Ned Smith, PhD
Harbor Branch Oceanographic Institution, Inc.
5600 North U.S. 1
Fort Pierce, FL 34946
Harbor Branch Oceanographic Institution had a current meter in the easternmost of the two channels that drain the southwestern part of Rabbit Key Basin. At the study site, the channel is about 20 m wide and 3 m deep. Currents were recorded 1 m above the bottom, using a General Oceanics Mark II inclinometer. Surface temperatures were being recorded at a second,nearby study site, about 80 m to the east of the channel. Temperatures were measured using a TidbiT recorder about 5 cm below the surface in about 0.5 m of water.
The plot of the current meter data (Figure 1) shows the passage of the storm on September 25, plus about five days of ebbs and floods before and after the storm. The effect of the hurricane was to eliminate two complete flood tide cycles. During the same time, however, ebb tide speeds were only slightly stronger than normal. It is noteworthy that no strong inflow occurred through the channel in advance of the storm. Nor was there any significant refilling of the bay through this channel after the storm had moved off to the northwest. Both before and after the storm, strongest ebb and flood current speeds are within the range of 40-50 cm/s.
The plot of surface temperatures (Figure 2) includes the entire time series recorded from August 26 to October 23. The storm effect appears as a pronounced, but transient dip in the plot. Part of the decrease in temperature is the effect of evaporative cooling with little solar heating.Also, part of the decrease is an effect of relatively cool rainfall arriving from higher levels in the atmosphere. Storm effects persisted for about a day, then the normal diurnal rise in fall of temperature resumed. Within about two weeks, the temperature was back down to about 25 deg C. In mid October, however, this is the result of some combination of cloud cover, evaporative cooling and lower midday sun angles.
In addition, one part of the observational study of Florida Bay circulation involves measurements of currents along the Intracoastal Waterway. It is hypothesized that the waterway plays an important, if local role in moving water between sub-basins in the northeast, east and southeast parts of the bay. A current meter was in operation in Cowpens Cut, near the bayside entrance to Tavernier Creek, from August 27 to December 10, 1998, and the data from September 22-29 show the effects of Hurricane Georges in forcing water first out of, then back into Cotton Key Basin.
The Cowpens Cut study site is located where the Intracoastal Waterway passes through Cross Bank. Because of the constricting effect of the shallow water on either side, the flow through the channel is an enhanced version of what would be recorded in the waterway further to the northwest or to the southeast. Tidal exchanges through Cowpens Cut are well defined, with strongest currents between 30 and 50 cm/s.
Effects of Hurricane Georges began on September 24 (Figure 3) with the disappearance of flow toward the southeast. Northwestward flow slowed somewhat, but the current did not reverse. The current remained northwestward until early on September 25, when it reversed and then flowed southeastward until early on the 26th. By late on the 26th, storm effects had diminished, and the ebb and flood of the tide reappeared as the dominant feature of the plot.
Throughout the record, northwestward flow appears to be stronger, and to last longer than southeastward flow. During the passage of Hurricane Georges in particular, flow out of Cotton Key Basin was considerably greater than the flow back into the basin as the storm moved away from the study area.