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NOAA TECHNICAL REPORT SERIES OCRM/SPD
by :
GREGORY C. HAN Ph.D. ROBERT W. CALVERT General Oceanics, Inc. 1295 NW 163 St. Miami, FL 33169
JACKSON 0. BLANTON Ph.D. Skidaway Institute of Oceanography Savannah, GA
ABSTRACT
A study was performed to observe the currents at Gray' Reef and a nearby coastal station for a period from June, 1984 to December, 1985. Current meters were deployed by General Oceanics at Gray's Reef (31.4 deg N,80.9 deg W) and "F" Reef (31.1 deg N,81.2 deg W) off the Georgia coast at depths near the middle and near the bottom of the water column. The purpose of the effort was to describe the reef energetics and nutrient dynamics representative of live bottom areas common to the South Atlantic Bight.
The results of the study consisted of records of current velocity and wind velocity" spanning the deployment period. Some gaps in the data occurred due to problems with instrumentation and damage from vessels in the area. Overall the data return was very good.
Analyses of the data were done using time series techniques. Means of winds and currents were compared on a seasonal basis. Correlations were computed between the records at each station and between each station and the winds at Savannah Navigational Light Tower.
Fluctuations in winds and currents occurred predominantly at 2-10 day periods. The most energetic fluctuations in ocean currents were generated by the astronomical tide at 0.5 day periods.
Comparisons were made of winds and currents for the spring, summer and autumn season for 1985. The seasonal mean current velocities at the top and bottom of each mooring were consistent with the mean wind speeds. Surface currents were predominately northward driven by the northward directed winds. A component of the flow was usually to the right of the wind at the upper instrument and to the left of the wind near bottom. This finding is consistent with other studies of currents on the continental shelf of the South Atlantic Bight.
There was usually good visual correlation in the data plots. However, there were episodes of southward currents during spring and summer when the wind stress is generally northward. These events result in correlation coefficients between wind and current that are lower than would occur under direct wind generation of shallow-water currents. Pressure gradient forces are likely responsible for a significant portion of the variance in ocean currents in the vicinity of Gray's Reef.
Gray's Reef is in an area which receives seasonal fluctuations of fresh water coastal runoff. The resulting pressure gradients can modify the response of the current to direct wind forcing. Seasonal fluctuations of freshwater runoff and changing sea level gradients have an important influence on coastal currents surrounding Gray's Reef. A key findings of this study, namely the rather low correlation between winds and currents, is evidence of forces other than wind that help force the observed currents at Gray's Reef. We recommend future studies designed to measure the cross-shelf and along shelf pressure gradient in the vicinity of Gray's Reef.
This work is the result of research sponsored by the U.S. Department of Commerce, National Oceanic and Atmospheric Administration, National Ocean Service, Office of Ocean and Coastal Resource Management, Sanctuary Programs Division Under Contract NA84AA-H-CZ027 and a Subcontract RR099-065 from University of Georgia.