W,29.5N) on March 8, 1996. During the 20 days period, there are two southward marches and two retreats after two marches. The modeled Lagrangian trajectory captures the observed trajectory quite well except for the last few days. To quantify the performance of the model simulation, we de"ne the distance between the observed and modeled Lagrangian trajectories by d(t)"((X !X )#(> !> ), (3.1) K M K M where X , > and X , > are the positions of model and observed Lagrangian K K M M trajectories, respectively. Fig. 10a shows the time evolution of the distance in km. In the "rst two days, the distance increases from zero on March 8 to 14 km on March 10; it then decreases to 1 km on March 12 and 3 km on March 14. In the next four days, the distance increases rapidly to 33 km on March 19. However, the distance decreases from the maximum on March 19 to 4 km on March 22 as the modeled and observed Lagrangian trajectories pass each other. After March 24, the two trajectories start to depart rapidly again. We can measure a cumulative departure between the modeled and observed trajectories during a period of time by computing a time mean distance. The time mean distance d is de"ned by the square root of the sum of squared distances divided K by the elapsed time q; that is
1 O d (q)" d(t) dt. (3.2) K q R The results show that the mean distance while increasing rapidly for "rst few days hovers at levels less than 10 km during "rst 10 days and increases to around 15 km after day 12 (Fig. 10b). For example, the mean distance is 8 km during the "rst week from March 8 to 15 and 16 km during a period of 16 days from March 8 to 24, 1996. The in situ current observation (Fig. 5) shows a good agreement with in situ drifter observation (Fig. 8). During March 1996, the near surface current is strongly
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Fig. 8. Five-day drifter trajectories from observations (on the left panel) and model results (on the right panel) during March 1996. Date denotes the last day of trajectories. The color in model results denotes time from blue to red as time increases.
northward before March 8, southeastward from March 9 to 16 and again after March 20. It then reverses direction toward the north on March 24. The northwestward retreat of the drifters matches the northward current during March 25}28. The near
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Fig. 8. (Continued.)
surface current attains speeds of almost 50}60 cm/s during the two southeastward drives. Unfortunately, these are the only current data available during March 1996, and spatial variability exists as seen from the model and drifter data. For example,
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Fig. 8. (Continued.)
during March 14}19, the northwestward retreat of drifters from the central West Florida Shelf is most evident. This retreat, nevertheless, is relatively weak and lasts only one day on March 17 at the ADCP location, so the Eulerian/Lagrangian comparison cannot be made more quantitative.
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Fig. 9. Twenty-day drifter trajectories from observation and model results. The drifter was located at (843W, 29.53N) on March 8. The small circle denotes the initial position of the drifter. Solid line and broken line are for the observed and modeled trajectories of that drifter.
Since the weather system has a scale much larger than the shelf and the low passed wind pattern changes little over the shelf, the non-uniformly distributed wind seems to have little e!ect on the circulation. This is in part due to the fact that there are no other o!shore meteorological station available over the region and only a few meteorological stations on the land over the shelf. The o!shore wind data is far better than those on the land. From our modeling experience, we noticed that the hourly o!shore wind data is far superior to even the 6 h ETA atmospheric model wind data for driving the shelf circulation. Part of the reason is that in the atmospheric model the wind information always favors land over the ocean since it is on the land that the atmospheric model has most observational data to be compared with and adjusted to. Therefore, the model shelf circulation driven by the hourly wind during March 1996 compares favorable with the in situ current observation (Fig. 5) and Lagrangian drifter observation (Fig. 8).
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Fig. 10. (a) The time evolution of the distance and (b) the mean distance between the modeled and observed Lagrangian trajectories for the drifter that was located at (843W, 29.53N) on March 8, 1996.
4. Possible mechanisms Dynamically, it is along-shore component of the wind stress that primarily produces the along-shore coastal jet. In addition, this coastal jet peels o!shore as it
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moves. Thus, the along-shore wind forcing is the primary dynamic mechanism for the observed features of the surface Lagrangian circulation, including the `forbidden zonea, on the West Florida Shelf. We can show this mechanism as follows. Consider a coastline which, at least locally, may be taken to be straight at x"x . Suppose that the ocean depth h is a function C only of distance from the coast, i.e., h"h(x). For the west coast with land to the east of the shelf, h decreases eastward with increasing x. The wind stress is assumed to have only along component. The inviscid, linearized, vertically integrated shallow water equations are *u *g !fv"!g , *x *t
(4.1)
*v *g q #fu"!g # *y h *t
(4.2)
where u, v are the depth-averaged components of velocity in the onshore and alongshore directions, respectively; q is the along-shore component of wind stress acting on the ocean surface, divided by the water density, being constant in time and uniform in space. f is the Coriolis parameter, g the sea surface height and g the gravity acceleration. x, y and t are the onshore and along variables and time. Taking the x and y derivatives of the two equations and forming the vertical vorticity equation, we have
*f *u *v dh "!f # !q , *t dx *x *y
(4.3)
where f"(*v/*x)!(*u/*y) is the vorticity. The left-hand side is the local vorticity tendency and the last term on the right-hand side represents the external forcing mechanism for the coastal jet. Thus, it is along-shore wind stress that is e!ective at inducing the vorticity tendency. The tendency is anti-cyclonic when the stress has along-shore component with the land to the left, i.e., southward on the West Florida Shelf, and cyclonic when the stress has along-shore component with the land to the right, i.e., northward on the West Florida Shelf. Therefore, with an along-shore wind stress toward the southeast on the West Florida Shelf, the forcing will induce a coastal jet and this jet will peel o!shore as it moves southward because of the anti-cyclonic vorticity tendency. The physical mechanism can be understood as follows. When the stress has an along-shore component of the wind stress, it produces an along-shore #ow. However, as the water column becomes shallow toward the shoreline, the along-shore #ow increases its magnitude and thus induces a coastal jet. Furthermore, the change in the along-shore #ow strength induces the cross-shore shear, which produces a vorticity tendency. Therefore, when the stress has an along-shore component with the land to the left it produces a coastal jet with anti-cyclonic vorticity, making the jet tend to leave the coast. When the stress has an along-shore component with the land to the right it produces a coastal jet with cyclonic vorticity, also making the jet tend to leave
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Fig. 11. Schematic of the local vorticity and coastal jet generation by the along-shore component of wind stress. (a) The vorticity tendency is anti-cyclonic when the stress has along-shore component with the land to the left; (b) The vorticity tendency is cyclonic when the wind stress has along-shore component with the land to the right. In both cases, the generated vorticity makes the coastal jet and the water parcel tend to leave the coast.
the coast. Fig. 11 shows the schematic of the formation of coastal jet and related vorticity by the along-shore wind stress forcing. However, there is also a bottom friction, inducing an Ekman bottom layer. This bottom layer produces an onshore bottom transport for southward #ow on the West Florida Shelf. Along with the surface o!shore Ekman transport, the coastal jet becomes three dimensional. The central region of the West Florida Shelf is therefore characterized by a fully three dimensional, accelerating coastal jet with local maximum upwelling. Water feeding this coastal jet comes from below as the surface waters are advected o!shore. Fig. 12 shows the schematic of the three-dimensional coastal jet and its relation to the `forbidden zonea. The three-dimensional Lagrangian view shows onshore movement of the water parcels near the bottom, then upwelling and o!shore transport near Tampa Bay. The second mechanism is related to both the coastal geometry and the bottom topography. A con#uence zone exists o!shore from Tampa Bay for southward #ow. This convergent zone enforces the coastal jet from north and the onshore transport near the bottom and o!shore transport near the surface, resulting in local maximum upwelling. The last possible mechanism is simply kinematic one, a Lagrangian view of the surface circulation. As frontal systems traverse the region there is short duration (a few days) of primarily along-shore #ow to the north or south, driven by the wind at the leading and trailing edge of the front. The net along-shore displacements are therefore generally small when averaged over a typical front. If, along with the passage of fronts, the prevailing winds tend to be easterly (and therefore more upwelling than downwelling favorable), the small net along-shore displacements are accompanied by persistent o!shore displacements that eventually translate #uid parcels o!shore.
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Fig. 12. Schematic of possible three-dimensional coastal jet and the large drifter free zone. The solid arrows indicate the onshore bottom transport whereas the double line arrow is for the o!shore surface transport. The dashed line is for the upwelling. Therefore, forced by the downward along wind stress, when moving southward, the shelf water parcels with the coastal jet move onshore near the bottom, then undergoes upwelling near Tampa Bay and "nally moves o!shore near the surface.
These three mechanisms attributed to prevailing winds and geometry are likely to work in concert to create the complex three-dimensional coastal jet and a drifter free zone along the coast and in the south for drifters released in the north. An example is given by the drifter trajectories during November 14}21, 1996 (Fig. 13). Along shore movement of short duration and small net displacement is accompanied by persistent o!shore displacement. Many drifters starting within the Big Ben region undergo the process as we discussed.
Fig. 13. Observed drifter trajectories during November 14}21, 1996. Initial positions are denoted by small squares.
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5. Discussions 5.1. The Loop Current Obviously, the Loop Current plays a critical role in bringing the drifters in to the Florida Straits and Florida Currents. The Loop Current which bounds the shelf circulation over the southwest may also facilitate the forbidden zone on the West Florida Shelf. The southward #owing Loop Current will induce a bottom onshore transport by the bottom Ekman dynamics near the shelf break. The bottom onshore transport then may create an upwelling favorable condition along the southern coast region of the West Florida Shelf. The upwelling brings an o!shore surface transport, which prevents the surface drifters entering the coastal region there. However, it is di$cult to assess how much the Loop Current may contribute in the formation and maintenance of the forbidden zone. Moreover, the Loop Current is varied greatly over year and does not have any permanent e!ective impact on the shelf circulation, maybe with exception over the southwest boundary area of the shelf. Thus, it is unlikely that the Loop Current will play a critical role in the shelf Lagrangian circulation and the `forbidden zonea on the West Florida Shelf. 5.2. Freshwater yux ewect There are some river run-o!s along the west coast of Florida. A low-salinity front created by the freshwater #ux would prevent the drifters from entering the coastal region. Could the freshwater from the river run-o! play a role in establishing and maintaining the `forbidden zonea? Since the `forbidden zonea exists over one year period, in order for the front to be e!ective, it has to be a permanent feature along the outer edge of the `forbidden zonea. Examining the monthly climatological salinity distributions on the West Florida Shelf reveals no such a permanent feature. Furthermore, during most time of year, the salinity contour lines are parallel to the coastlines on the southern portion of the West Florida Shelf. Therefore, it is unlikely that the salinity front, even if it existed during some time of the year, will play an important role in the establishment and maintenance of the `forbidden zonea. 5.3. Wind and wave ewect on the drifter The drift bottle has some known bias due to the wind and surface wave e!ect, even though the precise surface wave e!ect is still hard to assess (Williams et al., 1977). In contrast to the historical drifter data, the satellite tracked drifters are much better than the drift bottles in tracking the #owing water. One con"rmation comes from its reasonable description of the Loop Current and eddies that separated from the Loop Current system (Fig. 2). Another con"rmation comes from comparison of the Lagrangian circulation with the in situ ADCP observation. If the drifter did not follow the near surface current, the Lagrangian circulation from the drifter data may di!er from that interpreted from the observed ADCP data. To some extent, the good agreement
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between the in situ Eulerian current observation (Fig. 5) and Lagrangian drifter observation (Fig. 8) demonstrates that the drifter may largely follow the near surface water parcel with minor wind and wave e!ect on the drifter.
6. Summary This paper presents some results of the surface Lagrangian circulation on the West Florida Shelf from drifters released during 1996}1997 and compares with the numerical model results of the wind-driven circulation. Using modern satellite tracked surface drifter trajectories from February 1996 to February 1997, we found a large drifter free region, called the forbidden zone, over the southern portion of the West Florida Shelf, consistent with the historical drift bottle release data. This large drifter free zone is also consistent with a recent numerical model study of the West Florida Shelf circulation response to the climatological wind forcing (Yang and Weisberg, 1999). The model drifter simulations during March 1996 successfully capture all essential features of the observed drifter trajectories. Three mechanisms were proposed for the observed features of the Lagrangian circulation. The "rst is a dynamic one. It is along-shore component of the wind stress that induces the coastal jet, tending to move the water o!shore, and the onshore transport near the bottom and o!shore transport near the surface. The second one is the convergent coastal geometry and the bottom topography from south of Tampa Bay that strengthens the coastal jet and the bottom and surface transport, and induces a maximum local upwelling near Tampa Bay. The last is a kinematic one, due to the small net along-shore displacements that occur in response to typical synoptic weather systems over the shelf. The results of the Lagrangian circulation including the `forbidden zonea over the southern portion of the West Florida Shelf suggests that the near surface shelf waters over the north (northward from Tampa Bay) may not reach the southern coast of the West Florida (southward from Tampa Bay). One of the implications is that surface hazard such as oil spill that may occur outside of the southern West Florida shelf may not greatly impact the southern coastal region except Florida Keys year-around. However, the biological and chemical patches over the north that may occur in the water column such as red tides can easily reach the southern coastal region through the subsurface and bottom waters. The latter may be particularly true during the summer and fall when the water is strati"ed.
Acknowledgements This work was supported by the Minerals Management Service and by the USGS. We thank Eric Siegel, Byan Black and Jack Parrish for help on the current observational data preparation.
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References Blaha, J., Sturges, W., 1981. Evidence for wind-forced circulation in the Gulf of Mexico. J. Marine Res., 39 711}733. Blumberg, A.F., Mellor, G.L., 1983. Diagnostic and prognostic numerical circulation studies of the South Atlantic Bight. Journal of Geophysical Research 88, 4579}4592. Blumberg, A.F., Mellor, G.L., 1987. A description of a three-dimensional coastal ocean circulation model. In: Heaps, N. (Ed.), Three-Dimensional Coastal Ocean Models, vol. 4. Amer. Geophys. Union, Washington DC, pp. 1}16. Cragg, J., Mitchum, G., Sturges, W., 1983. Wind-induced sea surface slopes on the West Florida Shelf. Journal of Physical Oceanography 13, 2201}2212. Gaul, R.D., 1967. Circulation over the continental margin of the northeast Gulf of Mexico. Ph.D. Dissertation, Department of Oceanography, Texas A & M University, College Station, 156pp. Hellerman, S., Rosenstein, M., 1983. Normal monthly wind stress over the world ocean with error estimates. Journal of Physical Oceanography 13, 1093}1104. Huh, O.K., Wiseman, W.J., Rouse, L.J., 1981. Intrusion of Loop Current waters onto the West Florida continental shelf. Journal of Geophysical Research 86, 4186}4192. Gilbes, F., Tomas, C., Walsh, J.J., Muller-Karger, F.E., 1996. An episodic chlorophyll plume on the West Florida shelf. Continental Shelf Research 16, 1201}1224. Marmorino, G.O., 1982. Wind-forced sea level variability along the west Florida shelf. Journal of Physical Oceanography 12, 389}404. Mitchum, G.T., Clarke, A., 1986. Evaluation of frictional, wind-forced long-wave theory on the West Florida shelf. Journal of Physical Oceanography 16, 1029}1037. Mitchum, G.T., Sturges, W., 1982. Wind-driven currents on the west Florida shelf. Journal of Physical Oceanography 12, 1310}1317. Niiler, P.P., 1976. Observations of low-frequency currents on the West Florida continental shelf. Memoires de la Societe Royale des Sciences de Liege X 6, 331}358. Steidinger, K.A., 1983. A re-evaluation for toxic dino#agellate biology and ecology. Progress in Phycological Research 2, 147}188. Tolbert, W.H., Salsman, G.G., 1964. Surface circulation of the eastern Gulf of Mexico as determined by drift-bottle studies. Journal of Geophysical Research 69, 223}230. Vargo, G.A., Carder, K.L., Gregg, W., Shanley, E., Heil, C., 1987. The potential contribution of primary production by red tides to the West Florida shelf ecosystem. Limnology and Oceanography 32, 762}767. Weisberg, R.H., Black, B.D., Yang, H., 1996. Seasonal modulation of the west Florida continental shelf circulation. Geophysics Research Letters 23, 2247}2250. Williams, J., Grey, W.F., Murphy E.B., Crane, J.J., 1977. Memoirs of the Hourglass cruises. Report of the Marine Research Laboratory, Florida Department of National Research, St. Petersburg, IV, Part III, 134pp. Yang, H., Weisberg, R.H., 1999. Response of the West Florida Shelf circulation to climatological wind forcing. Journal of Geophysical Researh 104, 5301}5320.
Lagrangian circulation and forbidden zone on the West Florida Shelf Huijun Yang *, Robert H. Weisberg , P.P. Niiler, W. Sturges, W. Johnson Department of Marine Science, University of South Florida, 140 Seventh Avenue South, St. Petersburg, FL 33701-5016, USA Scripps Oceanographic Institute, La Jolla, CA, USA Department of Oceanography, Florida State University, Tallahassee, FL, USA Minerals Management Service, Herndon, VA, USA Received 13 August 1998; received in revised form 24 November 1998; accepted 2 February 1999
Abstract This paper presents some recent results of drifters released on the West Florida Shelf during 1996}1997 and compares with the numerical model results of the wind-driven circulation. Using satellite tracked surface drifters during the one year period from February 1996 to February 1997, a drifter free region, called the `forbidden zonea, is found over the southern portion of the West Florida Shelf. This "nding is consistent with historical drift bottle data and with a recent numerical model study of the West Florida Shelf circulation response to climatological wind forcing. Direct drifter simulations by numerical model during March 1996 show a good agreement with both the in situ ADCP current observation and drifter observation. Three mechanisms are proposed for the observed Lagrangian features. The primarily dynamic mechanism is the along-shore wind forcing, which induces a coastal jet that tends to leave the coast and the bottom onshore and near surface o!shore transports. The second one is the convergent coastal geometry and bottom topography for the southward #ow in central shelf near Tampa Bay that enforces the coastal jet and the bottom and near surface transport. The last is a kinematic one, simply due to the short along-shore Lagrangian excursion, driven by the typical synoptic weather systems. Thus near surface shelf waters over the north may not reach the southern coast of the West Florida. Implication is that surface hazard such as oil spill that may occur outside of the southern West Florida shelf may not greatly impact the southern coastal region except Florida Keys. However, the biological and chemical patches over the north that may occur in the water column such as red tides still can easily reach the southern coastal region through the subsurface and bottom waters. 1999 Elsevier Science Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected]. http://chaos.marine.usf.edu/yang (H. Yang) 0278-4343/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 8 - 4 3 4 3 ( 9 9 ) 0 0 0 2 1 - 7
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1. Introduction The West Florida Shelf is a broad, gently sloping region extending approximately 650 km between the Florida Panhandle in the north and the Florida Keys in the south (Fig. 1). Its isobaths generally tend to parallel the coastline that is oriented approximately along 333T over the middle portion of the region, and the 100 m isobath lies approximately 100}240 km o!shore. Shelf circulation is mainly driven by winds, tides, and buoyancy #ux. The shelf circulation is bounded in the southwest by the Loop Current, which enters the Gulf of Mexico through the Yucatan Straits and exits as the Gulf Stream through the Florida Strait. By virtue of transporting and mixing shelf and adjacent ocean waters, the coastal ocean circulation largely determines the biological and geochemical properties of the water. Important biological phenomena that occur on the West Florida Shelf include episodic, seasonal blooms of (red tide) toxic dino#agellates thought to originate at mid-shelf (Steidinger, 1983; Vargo et al., 1987) and the seasonal formation of high concentration pigment plumes near the shelf break (Gilbes et al., 1996). Relatively, little is known about the role of the shelf circulation in determining their onset, movement, growth, and ecosystem impacts, however. Observations show that the variations in the shelf circulation and sea level are highly correlated with wind stress (e.g., Niiler, 1976; Balaha and Sturges, 1981; Mitchum and Sturges, 1982; Cragg et al., 1983; Marmorino, 1982; Mitchum and Clarke, 1986). A reverse of the midshelf currents was also founded in a recent study (Weisberg et al., 1996). Until recently, drift bottle releases provided the only Lagrangian view of the surface circulation over the region (Tolbert and Salsman, 1964; Gaul, 1967 and the Hourglass report by Williams et al., 1977). However, these data only provided the launch sites and recovery locations of the drifters, not the drifter trajectories. Thus, value of the information is very limited. Starting in February 1996, the Mineral Management Service (MMS) launched a satellite tracked surface drifter program in the northeastern Gulf of Mexico. The program intended for one year yielded about 1.5 years of data. These data provided actual drifter trajectories during the course of the program, and thus are much more valuable than the drift bottle data. The purposes of this paper are (1) to describe some results from the program, (2) to compare them with the numerical modeling results of the wind-driven circulation, the historical data and other observational data, (3) to propose several possible mechanisms for the Lagrangian circulation, and (4) to shed some light on possible ecosystem impacts of the naturally or man-made disasters such as red tides and oil spill over the West Florida Shelf.
2. Surface drifter data 2.1. Recent drifter observations The SCULP-drifters designed to follow the upper meter of the water column, were released every two weeks at pre-determined locations, by airplane at 15 sites between
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Fig. 1. The bathymetric map of the West Florida Shelf region and the SCULP-drifters release area, Hourglass release area, and release sites of Tolbert and Gaul experiments.
the 20 and 60 m isobaths in the northeastern Gulf of Mexico, as shown Fig. 1. A 12-month long composite of the resulting surface drifter trajectories from February 1996 to February 1997 are shown in Fig. 2 with a total of 341 drifters released. The most evident is where these drifters did not go. Intriguing is the lack of any trajectories over a large portion of the shelf, south of Tampa Bay. This drifter free area, we called it the `forbidden zonea, ranges between the coast to about 50 km o!shore in the central region to 240 km o!shore in the southern region. The near coastal region of the Big Bend is also void of trajectories. The entire drifter free area occupies about 74,000 km, or about half of the West Florida Shelf. Furthermore, no drifters made landfall on the West Florida coast during the whole course of the program. Some drifters were entrained by the Loop Current, but most drifters remained in the
Fig. 2. Observed drifter trajectories from February 1996 to February 1997 with a total of 194 drifters.
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northern portion of the shelf. During the later half year many drifters went into the western Gulf of Mexico, which was particularly noticeable during tropical storm Josephine in early October 1996 when all drifters on the shelf went westward. The surface wind and wave e!ects may impact the drifters. However, we believe that these impacts are very limited and the drifter basically follows the near surface water. More supporting evidence was provided later in Section 3. 2.2. Historical bottle drifter data Three historical data sources exist for surface Lagrangian circulation information from drift bottle programs in 1960s (Tolbert and Salsman, 1965; Gaul, 1967; and Williams et al., 1977). By employing drift bottles, these studies only provide release location and points of landfall along the coast (Figs. 1 and 3), but not actual trajectories. The resulting data may also be biased by wind and surface wave e!ects. Both the Tolbert and Salsman (1965) and the Gaul (1967) drift bottles were released o! the Florida Panhandle coast, the former near shore south of Panama City and the latter o!shore beyond the 200 m isobath. The Williams et al. (1977) Hourglass drifter data are from a series of cruises on the continental shelf between Tampa Bay and Ft. Myers from August 1965 to November 1967. Each of the 16 permanent stations was sampled monthly to investigate water properties to a depth of about 73 m, and drift bottles were released at each station to study the surface circulation. A total of 4460 drift bottles were released and 1347 were retrieved along the coast. These data are summarized in Table 1, where the following de"nitions apply: WFS is for the coast between Key West and Apalachicola, North is for the coast between Apalachicola and
Fig. 3. Historical drifter bottle launch sites and recover zones.
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Table 1 Historical drifter data: percentage of recovered bottles in the West Florida Shelf (WFS), the North and East and West
Tolbert Gaul Hourglass
No. of bottles Released
No. of bottles Recovered
WFS(%)
North(%)
East(%)
West(%)
951 6919 4460
276 1452 1347
0 2 30
68 9 1
20 63 62
12 26 7
the Mississippi River Delta, East encompasses the cost to the east of Key West and West includes the coast west of the Mississippi River Delta (Fig. 3). The major drift bottle "ndings are as follows. A majority of the drift bottle landings were on the east coast of Florida during fall and winter seasons, and a substantial number also landed on the east coast during the remainder of the year. Furthermore, along with the Hourglass releases, many of these were from northern o!shore releases (Gaul, 1967). Virtually no landings were reported along the West Florida coast south of Apalachicola Bay at any time, except from release sites in the Hourglass region within 20 km of shore. The last observation is most remarkable, considering the fact that weather systems regularly sweep over the region each year and that the West Florida beaches are populated by tourists year-round. These historical data are consistent with the recent "nding of a large drifter free region over the southern portion of the West Florida Shelf (Fig. 2).
3. Numerical model results 3.1. Response to climatological wind forcing The response of the West Florida continental shelf circulation to monthly mean climatological wind forcing was recently investigated by Yang and Weisberg (1999), using the three-dimensional, primitive equation Princeton Ocean Model (Blumberg and Mellor, 1983,1987). The model domain included realistic coastal geometry and bottom topography over a rectangular region from 87 to 813W and 24 to 30.33N (see Fig. 1). An evenly spaced 4.6 km rectangular grid was used in the horizontal plane, with 21 sigma levels in the vertical arranged to provide higher resolution within the surface and bottom boundary layers. The model was forced solely by the monthly mean wind stress climatology of Hellerman and Rosenstein (1983), linearly interpolated to "t the model grid. The horizontal boundary conditions over the land in the north and the east were implemented by a land mask, which ensures that the velocity over the land and the normal velocity along the coastline are zero. The model domain had open boundaries in the south and west over water. The depth-averaged transports normal to these boundaries were speci"ed to be zero. Thus, there were no net horizontal transports into or out of the domain, so the total water mass within the
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domain was always conserved. However, depth-dependent #ows could enter into or leave out of the domain across the open boundaries according to a Sommer"eld radiation condition (see Yang and Weisberg, 1999 for details). Twelve model runs were made for the 12 calendar months. The results were obtained after the model reached the equilibrium with the wind forcing and was in quasi-steady state for each run. Two basic seasonal patterns of circulation and sea surface elevation were found: a winter pattern from October to March and a summer pattern from April to September. The winter pattern includes an anticyclonic gyre, called the Big Bend Gyre, over the northeastern Florida Big Bend region that merges with a northwestward #ow from the south. The winter circulation also includes o!shore surface transport, coastal upwelling, and relatively low coastal sea level. The summer pattern features a continuous northwestward directed #ow, onshore surface transport, coastal downwelling, and relatively high coastal sea level. Transitions between these two seasonal patterns show either a development or relaxation of the Big Bend Gyre. In all cases the circulation patterns are fully three dimensional, and they result from wind-driven Ekman transports and surface-slope-induced geostrophic #ows. The model surface circulation patterns for each month are shown in Fig. 4, where the latitude is 24 to 30.3N from bottom to top and longitude is 87 to 81W from west to east. From April to September, the surface shelf circulation is predominantly alongshore toward the northwest. The model results are therefore consistent with the Hourglass drift bottle retrievals found to the north. In contrast, from October to March, the surface shelf circulation has an o!shore component over most of the shelf. The surface circulation is consistent with the existence of a large drifter free region in the southern portion of the West Florida Shelf. 3.2. Drifter simulation during March 1996 To further explore the surface Lagrangian circulation, we performed numerical model experiment for comparison of model drifters with observed drifters. The three dimensional, primitive equation POM is again applied to the West Florida Shelf during March 1996 with the model domain being the entire northeastern Gulf of Mexico region from 89 to 813W and 24 to 30.33N (see Fig. 3). The horizontal resolution is 6 km; there are 18 layers in the vertical, realistic coastal geometry and bottom topography are included; and the shelf strati"cation is speci"ed by a climatological monthly mean density "eld for March. The distribution of the salinity in March shows little di!erence from the north to south with contour lines parallel to the coastlines over the shelf while there is some temperature di!erence. Horizontal boundaries are open on the west and south for both the vertically integrated #ow and depth-dependent #ow, according to the radiation condition. Examining AVHRR images during March 1996 shows that the sea surface temperature (SST) is always relatively low near the coast compared with o!shore (not shown). This SST pattern favors a southeastward #ow and the SST seems to change only passively in response to the surface shelf circulation. Therefore, the baroclinicity
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during this time played a minor role, tending to enforce the southeastward #ow and weaken the northwestward #ow. The model thus, was driven solely by the low passed (36-h) hourly wind forcing (wind data were derived from NOAA Buoy station shown in top panel of Fig. 5). March 1996 starts with weak northerly winds that gradually change to southerly from March 4 and reach a maximum speed of 9 m/s on March 7. On the next day, the wind suddenly changes to northwesterly with maximum speed of 15 m/s, to northerly, and to northeasterly, as a front traverses the domain from northwest to southeast. On March 14, the wind becomes southerly. During March 19 and early March 20 the wind again becomes strongly northwesterly. Thus, two periods exist when wind is primarily northerly. However, due to the nature of the weather systems on the West Florida Shelf, the duration of continuous northerly wind is generally less than four to "ve days. This limits the time for which the shelf surface circulation can #ow continuously southward in response to winds. The shelf circulation responds rapidly to the wind changes, particularly during the two strong northwesterly wind events (Fig. 5). For example, on March 5 the surface #ow is onshore and northeastward in the south and northeastward in the north in response to the southerly wind. The surface #ow then rapidly changes direction in response to strong northwesterly wind after March 8. The surface current again moves southeastward after March 19 and moves back toward northwest on March 25. Fig. 6 shows the sample model surface circulation over the West Florida Shelf. The vertical structure of these time varying shelf circulation is also rather interesting (Fig. 7). On March 7, surface transports are onshore over most shelf. However, on the mid level and bottom onshore transports only occur in the inner shelf and the rest are basically along shore. On the mid-level on March 7, there is a counter current at the mid shelf moving toward the southeast to south of Tampa Bay. This may be explained by a pressure "led adjustment time before the wind e!ect can be felt at depth. But this changes on March 8 when #ows at all depth move toward the south (on the left half panel of Fig. 7). Thus, in response to northerly winds, the surface and mid-level currents near Tampa Bay are o!shore while the bottom currents are onshore. The strong coastal jet is evident in the shelf circulation, in particular near Tampa Bay. The same is true on March 20 (on right half panel of Fig. 7). The DeSoto Canyon is also an interesting region. The mid-level and bottom currents are all strongly onshore over the DeSoto Canyon and near coast. This strong bottom onshore transport may provide a mechanism for the observed coastal upwelling over the Florida Pandhandle coast during summer 1998 in the sea surface temperature (not shown). The strong onshore transport, in particular in the subsurface south of the DeSoto Canyon may explain the warm eddy intrusion over the DeSoto Canyon (Huh et al., 1981). Therefore, the shelf circulation is fully three dimensional and rather complex. 䉳&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& Fig. 4. The model surface shelf circulation patterns (Yang and Weisberg, 1999), showing an o!shore surface transport during the period from October to March and a northwestward and an onshore surface transport during the period from April to September over the central and southern West Florida Shelf. The longitudes are from 87 (left) to 813W (right) and latitudes are from 24 (bottom) to 30.33N (top).
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Fig. 5. Wind observation at NOAA Buoy located at (28.783N, 86.043W)(top panel); in situ ADCP current observation at 3 m depth (middle panel) and model surface current located at (28.833N,84.853W)(bottom panel) during March of 1996, after applied 36 h "lter.
Fig. 6. Model surface circulation of the West Florida Shelf during March 1996. Colors denote the water depth.
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Fig. 7. Model three-dimensional structure of the West Florida Shelf circulation. Top, middle and bottom panels are for the surface, mid-level and near bottom circulation. Colors denote the water depth.
Using the hourly model Eulerian velocity "eld, we can calculate Lagrangian drifter trajectories by forward time integration. The integration is carried out by a modi"ed fourth-order RK method. Simple linearly temporal and bilinearly spatial interpolation was used. Tests with di!erent interpolation schemes show no noticeable e!ect on the Lagrangian calculation.
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Model drifters were launched on March 8 for the comparison because there were only few SCULP-drifters in water before that time. A "ve-day model and in situ drifter trajectory comparison is shown in Fig. 8. Two major events are noted. As the wind direction turns toward the southeast, the drifters also move rapidly toward the southeast primarily in the along-shore direction from March 9 until March 13. Then they reverse direction, moving back north until March 19. On the way back they retrace the paths on which they came south. Thus, on March 19, the "ve-day drifter trajectories are all very short and are located north of the original launch sites. A second southward drive occurs immediately after the wind turns toward the southeast on March 19, and the drifters continue this drive until March 23. By this time some of them are positioned o!shore to south of Tampa Bay. The repetition process results in the model drifters that are in qualitative and quantitative agreement with the observations. For example, Fig. 9 zooms in the comparison for one typical drifter. This drifter was released in the Florida Big Bend at (84W,29.5N) on March 8, 1996. During the 20 days period, there are two southward marches and two retreats after two marches. The modeled Lagrangian trajectory captures the observed trajectory quite well except for the last few days. To quantify the performance of the model simulation, we de"ne the distance between the observed and modeled Lagrangian trajectories by d(t)"((X !X )#(> !> ), (3.1) K M K M where X , > and X , > are the positions of model and observed Lagrangian K K M M trajectories, respectively. Fig. 10a shows the time evolution of the distance in km. In the "rst two days, the distance increases from zero on March 8 to 14 km on March 10; it then decreases to 1 km on March 12 and 3 km on March 14. In the next four days, the distance increases rapidly to 33 km on March 19. However, the distance decreases from the maximum on March 19 to 4 km on March 22 as the modeled and observed Lagrangian trajectories pass each other. After March 24, the two trajectories start to depart rapidly again. We can measure a cumulative departure between the modeled and observed trajectories during a period of time by computing a time mean distance. The time mean distance d is de"ned by the square root of the sum of squared distances divided K by the elapsed time q; that is
1 O d (q)" d(t) dt. (3.2) K q R The results show that the mean distance while increasing rapidly for "rst few days hovers at levels less than 10 km during "rst 10 days and increases to around 15 km after day 12 (Fig. 10b). For example, the mean distance is 8 km during the "rst week from March 8 to 15 and 16 km during a period of 16 days from March 8 to 24, 1996. The in situ current observation (Fig. 5) shows a good agreement with in situ drifter observation (Fig. 8). During March 1996, the near surface current is strongly
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Fig. 8. Five-day drifter trajectories from observations (on the left panel) and model results (on the right panel) during March 1996. Date denotes the last day of trajectories. The color in model results denotes time from blue to red as time increases.
northward before March 8, southeastward from March 9 to 16 and again after March 20. It then reverses direction toward the north on March 24. The northwestward retreat of the drifters matches the northward current during March 25}28. The near
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Fig. 8. (Continued.)
surface current attains speeds of almost 50}60 cm/s during the two southeastward drives. Unfortunately, these are the only current data available during March 1996, and spatial variability exists as seen from the model and drifter data. For example,
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Fig. 8. (Continued.)
during March 14}19, the northwestward retreat of drifters from the central West Florida Shelf is most evident. This retreat, nevertheless, is relatively weak and lasts only one day on March 17 at the ADCP location, so the Eulerian/Lagrangian comparison cannot be made more quantitative.
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Fig. 9. Twenty-day drifter trajectories from observation and model results. The drifter was located at (843W, 29.53N) on March 8. The small circle denotes the initial position of the drifter. Solid line and broken line are for the observed and modeled trajectories of that drifter.
Since the weather system has a scale much larger than the shelf and the low passed wind pattern changes little over the shelf, the non-uniformly distributed wind seems to have little e!ect on the circulation. This is in part due to the fact that there are no other o!shore meteorological station available over the region and only a few meteorological stations on the land over the shelf. The o!shore wind data is far better than those on the land. From our modeling experience, we noticed that the hourly o!shore wind data is far superior to even the 6 h ETA atmospheric model wind data for driving the shelf circulation. Part of the reason is that in the atmospheric model the wind information always favors land over the ocean since it is on the land that the atmospheric model has most observational data to be compared with and adjusted to. Therefore, the model shelf circulation driven by the hourly wind during March 1996 compares favorable with the in situ current observation (Fig. 5) and Lagrangian drifter observation (Fig. 8).
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Fig. 10. (a) The time evolution of the distance and (b) the mean distance between the modeled and observed Lagrangian trajectories for the drifter that was located at (843W, 29.53N) on March 8, 1996.
4. Possible mechanisms Dynamically, it is along-shore component of the wind stress that primarily produces the along-shore coastal jet. In addition, this coastal jet peels o!shore as it
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moves. Thus, the along-shore wind forcing is the primary dynamic mechanism for the observed features of the surface Lagrangian circulation, including the `forbidden zonea, on the West Florida Shelf. We can show this mechanism as follows. Consider a coastline which, at least locally, may be taken to be straight at x"x . Suppose that the ocean depth h is a function C only of distance from the coast, i.e., h"h(x). For the west coast with land to the east of the shelf, h decreases eastward with increasing x. The wind stress is assumed to have only along component. The inviscid, linearized, vertically integrated shallow water equations are *u *g !fv"!g , *x *t
(4.1)
*v *g q #fu"!g # *y h *t
(4.2)
where u, v are the depth-averaged components of velocity in the onshore and alongshore directions, respectively; q is the along-shore component of wind stress acting on the ocean surface, divided by the water density, being constant in time and uniform in space. f is the Coriolis parameter, g the sea surface height and g the gravity acceleration. x, y and t are the onshore and along variables and time. Taking the x and y derivatives of the two equations and forming the vertical vorticity equation, we have
*f *u *v dh "!f # !q , *t dx *x *y
(4.3)
where f"(*v/*x)!(*u/*y) is the vorticity. The left-hand side is the local vorticity tendency and the last term on the right-hand side represents the external forcing mechanism for the coastal jet. Thus, it is along-shore wind stress that is e!ective at inducing the vorticity tendency. The tendency is anti-cyclonic when the stress has along-shore component with the land to the left, i.e., southward on the West Florida Shelf, and cyclonic when the stress has along-shore component with the land to the right, i.e., northward on the West Florida Shelf. Therefore, with an along-shore wind stress toward the southeast on the West Florida Shelf, the forcing will induce a coastal jet and this jet will peel o!shore as it moves southward because of the anti-cyclonic vorticity tendency. The physical mechanism can be understood as follows. When the stress has an along-shore component of the wind stress, it produces an along-shore #ow. However, as the water column becomes shallow toward the shoreline, the along-shore #ow increases its magnitude and thus induces a coastal jet. Furthermore, the change in the along-shore #ow strength induces the cross-shore shear, which produces a vorticity tendency. Therefore, when the stress has an along-shore component with the land to the left it produces a coastal jet with anti-cyclonic vorticity, making the jet tend to leave the coast. When the stress has an along-shore component with the land to the right it produces a coastal jet with cyclonic vorticity, also making the jet tend to leave
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Fig. 11. Schematic of the local vorticity and coastal jet generation by the along-shore component of wind stress. (a) The vorticity tendency is anti-cyclonic when the stress has along-shore component with the land to the left; (b) The vorticity tendency is cyclonic when the wind stress has along-shore component with the land to the right. In both cases, the generated vorticity makes the coastal jet and the water parcel tend to leave the coast.
the coast. Fig. 11 shows the schematic of the formation of coastal jet and related vorticity by the along-shore wind stress forcing. However, there is also a bottom friction, inducing an Ekman bottom layer. This bottom layer produces an onshore bottom transport for southward #ow on the West Florida Shelf. Along with the surface o!shore Ekman transport, the coastal jet becomes three dimensional. The central region of the West Florida Shelf is therefore characterized by a fully three dimensional, accelerating coastal jet with local maximum upwelling. Water feeding this coastal jet comes from below as the surface waters are advected o!shore. Fig. 12 shows the schematic of the three-dimensional coastal jet and its relation to the `forbidden zonea. The three-dimensional Lagrangian view shows onshore movement of the water parcels near the bottom, then upwelling and o!shore transport near Tampa Bay. The second mechanism is related to both the coastal geometry and the bottom topography. A con#uence zone exists o!shore from Tampa Bay for southward #ow. This convergent zone enforces the coastal jet from north and the onshore transport near the bottom and o!shore transport near the surface, resulting in local maximum upwelling. The last possible mechanism is simply kinematic one, a Lagrangian view of the surface circulation. As frontal systems traverse the region there is short duration (a few days) of primarily along-shore #ow to the north or south, driven by the wind at the leading and trailing edge of the front. The net along-shore displacements are therefore generally small when averaged over a typical front. If, along with the passage of fronts, the prevailing winds tend to be easterly (and therefore more upwelling than downwelling favorable), the small net along-shore displacements are accompanied by persistent o!shore displacements that eventually translate #uid parcels o!shore.
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Fig. 12. Schematic of possible three-dimensional coastal jet and the large drifter free zone. The solid arrows indicate the onshore bottom transport whereas the double line arrow is for the o!shore surface transport. The dashed line is for the upwelling. Therefore, forced by the downward along wind stress, when moving southward, the shelf water parcels with the coastal jet move onshore near the bottom, then undergoes upwelling near Tampa Bay and "nally moves o!shore near the surface.
These three mechanisms attributed to prevailing winds and geometry are likely to work in concert to create the complex three-dimensional coastal jet and a drifter free zone along the coast and in the south for drifters released in the north. An example is given by the drifter trajectories during November 14}21, 1996 (Fig. 13). Along shore movement of short duration and small net displacement is accompanied by persistent o!shore displacement. Many drifters starting within the Big Ben region undergo the process as we discussed.
Fig. 13. Observed drifter trajectories during November 14}21, 1996. Initial positions are denoted by small squares.
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5. Discussions 5.1. The Loop Current Obviously, the Loop Current plays a critical role in bringing the drifters in to the Florida Straits and Florida Currents. The Loop Current which bounds the shelf circulation over the southwest may also facilitate the forbidden zone on the West Florida Shelf. The southward #owing Loop Current will induce a bottom onshore transport by the bottom Ekman dynamics near the shelf break. The bottom onshore transport then may create an upwelling favorable condition along the southern coast region of the West Florida Shelf. The upwelling brings an o!shore surface transport, which prevents the surface drifters entering the coastal region there. However, it is di$cult to assess how much the Loop Current may contribute in the formation and maintenance of the forbidden zone. Moreover, the Loop Current is varied greatly over year and does not have any permanent e!ective impact on the shelf circulation, maybe with exception over the southwest boundary area of the shelf. Thus, it is unlikely that the Loop Current will play a critical role in the shelf Lagrangian circulation and the `forbidden zonea on the West Florida Shelf. 5.2. Freshwater yux ewect There are some river run-o!s along the west coast of Florida. A low-salinity front created by the freshwater #ux would prevent the drifters from entering the coastal region. Could the freshwater from the river run-o! play a role in establishing and maintaining the `forbidden zonea? Since the `forbidden zonea exists over one year period, in order for the front to be e!ective, it has to be a permanent feature along the outer edge of the `forbidden zonea. Examining the monthly climatological salinity distributions on the West Florida Shelf reveals no such a permanent feature. Furthermore, during most time of year, the salinity contour lines are parallel to the coastlines on the southern portion of the West Florida Shelf. Therefore, it is unlikely that the salinity front, even if it existed during some time of the year, will play an important role in the establishment and maintenance of the `forbidden zonea. 5.3. Wind and wave ewect on the drifter The drift bottle has some known bias due to the wind and surface wave e!ect, even though the precise surface wave e!ect is still hard to assess (Williams et al., 1977). In contrast to the historical drifter data, the satellite tracked drifters are much better than the drift bottles in tracking the #owing water. One con"rmation comes from its reasonable description of the Loop Current and eddies that separated from the Loop Current system (Fig. 2). Another con"rmation comes from comparison of the Lagrangian circulation with the in situ ADCP observation. If the drifter did not follow the near surface current, the Lagrangian circulation from the drifter data may di!er from that interpreted from the observed ADCP data. To some extent, the good agreement
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between the in situ Eulerian current observation (Fig. 5) and Lagrangian drifter observation (Fig. 8) demonstrates that the drifter may largely follow the near surface water parcel with minor wind and wave e!ect on the drifter.
6. Summary This paper presents some results of the surface Lagrangian circulation on the West Florida Shelf from drifters released during 1996}1997 and compares with the numerical model results of the wind-driven circulation. Using modern satellite tracked surface drifter trajectories from February 1996 to February 1997, we found a large drifter free region, called the forbidden zone, over the southern portion of the West Florida Shelf, consistent with the historical drift bottle release data. This large drifter free zone is also consistent with a recent numerical model study of the West Florida Shelf circulation response to the climatological wind forcing (Yang and Weisberg, 1999). The model drifter simulations during March 1996 successfully capture all essential features of the observed drifter trajectories. Three mechanisms were proposed for the observed features of the Lagrangian circulation. The "rst is a dynamic one. It is along-shore component of the wind stress that induces the coastal jet, tending to move the water o!shore, and the onshore transport near the bottom and o!shore transport near the surface. The second one is the convergent coastal geometry and the bottom topography from south of Tampa Bay that strengthens the coastal jet and the bottom and surface transport, and induces a maximum local upwelling near Tampa Bay. The last is a kinematic one, due to the small net along-shore displacements that occur in response to typical synoptic weather systems over the shelf. The results of the Lagrangian circulation including the `forbidden zonea over the southern portion of the West Florida Shelf suggests that the near surface shelf waters over the north (northward from Tampa Bay) may not reach the southern coast of the West Florida (southward from Tampa Bay). One of the implications is that surface hazard such as oil spill that may occur outside of the southern West Florida shelf may not greatly impact the southern coastal region except Florida Keys year-around. However, the biological and chemical patches over the north that may occur in the water column such as red tides can easily reach the southern coastal region through the subsurface and bottom waters. The latter may be particularly true during the summer and fall when the water is strati"ed.
Acknowledgements This work was supported by the Minerals Management Service and by the USGS. We thank Eric Siegel, Byan Black and Jack Parrish for help on the current observational data preparation.
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