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Numerical Modeling of the Ocean Circulation on the West Florida Shelf

West-Central Florida Coastal Studies Home
Open File Report: Second West-Central Florida Coastal Studies Workshop
Introduction
Agenda
Processes
Framework
Morphodynamics
Attendees
Contact:
Chief Scientist
Zhenjiang Li, Department of Marine Science, University of South Florida, St. Petersburg, Florida
Robert H. Weisberg, Department of Marine Science, University of South Florida, St. Petersburg, Florida

The Princeton Ocean Model (POM) is used to investigate the Ocean Circulation over the west Florida shelf. The POM, described by Blumberg and Mellor (1987) and Mellor (1992), is a time-dependent, three dimensional, nonlinear, free-surface, primitive equation model. It employs a sigma coordinate in the vertical and coast-following coordinates in the horizontal, and embeds a turbulence closure submodel for parameterization of the vertical turbulent mixing. The POM has been used to diagnostically study the shelf responses to uniform winds blowing from the four coordinate directions: on- and off-shore and southeastward and northwestward alongshore under the barotropic setting. The magnitude of the wind stress is 1 , which is representative of the quantitative scale of prototypical synoptic front system over the shelf. The water temperature is 20 , salinity is 35. These experiments provide a framework for understanding the circulation response of the shelf to wind forcing. Some of the findings from the offshore and southeastward alongshore wind forcing experiments are presented in the following subsections. The results for the onshore and the northwestward alongshore wind forcing cases are generally similar, but opposite in sign, to those of the offshore and southeastward alongshore wind cases.

Off-shore Wind Forcing

In this case the wind forcing is spatially uniform and oriented anticlockwise from east with a magnitude of 1 . Fig. 2 shows the time series of kinetic energy at a west-central Florida continental shelf location. The model spins-up rapidly, peaking after about 1.6 days and then oscillating about a near steady state. Sea level and circulation pattern as a whole do not vary significantly after their initial rapid set up. Figures 3-6 show these nearly steady fields at day 12. The contours of the sea surface elevation (Fig. 3) show that the sea surface along the Florida peninsula coast drops and that along the Florida panhandle rises. The difference of the response along the west Florida coast means that while flooding would occur along the Florida panhandle, drying could happen simultaneously along the west Florida peninsula coast. The difference between the highest, near Pensacola, and the lowest, which is in the Florida Keys, is about 21 cm. In the offshore direction, the sea surface is piling up with a sharp deformation near the Florida peninsula coast. The deformation of the sea surface with an e-folding scale of 60 km which is within the 30 m isobath line corresponds to that determined by Marmorino (1982) and that calculated with Csanady's theoretical analysis. The depth-averaged velocity field and the middle layer circulation are similar to each other. The coastal jet geostrophically induced by the deformation of the sea surface bifurcates in the middle of the Florida Big Bend into two branches; each forms an independent circulation system. The southward coastal jet is blocked by the Florida Keys and veers northwestward to join the northward middle and outer shelf flow that rejoins the coastal jet in the west-central shelf to complete the anticyclonic circulation. The other circulation system consists of a northwestward coastal jet and a narrow, weaker nearby southeastward flow that is also the response to the sea surface deformation. At the surface and bottom, although the inner shelf shows a surface-tilting-induced geostrophic coastal jet, the flows are also controlled by Ekman dynamics. This can be seen particularly in middle shelf. The resulting consequences of the two Ekman boundary layers are that the cross-shelf transport proceeds in the surface and bottom layers, which has important implication for material property transport. In the Florida Bay, the circulation is ageostrophic due to the closed boundary associated with the Florida Keys. A vertical cross-shore section, located as shown in Fig. 2, of the velocity field is given in Fig. 5. It shows that the circulation is fully three-dimensional. Two distinct vertical regions come into being. The currents in the upper and lower depths are of opposite sign. A southward coastal jet whose core is clearly evident is located over the 20-30 m isobath with a width of about 30 km. A surface boundary layer appears within the upper 10-20 meters. The bottom boundary layer is relatively very thin. From the cross-shore velocity component (bottom at Fig. 5), we also can see a substantial cross-shelf circulation with water moving offshore near the surface and onshore at depth suggestive of upwelling near the coast. The region of maximum onshore flow roughly coincides with the core of the alongshore jet. In these core regions, large horizontal velocity gradients lead to substantial divergence as confirmed by the vertical velocity component shown near the bottom in Fig. 6. Upwelling occurs over most of the west Florida continental shelf region. A relative upwelling maximum exists as a continuous feature along the west-central Florida coast from Tampa Bay to the southwest of Naples, which corresponds to the horizontal velocity cores addressed above. This vertical upwelling feature is observed in satellite observations (Weisberg, et al., 1996). Along the shelf break, there are alternative upwelling and downwelling eddies that also appear in the Hsueh (1982) model and are related to waves at the shelf break.

Southward Alongshore Wind Forcing

In this case the wind forcing is spatially uniform and oriented anticlockwise from the east. The magnitude is the same as that in offshore wind forcing case. Southeastward alongshore wind forcing lowers the free surface elevation at the coast via an offshore Ekman transport. This occurs over the shelf width causing the flow field at the mid-shelf location shown to be more energetic (compare Fig. 7 and Fig. 2). The spin-up time is also slightly different. For the alongshore wind the response peaks after a pendulum day (approximate 25 hours here) and then oscillates at slightly larger period due to bottom friction. As will the offshore wind case the sea level and initial spin-up, Fig. 8 shows that wind lowers the sea surface from the Florida panhandle coast to the south of the Naples and causes higher sea level in the shallow Florida bay area. The difference between the lowest near Shell Point and the highest near the Florida Keys is about 42 cm which is much larger than that in offshore wind forcing case. Compared to the offshore wind forcing case, the sea surface deformation area is also larger, so the free surface-induced coastal jet is much wider than that of the offshore forcing cases. Another remarkable feature is that there is a long higher sea elevation strip in the middle shelf. Fig. 9 shows the horizontal velocity fields. The depth-averaged circulation as well as the middle level circulation consists of a wide coastal southward alongshore flow and a relatively weaker isobath-following northward flows beyond the middle shelf. The wide southward coast jet is blocked by the Florida keys and ultimately veers northwestward to merge with middle and outer shelf water. Although the wide coastal jet reaches the bottom from the surface, the circulation over the shelf is still fully three-dimensional. The surface flow is the combination of the surface-deformation-induced geostrophic flow along shore and the Ekman transports across shore. Near the bottom, there is a bottom boundary layer induced by the interior quasi-geostrophic flow. This results in a convergent flow offshore from Tamp Bay and along the central-west Florida continental shelf. Its consequence is that an upwelling should occur there. Fig. 10 is the contours of the horizontal velocity in the specified west-central Florida continental vertical section. An interesting northward undercurrent forms in the middle shelf. Its core lies over the 100 m isobath with a width of about 20 km. The coastal jet is wide and extends to the bottom. In the cross-shelf direction, we see that the cross-shelf circulation consists of the offshore flow over the top 30 m and the onshore flow at depth. Fig. 11 is the vertical velocity field near the bottom. Maximum upwelling occurs where the cross-shelf flow is maximum. This is observed along the Florida panhandle and south of Apalachicola Bay. There is another upwelling center in the west-central Florida continental shelf just off Tampa Bay. Almost half of the broad shelf in the southern portion of the west Florida continental shelf shows downwelling which corresponds to the ageostrophic turn-back flow of the coastal jet because of the existence of the Florida Keys. Shelf wave-related upwelling and downwelling also exist at the shelf break.

model domain west Florida shelf bathymetry

Figure 1: The bathymetry of the west Florida shelf (right) and the model domain with the grids (left). The size of grids is about 9 km. The green line is the position of the vertical section used in later analyses.

time series of kinetic energy

Figure 2: The time series of the kinetic energy at 0.65 m (green) and 31.3 m (red) of a 47m water column at 28.04N,84.01W from the offshore wind experiment.

contour map

Figure 3: The contours of the free surface elevation from the diagnostic offshore wind experiment.

horizontal velocity arrows
(a) depth-averaged(day 12). max: 15.5 cm/s
horizontal velocity arrows
(b) subsurface(level 2, day 12). max: 24.1 cm/s
horizontal velocity arrows
(c) middle(level 8, day 12). max: 15 cm/s
horizontal velocity arrows
(d) near bottom(level 15, day 12). max: 4.5 cm/s

Figure 4: Horizontal velocity arrows. The offshore wind forcing is applied. The arrows have magnitudes as followings: cyan: (0 - 1/4) maximum; green: (1/4 - 2/3) maximum; red: (2/3 - 1) maximum.
fields of velocity components fields of velocity components

Figure 5: Fields of the velocity components at offshore section j=33 from the diagnostic offshore wind (1 dyn/cm 2 ) experiment. The coarse green line is with zero value. Positive values indicate the southward alongshore flow (top) and onshore flow (bottom).

vertical velocity field

Figure 6: The vertical velocity field (z_sense) at the sigma level 15. The offshore wind (1 dyn/cm 2 ) is applied. The red and cyan lines indicate upwelling and downwelling, respectively.

time series of kinetic energy

Figure 7: The time series of the kinetic energy at 0.65 m (green) and 31.3 m (red) of a 47m water column at 28.04N,84.01W from the southeastward alongshore wind experiment.

contour map

Figure 8: The contours of the free surface elevation from the diagnostic southeastward alongshore wind experiment.

horizontal velocity arrows
(a) depth-averaged(day 12). max: 29.4 cm/s
horizontal velocity arrows
(b) subsurface(level 2, day 12). max: 39.3 cm/s
horizontal velocity arrows
(c) middle(level 8, day 12). max: 31.8 cm/s
horizontal velocity arrows
(d) near bottom(level 15, day 12). max: 12.2 cm/s

Figure 9: Horizontal velocity arrows. The southeastward alongshore wind forcing is applied. The arrows have magnitudes as followings: cyan: (0 - 1/4) maximum; green: (1/4 - 2/3) maximum; red: (2/3 - 1) maximum.

fields of velocity components fields of velocity components

Figure 10: Fields of the velocity components at offshore section j=33 from the diagnostic southeastward alongshore wind (1 dyn/cm 2 ) experiment. The coarse green line is with zero value. Positive values indicate southward alongshore flow (top) and onshore flow (bottom).

vertical velocity field

Figure 11: The vertical velocity field (z_sense) at the sigma level 15. The southeastward alongshore wind (1 dyn/cm 2 ) is applied. The red and cyan lines indicate upwelling and downwelling, respectively.

Coastal & Marine Geology Program > St. Petersburg Coastal and Marine Science Center > West-Central Florida Coastal Studies Project > Second West-Central Florida Coastal Studies Workshop > Processes > Numerical Modeling of the Ocean Circulation on the West Florida Shelf


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