|Robert H. Weisberg, Department of Marine Science, University of South Florida, St. Petersburg, FL
Huijun Yang, Department of Marine Science, University of South Florida, St. Petersburg, FL
Bryan D. Black, Department of Marine Science, University of South Florida, St. Petersburg, FL
Zhenjiang Li, Department of Marine Science, University of South Florida, St. Petersburg, FL
E. Siegel, Department of Marine Science, University of South Florida, St. Petersburg, FL
R. Cole, Department of Marine Science, University of South Florida, St. Petersburg, FL
J. Donovan, Department of Marine Science, University of South Florida, St. Petersburg, FL
An important factor affecting coastal erosion on Florida's west coast is the seasonal variation of sea level. Sea level, on average, tends to be lower in winter, when synoptic storm and wave activities are largest, than in summer, when these activities are smallest. This results from a combination of the shelf-wide density distribution deriving from both water circulation and air-sea interactions, seasonally varying wind stress distributions and atmospheric pressure. Since the offshore scale for the circulation is controlled by the overall shelf topography, not just the nearshore part, an understanding the coastal zone requires an understanding of the shelf-wide circulation and its response to seasonal and synoptic wind and buoyancy forcing. The "West-Central Florida Shelf Hydrography and Circulation" element of the "West-Central Florida Coastal Studies Project" is therefore aimed at an improved description and understanding of the relevant physical processes that control the shelf circulation and hydrography. This is prerequisite to understanding physical oceanographic impacts upon coastal erosion.
The circulation is driven by tides, winds and buoyancy (surface, coastal and offshore) fluxes, and it is steered by the joint effects of the earth's rotation and the local topography. The time scales over which the circulation varies range from semi-daily to inter-annual, and these include tidal variations, variations owing to synoptic weather system passage and seasonal variations due to the distribution of water properties and winds. Episodic events associated with tropical cyclones and loop current intrusions are also important. The spatial scale of these variations range from a few kilometers for the tidal currents to the full width of the shelf for those factors affecting the seasonal variations in sea level. Our primary focus is on those circulation time and space scales that impact sea level, since these are among the most important factors affecting the present day coastal geology. For example, if sea level, on average, wasn't about 0.5 ft lower in winter than in summer the beach geomorphology might be quite different. Additionally, the phasing between the synoptic wind induced sea level variations and the wind waves incident on the beach are also very important.
Compared with other continental shelf regions relatively little is known about the west Florida shelf circulation. From previous observations and models (focused mostly on synoptic weather and tide-forced aspects) the shelf may be discussed in terms of three regimes: 1) a coastal regime influenced by estuaries, point sources of fresh water and inlets and shoals; 2) a broad, nearly homogeneous, mid-shelf regime largely wind-forced and 3) a transitional, large gradient regime separating the shelf from the Gulf of Mexico. These regimes all vary seasonally.
The proposed research continues the development of a quantitative description and understanding of the circulation and water properties on the west Florida continental shelf; their responses to seasonal, synoptic weather and tidal forcing; the impact of the circulation upon sediment transport and coastal processes and to assist with interpretations of the many bio-geochemical questions that are related to the circulation. In order to accomplish this the West-Central Florida Shelf Hydrography and Circulation study has been developed along three parallel tracks: 1) in-situ measurements, 2) numerical modeling and 3) satellite remote sensing. The study objectives remain unchanged from previous years. They are enumerated below.
West-Central Florida Shelf Hydrography and Circulation study objectives.
1)Develop a description of the seasonally varying circulation over the west Florida continental shelf
2)Improve upon the description of how the various forcing functions: surface, coastal and offshore buoyancy fluxes; synoptic weather systems and tides affect the west Florida shelf circulation.
3)Determine how these processes, in combination, affect both the along- and across-shelf transports of material properties.
4)Determine the relative importance of the surface fluxes and the coastal ocean dynamics in the distribution of water properties on the west Florida shelf, particularly during the seasonal transitions.
5)Relate these physical oceanographic findings to questions of geological, biological and chemical importance; for example sediment distributions and coastal erosion, nutrient distributions, species migrations and successions, and the appearance of large scale regions of enhanced primary productivity evident in CZCS imagery.
Highlights of work in progress
A brief synopsis of progress made on our work plan through FY96 is listed below.
1)Deploy a pilot mooring at a mid-shelf location to initiate a long time series, test mooring behavior, determine biofouling and fish effects on velocity profiles, and provide a test platform for bio-optical or other sensors.
2)Deploy a trans-shelf array of surface and subsurface moorings between the near shore and the shelf slope.
3)Develop a numerical circulation model of the west-Florida continental shelf for use in Eulerian and Lagrangian studies of the large scale shelf circulation and smaller, regional scale studies.
4)Collect satellite SST images for interpretation relative to the moored and shipboard measurements and interact with SEAWIFS investigators once those ocean color data become available.
5)Collect hydrographic data on mooring deployment cruises and collaborate on other hydrographic measurements.
Statement of Work for FY97:
1) Coastal Jets
Sea level is observed to set up and down along the coast in response to synoptic weather fronts. The scale width is that of the shelf and indeed we observe coherent mid-shelf motions when this occurs. However, we have also observed (in satellite imagery) concurrently well defined, near coast features that are continuous between Tampa Bay and Key West and that may be interpreted as coastal upwelling. One particularly clear event reported on at our June 7, 1996 workshop occurred in May 1994. We have attempted to analyze such features by performing numerical model experiments and have found that they are related to coastal jet and upwelling responses to both off-shore and alongshore wind stress components. We therefore intend to sample these where they appear to have a maximum signature, offshore from Manatee/Sarasota Counties on about the 15 m isobath. We will utilize a new RD-Instruments "workhorse" ADCP along with our RD-Instruments "BroadBand" ADCP between the 10 m to 25 m isobaths, The actual location along the coast will be coordinated with other project investigators to maximize cooperative interactions. We would like to deploy in September 1996 to sample the onset of the frontal passage season and to maintain these measurements through spring 1997. Having achieved these measurements we will have satisfied our original goal of describing currents across the entire shelf from the near shore region to the shelf break.
2) Tidal Current Transition
Tidal current ellipses are observed to be oriented perpendicular to the isobaths at mid-shelf while aligning with the isobaths near the beach. Using single point measuring S4 current meters we intend to explore the transition from shore normal to shore perpendicular tidal currents. The logistically most convenient region is directly seaward of St. Pete Beach, FL.; however, we will coordinate these measurements with our project colleagues to maximize scientific interactions. The measurement duration will be between 2 weeks to 1 month and we anticipate deployments on the 5 m, 10 m and 15 m isobaths. Another strategy would be to combine these measurements with those in 1) above.
3) Site specific geological sampling
Several interesting sedimentological features have been reported, such as shore-normal sand waves near Anna Maria Island, shore-oblique ridges and troughs near Longboat and Siesta Keys and flow regimes associated with inlets, channels and islands. In collaboration with our project colleagues we will attempt to assist in sampling some of these features to help ascertain currents, sediments interactions. For example, Dr. Davis has already requested the use of our S4 current meters for his Egmont Key studies.
4) Long-term currents monitoring at mid-shelf
We intend to redeploy a surface mooring on about the 50 m isobath offshore from Tampa Bay with a downward looking 600 kHz ADCP, surface meteorological instruments, limited bio-optical instruments and subsurface temperature and salinity measurements. The observation of a seasonally varying background circulation on the west Florida shelf apparently driven in part by baroclinicity, has been a very important contribution of our work thus far. It is important to develop replicate years of such observations to determine whether or not this is really a statistically significant feature. We also believe it to be very important to begin developing quasi-synoptic, repeated transects of hydrographic measurements which has been a very weak element of our program and of previous studies on the west Florida continental shelf. While this cannot be accomplished with present project funding we will be attempting to help coordinate such measurements by others.
Ancillary data for all of the above will include coastal sea level and meteorological stations and meteorological data from the NOAA offshore buoys. We will interact with the Tampa Bay PORTS program, assisting with their measurements offshore from Tampa Bay. We are also involved with the Minerals Management Service, Northeast Gulf of Mexico Study, making similar moored measurements just south of Apalachicola Bay, FL. through a subcontract from the Florida State University. These interactions will result in a much richer data set than would be available through this particular project alone. They are also allowing us to perform the work herein on a very cost-effective basis.
Numerical Modeling We are proceeding with two versions of the public domain three-dimensional, primitive equation, numerical ocean model developed by Blumberg and Mellor (1987) at Princeton (the so-called Princeton Ocean Model) that contains an imbedded second moment turbulence closure sub-model to provide vertical mixing coefficients, employs a sigma coordinate system with a free surface and uses a split time step to efficiently calculate vertically averaged and vertically dependent flow components. We have implemented and experimented with both cartesian coordinate system and curvilinear coordinate system versions and have found that each has its advantages depending upon the type of experiment being performed.
Having completed response calculations to monthly wind stress climatologies, with and without seasonal temperature and salinity climatologies, we have learned that the hydrographic data base for the region is woefully lacking. We received a revised set of hydrography through our MMS interactions and will be employing that to see what improvements are achieved. In essence, the climatological winds fail to account for the observed seasonally varying background currents, suggesting the importance of baroclinicity. However, the existing, very coarsely resolved hydrographic climatology for the shelf shows much too much vertical stratification and not enough horizontal gradient to be useful. Model runs utilizing this are inconsistent with the observed currents. Whether or not this improves with the revised hydrographic climatology remains to be determined.
Thus far we have completed on manuscript for publication (Yang et al., 1996) on this topic. We continue to use this version of the model for experiments on responses to prototypical fronts and cyclones as they propagate across the shelf and for particle trajectory modeling.
The curvilinear coordinate system model has been used to determine shelf responses to uniform winds blowing from the four coordinate directions: on- and off-shore and along-shore from the northeast and southwest. We have been analyzing the kinematics and we are in the process of analyzing the dynamics. Having accomplished this we will be able to run more realistic winds to be supplied by NOAA, NWS, Ruskin, FL. for the purpose of simulating responses to real winds for comparisons with the real in-situ currents observations. These experiments will be part of Z. Li's PhD dissertation. Most of our support for the numerical modeling part of the project is being provided by an MMS cooperative grant (subcontract from the Florida State University).
Satellite SST Along with in-situ measurements and numerical modeling of the west-central Florida continental shelf circulation and hydrography we have been collecting satellite AVHRR imagery of the sea surface temperature (SST) field in order to describe and better understand how the SST field evolves in time and space relative to the processes that we are observing and modeling. Such SST imagery has been the subject of numerous scientific papers available in the published literature. Previous studies, however, have generally been without the benefit of in situ measurements of the type that we are collecting. The imagery that we have been collecting provides examples of features having time and space scales owing to loop current influences, wind induced Ekman transports, coastal upwelling and offshore upwelling. These not only lend themselves to interpretation, but, more importantly, they extend the interpretations from our limited measurements to a much larger spatial domain.
A specific example that has motivated some of our FY97 proposed field work was a narrow band of coastal upwelling owing to the coalescence of Ekman induced upwelling off of Key West/Dry Tortugas and off of Pinellas/Manatee/Sarasota Counties. We discussed the observation in detail at the June 7, 1996 workshop, including the consistencies found in our numerical model experiments. This highlights out the importance of maintaining the three elements of the physical study: in-situ measurements, numerical modeling and remote sensing, since each element adds constructively with the other, allowing us to gain a much broader interpretation than would be available from any one element alone.
The west-central Florida shelf hydrography and circulation study is proceeding as planned. We are accomplishing most of our measurement objectives, although the hydrography has suffered due to financial and manpower constraints. Our goal of achieving a shelf-wide set of currents observations is on course. We presently have measurements from the 300 m to the 30 m isobath and work during FY97 will concentrate closer to the coast. Our numerical model experimentation and analyses are on track and along with remotely sensed SST we are developing a better description and understanding of the complex, three-dimensional, time dependent circulation of the west Florida continental shelf and how this impacts material property distributions.
Acknowledgments: The USF Division of Sponsored Research assisted with equipment matching funds, The Florida Department of Environmental Protection provided an equipment support grant and the Florida Institute of Oceanography provided partial ship support. We would also like to thank Dr. F. Muller-Karger and his research group at USF for sharing their expertise and resources in the field of satellite remote sensing.
Appendix: List of Accomplishments
Zhenjiang Li - PhD - Dissertation work in progress
Bryan Black - MS - Thesis work in progress
Eric Siegel - Senior thesis, Eckerd College
Publications and Reports:
Yang, H. and R. H. Weisberg (1996). West Florida continental shelf circulation response to climatological wind forcing (submitted to Jour. Geophys. Res.)
Weisberg, R. H. 1994: Transport of Mississippi River water to the west Florida shelf Special NOAA Rep., Coastal oceanographic effects of summer 1993 Mississippi River flooding, M. J. Dowgiallo, ed., NOAA Coastal Ocean Office, March 1994, pp. 55-59.
Weisberg, R H, B D Black, I C. Donovan and R. D. Cole (1996). The west-central Florida shelf hydrography and circulation study: a report on data collected using a surface moored acoustic Doppler current profiler, October 1993-January 1995. DMS-USF tech. rep., Jan 1996
Weisberg, R. H., H. Yang and B. Black (1995). West-central Florida shelf hydrography and circulation, in West-Central Florida Coastal Studies Workshop, April 24, 1995, USGS St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL. Open File Rep. 95- 840, G. Gelfenbaum, ed.
Black, B., R. LI. Weisberg and H. Yang (1995). Seasonal variations of the west-central Florida shelf circulation from a process prospective, in West-Central Florida coastal Studies Workshop, April 24, 1995, USGS St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL. Open File Rep. 95-840, G. Gelfenbaum, ed.
Yang, H., R. H. Weisberg and B. Black (1995). Numerical investigations on the three dimensional west Florida shelf circulation, in West-Central Florida Coastal Studies Workshop, April 24, 1995, USGS St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL. Open File Rep. 95-840, G. Gelfenbaum, ed.
Weisberg, R. H., B. Black and H. Yang (1995). West-central Florida shelf hydrography and circulation: inferences from satellite AVHRR imagery, in West-Central Florida Coastal Studies Workshop, April 24, 1995, USGS St. Petersburg Coastal and Marine Science Center, St. Petersburg, FL. Open File Rep. 95-840, G. Gelfenbaum, ed.
Weisberg, R. H., J. C. Donovan and R. D. Cole (1996). Salinity temperature and velocity measurements at the mouth of the Suwannee River, FL. Department of Marine Science, University of South Florida, Technical Report, complete except for printing.
Siegel, E. and R. H. Weisberg (1996). Physical factors affecting the Suwannee River estuary, Department of Marine Science, University of South Florida, Technical 5 Report, draft complete.
Presentations at National Meetings with Published Abstracts:
Black, B. D., R. H. Weisberg and H. Yang (1996). Observations of currents over the west Florida continental shelf. Ocean Sciences '96, San Diego CA., Feb. 12-16 1996.
Yang, H., R. H. Weisberg and B. D. Black (1996). Three dimensional modeling of the west Florida continental shelf circulation. Ocean Sciences '96, San Diego CA., Feb. 12-16 1996.
Other Scholarly Presentations:
4/24/95, St. Petersburg, FL. USGS WFS workshop, participant and presenter of 4 talks on in-situ measurements, satellite remote sensing and numerical modeling of the west Florida continental shelf circulation.
3/28/96, St. Petersburg, FL. MMS NEGOM workshop, convener, participant and presenter of 2 talks on in-situ measurements, satellite remote sensing and numerical modeling of the west Florida continental shelf circulation.
4/9/96, Ft. Pierce, FL. Invited speaker, Florida Coastal Ocean Sciences and Technology Symposium FCOSTS '96, Harbor Branch Oceanographic Institution, West Florida Shelf Circulation Studies.
6/7/96, St. Petersburg, FL. USGS WFS workshop, participant and presenter of 4 talks on in-situ measurements, satellite remote sensing and numerical modeling of the west Florida continental shelf circulation.
6/10/96, Princeton N. J. Princeton Ocean Model Meeting, Presentation by H. Yang on WFS circulation modeling.