|Bryan D. Black, Department of Marine Science, University of South Florida, St. Petersburg, FL.
Robert H. Weisberg, Department of Marine Science, University of South Florida, St. Petersburg, FL.
The broad, gently sloping west Florida continental shelf has isobaths that generally parallel the coastline with the 100 m isobath located some 150-200 km offshore. The circulation on the west Florida shelf is driven by tides, winds and buoyancy fluxes, and it is steered by the joint effects of the earth's rotation and topography. The Loop Current, by introducing water of different density, provides a dynamic height difference which also influences the shelf circulation.
Velocity data from 28°N, 84°W on the west Florida continental shelf are presented. The data were sampled with an RD Instruments acoustic Doppler current profiler (ADCP) from October 1993 through January 1995 at 1 m intervals between 3 m and 42 m in a total water depth of 47 m. The continuation of this project included the deployment of a broad-band ADCP deployed on the 30 m isobath at 27°48.5'N, 83°24.9'W from June 1995 through January 1996. The positions of these assets have been marked in Figure 1. This report will briefly compare the monthly means of both deployments but will focus on analyses performed on the first deployment, and builds upon earlier measurements such as Niiler (1976), Koblinsky (1981), Mitchum and Sturges (1982), Marmorino (1983) and Halper and Schroeder (1990) that were of shorter duration or stationed relatively far offshore.
The monthly means of the first deployment have been offered in Figure 2 as horizontal velocity vectors at 10, 20, 30, and 40 m depths. The monthly mean vectors tend to be oriented along-shore with the largest magnitudes (about 10 cm s -1 ) directed toward the southeast (northwest) in spring (late summer/early fall). When the monthly means are largest; for example, 10/93, 4/94, 9/94, they also show a systematic decrease with depth in the along-shore component consistent with a thermal wind balance. The monthly means for measured by the broad-band ADCP are shown in Figure 3 as horizontal velocity vectors at 5, 10, 15, 20, 25, and 30 m. We see this same trend repeated a year later and closer to shore. The most striking feature, however, is the repeated along-shore vector in the late summer. This suggests a seasonal signal that replicates over the three years sampled.
A progressive vector approximation is adopted for estimating the particle excursions at 3 m depth based upon the Eulerian observations. Position time series in the east and north coordinate directions [x(t),y(t)] were calculated according to dx/dt=u(t) and dy/dt=v(t), where (u,v) are the Eulerian velocity components in (x,y). This was done for the semi-diurnal, diurnal and synoptic time scales after bandpass filtering the hourly data set, and for the unfiltered hourly data. The months of January, April, July, and October of 1994 were chosen as representative of the seasonal evolution and progressive vector diagrams are presented in Figure 4. The format in each set of panels is similar, the only difference being the spatial scale which increases with the time scale. Well defined, eccentric ellipses are observed each month at the semi-diurnal time scale due primarily to the M2 and S2 tidal constituents. The semi-major axes of these ellipses are oriented across-shelf and their associate particle excursions over a tidal cycle are about plus and minus 1 km in the across-shelf direction. The diurnal time scale shows a much larger seasonal modulation. At the a latitude of 28°N, the inertial and diurnal time scales are close to each other. During the winter months, the diurnal ellipses are forced by the O1 and K1 tidal constituents, and are less eccentric than the semi-diurnal ellipses. The particle excursions are similar in direction and magnitude as those in the semi-diurnal time scale. Once the water column stratifies however, the surface waters are isolated from the frictional effects of the bottom allowing for the more circular patterns of inertial oscillations. This is especially evident in July when particle excursions of approximately 5 km are observed with no preferred direction. Since the inertial oscillations are usually set up by a wind impulse, the inertial time scale motions are both tide and wind forced.
At synoptic time scales, the particle motions are primarily wind forced. Frontal systems are more pronounced in fall and winter, and thus the displacements during this period tend to be larger than during the spring and summer. A typical winter frontal passage can result in along-shore displacements of plus and minus 10 km, which is almost twice those found in the summer. When these time scales are combined with the monthly varying mean flows, we see the monthly, along-shore, particle displacements increase to as much as 300 km. This large increase can be attributed to a mean background flow caused by the horizontal density gradients which reverse seasonally. A simplistic way of thinking about this is to recognize that during winter (summer) months, the waters at the shelf break are relatively warm (cold) compared with the waters at the coast. From this, one can hypothesize a reversal in the across-shore density gradient consistent with a geostrophic reversal in the currents.
The complete record of currents for the first deployment along with ancillary wind, sea level and temperature data are available in Weisberg et al. (1996a) and a more detailed analysis of these data is offered in Weisberg et al. (1996b).
Weisberg, R. H., B. D. Black, J. C. Donovan and R. D. Cole, The west-central Florida shelf hydrography and circulation study: a report on data collected using a surface moored acoustic Doppler current profier, 129 pp., Department of Marine Science, University of South Florida, St. Petersburg, FL, 1996a.
Weisberg, R. H., B. D. Black, H. Yang, Seasonal modulation of the west Florida continental shelf circulation, Geophys. Res. Let., 23:2247-2250, 1996b.