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Kevin D. Leaman and Peter S. Vertes

Abstract

Over a period of several years, RAFOS floats were launched into three levels of the deep western boundary current (DWBC) east of the northern Bahamas in order to identify and study any local recirculations that might be present in addition to the thermohaline-driven component of the current. These float trajectories reveal the presence of recirculations that are clearly caused by features of the lateral and bottom topography. In particular, the San Salvador Spur exerts a major influence on the paths of these floats. Although the floats exhibit a complicated set of motions, some order is imposed by relating periods when floats move directly along the boundary versus periods when they leave the launch site “anomalously” (i.e., to the cast or northeast) due to motions of the DWBC core. Comparison to current meter records along 26°30(N near the launch site shows that floats in the latter group were deployed when the DWBC core was located offshore.

The “eruption” of floats into the interior recirculation at the San Salvador Spur causes a reduction (by a process similar to what elsewhere has been termed “arrested dispersion”) in the mean rate at which the floats, and presumably other tracers, move southward along the boundary. The ”effective southward spreading rate” of these floats is estimated as 1.97 cm s−1, in reasonable agreement with analogous results from tracer studies in the same region.

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Kevin D. Leaman, Peter S. Vertes, and Chris Rocken

Abstract

Results from an initial feasibility study to test whether Global Positioning System (GPS) navigation can be combined with more traditional acoustic methods to measure ocean current profiles are presented. A typical acoustic current profiler such as PEGASUS measures currents by ranging on two acoustic sources (one-way beacons or two-way transponders) as it falls or rises through the water column. These sources must be previously deployed on the bottom, and their positions accurately determined via the attending research vessel. As discussed below, this procedure introduces a number of complications. In particular, any unresolved errors in the source deployment will remain as systematic errors in the resulting velocity data.

The method described here replaces the bottom-mounted sources with hydrophones drifting near the ocean surface. The positions of these hydrophones are computed every few seconds using GPS.

This feasibility test shows that a combined GPS-acoustic system can approach accuracy levels found in the standard method. Furthermore, systematic errors can be significantly reduced. Random errors are estimated to be approximately ±1–2 cm s−1 dependent on station geometry.

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Kevin D. Leaman, Robert L. Molinari, and Peter S. Vertes

Abstract

Results of a two-year field experiment as part of the SubTropical Atlantic Climate Studies (STACS) program in the Straits of Florida are presented. Temperature and absolute ocean current observations were obtained by PEGASUS acoustic current profilers over 16 cruises during which repeated cross sections of the Florida Current were made at 27°N. Results are shown for the mean velocity and temperature fields, the perturbation horizontal kinetic energy and potential energy fields and for those energy conversion terms that could be computed directly from the data. The barotropic and baroclinic energy conversion terms, although small, indicate that the flow is stable for both types of perturbations. A 1arge part of the variability is contributed by short time scales (one week or less).

The average and standard deviation of northward volume transport by the Florida Current during these cruises was (31.7 ± 3.0) × 106 m3 s−1. Barotropic and baroclinic contributions to the total heat flux across the North Atlantic Ocean at 27°N are computed for each cruise and for the two-year average of all cruises. With the use of previous estimates of the midbasin baroclinic and Ekman heat fluxes, the total average northward heat flux from the observations is (1.29 ± 0.21) × 1015 W.

To compare STACS data with results from a recent numerical model by Anderson and Corry, Florida Current transport are resolved in a simple manner into barotropic and baroclinic modes. Although the barotropic mode is considerably more variable than the baroclinic, the basic annual signal obtained from the model also appears in the STACS observations. In particular, a rapid transport decrease in the fall with a secondary decrease in the spring are found in both model and observations.

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