DIRECT OBSERVATIONS IN SUPPORT OF OPERATIONAL MONITORING
To obtain direct measurements of the volume of water
passing through the Straits of Florida between the Florida
peninsula and the Bahamas.
An undersea cable is being
used to continuously measure the transport (volume of water
flowing) through the Straits of Florida. As sea water flows north,
the earth's magnetic field causes an electrical voltage to be
generated across the flow. By measuring the voltage difference
between one end of the cable near Palm Beach, Florida and the other
end at Settlement Point in the Bahamas, the transport can be
estimated. In order to calibrate and verify the cable
measurements, periodic direct measurements of the transport are
made by AOML personnel.
The transport is computed by multiplying the average
current by the cross sectional area through which it flows. To
estimate the transport through the Straits of Florida, average
current measurements are made at 9 fixed locations between Florida
and the Bahamas. The cross sectional area is known from the
station spacing and the water depth. To measure the average
current at each station, a small float is allowed to sink to the
bottom where it releases a weight and then floats back to the
surface. By measuring the time from release until it resurfaces
(DT) and the distance it moves (DX), the average current can be
computed (DX/DT). To measure the time and distance accurately, a
Global Positioning System (GPS) navigation receiver is mounted in
the float which continuously stores position and very accurate time
information whenever the float is on the surface.
A series of 10 - 12 direct transport measurements
spaced over a period of 20 to 30 days are made twice each year plus
single trip measurements are made several other times to provide
sufficient data to evaluate the performance of the cable
measurements. The transport measurements have been performed
continuously since 1982 and an extensive historical database of the
Gulf Stream transport has been developed.
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