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1. Processes
West-Central Florida Shelf Hydrography and Circulation Study
Introduction
Continental shelves may be discussed in terms of three regions, each controlled by different physical processes. The inner-shelf is where divergent surface and bottom frictional boundary layers play a major role; the middle-shelf is where these boundary layers are separated by a geostrophic interior; and the outer-shelf (shelf-break) is where strong interactions occur with the adjacent ocean. Of these regions, the inner-shelf is the least studied, either through in-situ observations, or through the application of analytical and numerical models. Yet, in many respects, depending upon the shelf width, this may be the scientifically most interesting region because of its physical, biological, chemical, and geological interactions in response to synoptic and seasonal forcing.
The west Florida shelf (WFS) is broad enough so that its inner, middle, and outer-shelf regions are relatively distinct, allowing the dominant processes within each region to be viewed somewhat independently from one another. Important inner-shelf scientific questions can therefore be addressed through the combination of in-situ measurements and models. These questions include the nature of: (1) the coastal jet's three-dimensional structure, time dependence, and turbulence evolution; (2) its rectification by stratified boundary layer effects and how this impacts along and across-shelf transports; and (3) the seasonal modulation of these processes by local and offshore forcing. Thus, the gently sloping WFS provides an excellent natural laboratory for studying the physical processes that set the stage for biological (blooms), chemical (nutrient concentrations), and geological (e.g., sediment resuspension) events.
This study was originally designed to include both in-situ measurements and numerical circulation modeling. It subsequently evolved with support from the MMS, the State of Florida, NOAA, and ONR. Presently ongoing is an Ecology of Harmful Algal Blooms (ECOHAB) regional field study to advance our understanding and forecasting ability for red-tides; a State of Florida supported Coastal Ocean Monitoring and Prediction System (COMPS) to provide real-time data for emergency management use; and ONR supported in-situ measurements and modeling. We adhere to the tenet that the in-situ measurements and modeling must be interactive. Measurements are sparse and require a multiple scale context. Models are critically dependent upon their parameterizations, initial state, and boundary conditions, so they must be pegged to measurements to be useful.
Accomplishments
Accomplishments may be organized in terms of in-situ measurements and models, both having a common theme: local forcing and its effects on the inner-shelf. We note that continental shelves respond to both local and offshore forcing. Local forcing is by the combined effects of momentum and buoyancy inputs at the sea surface and buoyancy inputs at the coast. Offshore forcing is by momentum and buoyancy inputs from the deep ocean at the shelf-break. By virtue of its width, the WFS is an end member in the spectrum of continental shelves for which local forcing should be important. Thus, our emphasis has been on local forcing and its effects on the inner-shelf. For completeness, we are making more limited measurements out to the shelf break, and our model domains [each employing the Princeton Ocean Model (POM)], while regional, extend out beyond the shelf break. Accomplishments are now listed and discussed in terms of: (1) in-situ data, (2) publications, and (3) dissertations/theses and other reports.
In Weisberg et al. (1996) we identify a seasonality to the circulation that previously had not been appreciated. We also discuss typical length scales for motions at semi-diurnal, diurnal, synoptic and monthly time scales and how these are modulated by season.
Li and Weisberg (1999a,b) describe the responses of the WFS to upwelling-favorable winds that are directed either along-shore or offshore. The model-related studies use a grid that extends from just west of the DeSoto Canyon region to the Florida Keys, and sufficiently far offshore so that the open boundary there does not interfere with the model responses over the continental shelf. Realistic coastline and isobath geometries are employed in fully three-dimensional model runs under a constant density setting. One of these describes the model response kinematics, and the other companion paper describes the model response dynamics. Relevant findings are as follows. The model spin-up follows an Ekman-geostrophic route resulting in a rapid adjustment over the course of a pendulum day. Surface Ekman layer divergence effects a sea surface slope that causes a geostrophic coastal jet that, in turn, causes a bottom Ekman layer response. While the WFS is gently sloping, subtle variations in coastline geometry have major effects on the fully three-dimensional circulation fields. In particular, partial closure in the south by the Florida Keys and the coastline curvatures of the Florida Big Bend conspire to yield strong responses to uniform winds directed either along-shore or offshore with respect to the central west Florida coastline. This may seem surprising at first, based on two-dimensional arguments that offshore winds should not effect a coastal jet/upwelling response. Nevertheless, these findings are borne out by observations, and they are attributable to the coastline and isobath geometries. These geometry effects result in fully three-dimensional circulation patterns that are relevant to material property transports. The dynamics analyses help to define the inner-shelf as the region over which the sea-surface slope sets up. We describe, in a vertically integrated sense, the interactions that occur between the along-shelf and across-shelf momentum balances in establishing the flow fields. The offshore scale of the inner-shelf is a frictional one associated with the divergence of the surface and bottom Ekman layers. This varies with bottom slope as shown for different regions along the coast. Offshore of the central west Florida coastline, where we have corroborating data, the inner-shelf under a constant density setting extends out to about the 50-75m isobath. Fully three-dimensional momentum balance analyses provide further diagnoses of the model responses and where the Ekman and geostrophic regions are situated.
Yang and Weisberg (1999) ask the question of whether or not the seasonal variations in the circulation can be accounted for by seasonally varying winds alone. We used climatological monthly mean winds to drive monthly model simulations. While interesting patterns are found suggesting that the WFS has wind-driven seasons of predominantly upwelling and downwelling circulations, monthly mean winds alone can not account fully for the seasonal variations in the circulation. Baroclinic effects must also be important along with the surface heat fluxes that facilitate these.
Yang et al. (1999) compares observed Lagrangian surface drifter tracks with numerical model simulations. We demonstrate that model drifters can mimic in-situ drifters over time scales of the synoptic weather events, at least on the shelf where the model responses to these synoptic weather events exceed the effects of the Loop Current or eddies positioned on or offshore of the shelf-break. We also attempt to explain the curious finding that no drifters deployed to the north of the west central Florida shelf tracked toward the shoreline Ð hence the title of the paper.
Weisberg et al. (2000) is a specific case study of an upwelling event recorded both in satellite AVHRR imagery and in-situ data. While upwelling (and downwelling) occurs regularly on continental shelves with the passage of each weather front, rarely do all the necessary ingredients of nature coincide to make the upwelling response visible in satellite imagery. In this case, the winds were light for several days prior to a strong, upwelling favorable wind event, and we had in-situ velocity measurements for comparison. Through a combination of data and model analyses we demonstrate the Ekman-geostrophic spin-up route described earlier. Using both constant density and stratified runs (stratification estimated from velocity shear by thermal wind) we describe the relative effects and explain the finding of maximum upwelling just offshore and to the south of Tampa Bay. We also explain other regions of local upwelling maxima south of Cape San Blas.
Spring and summer of 1998 exhibited large stratification over the inner-shelf, even right up to the shoreline. In-situ data shows that the circulation is very sensitive to stratification. By separating the surface and bottom Ekman layers, stratification in increases across-shelf transports. Weisberg et al. (2001) describes the in-situ data and the numerical model simulation for the month of April, 1998 and, buoyed by the fidelity between the two, uses the model to analyze the dynamics. A new result is found. The data show a rectification of the inner-shelf responses to synoptic wind forcing wherein upwelling favorable winds produce disproportionately larger responses in both sea level and currents than downwelling favorable winds. Stratification accounts for the rectification, and this is most readily understood in terms of the streamwise component of vorticity. For downwelling favorable winds, the buoyancy torque due to isopycnals bending into the sloping bottom opposes the tendency by planetary vorticity tilting due to the vertically sheared coastal jet. This thermal wind effect negates the need for large relative vorticity dissipation by the across-shelf flow in the bottom Ekman layer. The opposite occurs for upwelling favorable winds. Buoyancy torque adds constructively with planetary vorticity tilting requiring larger dissipation of relative vorticity by the bottom Ekman layer. By enhancing (upwelling) or suppressing (downwelling) the bottom Ekman layer the entire response is reduced or increased, respectively. Such rectification is evident on the WFS because the WFS is wide enough to distinguish the frictional inner-shelf (which this paper also helps to better define) from the shelf break.
Meyers et al. (2000) addresses the region of the shelf break based on the in-situ data collected between the 75 m and the 300 m isobaths over a one year time span. Large currents are found on occasion when the Loop Current or its eddies impinge on the shelf break. These effects are localized in the across-shelf direction due to the vorticity constraint of the shelf topography. Analysis of available satellite altimetry data also suggests local forcing. Loop Current impingement farther south did not force a large shelf-wide response.
Nearshore/Coastal Processes
A series of transverse bars has been observed off Anna Maria Island, on the west-central coast of Florida. These bars are unique in that they are oriented normal to shore, extend a long distance in the across shore direction, and they have amplitudes that become a significant fraction of the water depth. The bars come close to the beach but do not seem to effect the morphology of the beach.
Through the use of historical aerial photographs and repeated bathymetric surveys, these bars have been shown to migrate to the south. The long-term rate of migration averaged over 40 years is about 8 m/yr, whereas the short-term rate as determined by the bathymetric surveys is substantially higher. Sand supplied to the bars comes from a shallow flank of the Tampa Bay ebb-tidal delta.
The migration of the transverse bars off Anna Maria Island seems to be associated with the passage of cold fronts as they move from north to south along the west Florida shelf and not related to tidal currents. The northerly directed winds ahead of the cold front drive a near-bed flow in a northerly direction normal to the transverse bar crests. This flow may or may not be strong enough to induce sediment transport. After the front passes, the wind reverses and drives a stronger flow to the south, again normal to the bar crests. This stronger flow, and the associated larger waves force sediment transport to the south and results in the net southerly migration of the transverse bars. The enhanced sediment transport due to the combined waves and currents, along with the reversing nature of the sediment transport associated with the passage of the cold front may explain how the bars attain the large bar height to water depth ratios that were observed.
continue to Underlying Geologic Template/Antecedent Topographic Control
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