|H. Yang, Department of Marine Science, University of South Florida, St. Petersburg, FL
R.H. Weisberg, Department of Marine Science, University of South Florida, St. Petersburg, FL
A three-dimensional, primitive equation, numerical ocean model is used in conjunction with an in-situ measurement program to investigate the west Florida shelf circulation. We have implemented the Princeton Ocean Model (Blumberg and Mellor, 1987). The model contains an imbedded second moment turbulence closure sub-model to provide vertical mixing coefficients, is in sigma coordinate and has a free surface and a split time step. The domain of the model is from 87W to 81W in zonal direction and 24N to 30.3N in the meridional direction. The evenly spaced rectangular grid is adapted with very high horizontal resolution of about 4.6km by 4.6 km and 21 layers in the vertical.
By using climatologies of monthly wind stress, twelve monthly depth-averaged horizontal current patterns, elevations and three-dimensional circulation structures are obtained. The resulting annual cycle of the shelf circulation shows two basic patterns; they are the winter pattern from October to March, and the summer pattern from April to September. The winter circulation pattern features a strong anticyclonic recirculation over the northeastern shelf region, two oppositely directed coastal jets, one northwestward flowing mid-shelf jet, strong vertical shear, lower surface elevation near the coast, offshore horizontal transport and upwelling. The existence of the Big Bend Eddy owes to a joint effect of wind curl and coastal geometry. The summer circulation pattern features a strong single coastal jet, flowing northward or northwestward along the coast, higher surface elevation near the coast, vertical uniform mid-shelf northwestward flowing currents, onshore transport and downwelling. The resulting circulation is in a combined Ekman and geostrophic balance. The model captures many observed features in the region, including the coastal jets, seasonal variations of the elevation near the coast and strong seasonal variability of the direction and the magnitude of the current along the shore. The wind climatology accounts one third of observed annual sea level change. See Yang et al. (1996) for detailed discussions.
However, the wind stress alone fails to account for the southward current prevailing during the spring and summer over the mid shelf (Weisberg et al., 1996). The hydrographic effect is thus considered by using the monthly temperature and salinity climatology. Hydrographic effect explains only part of the observed southward current over the mid shelf during spring and summer. There are two possibilities. One is that the hydrographic data are not accurate enough to account for the current. The other may be the off shore large scale circulation (mainly the Loop Current). Therefore, the annual cycle of the circulation may be a result of several factors. During the fall and winter, the wind is strong and the shelf is less stratified. Thus the wind-driven circulation may dominate the shelf circulation. As the season changes to spring and summer, the wind weakens, the Loop Current marches northward and the shelf water becomes better stratified. Thus the hydrographic and the Loop Current effects may overcome the wind stress factor and dominate the shelf circulation. These ideas will be tested further in subsequent years.
The experiments of frontal system passage over the shelf were also conducted. Significant difference of the impacts of the frontal system on the sea surface elevation and shelf circulation was found between two differently oriented frontal systems that translated in different directions. When a frontal system is translated from west to east over the shelf with a speed 5 m/s, the resulting surface elevation evolution is shown in Fig.1, where the bright and dark color represent high and low elevation, respectively. The corresponding vertical structure of the currents are shown in Fig.2. When a frontal system is translated from north to south across the shelf, the results are shown in Fig.3 for the surface elevation and in Fig.4 for the vertical structure of the resulting currents. Both the continental shelf waves over the inner and mid shelf and over the shelf break, propagating toward the northwest are observed. The forcing dominates the circulation until the forcing system passes the region. Then the imbalance will induce a large perturbation over the Florida keys and thus the perturbation propagates upward as continental shelf waves. On the other hand, there is indication that the Dry Tortugas and Key West, seem also important for inducing the initial perturbation for the continental shelf wave propagation. The results also show that the low frequency variation of the surface elevation in the Florida Keys are much smaller than that in the north due to the northwestward propagation of the continental shelf waves.
Blumberg, A.F. and G.L. Mellor, 1987: A description of a three-dimensional coastal ocean circulation model, in Three-Dimensional Coastal Ocean Models, Vol.4, edited by N. Heaps, pp.208, American Geophysical Union, Washington, D.C.
Weisberg, R.H., B.D. Black and H. Yang, 1996: Seasonal modulation of the west Florida continental shelf circulation. Geophysical Research Letters, 23, 2247-2250.
Yang, H. and R.H. Weisberg, 1996: Climatological wind response of the three-dimensional west Florida continental shelf circulation. Journal of Geophysical Research, in revision.