A Comparison of Techniques for Referencing Geostrophic Velocities

Abstract

A geostrophic velocity section across the Gulf Stream and deep western boundary current near 35°N is referenced four different ways: using a POGO float (acoustically tracked transport float), shipboard acoustic Doppler current profiler (ADCP), and bottom current meters, and by assuming an isotherm level of no motion. The comparison between the first two techniques is emphasized because they are most easily applied. In general, reference velocities calculated using the float data agree well with those obtained from the ADCP data. However, there is disagreement at locations where the ADCP velocity is not in thermal wind balance, in which case the POGO value is deemed more accurate because the float samples deeper into the subsurface geostrophic flow. Disagreement is also caused by insufficient cross-stream POCSO spacing (although this could be avoided). The isotherm- and current meter-referenced sections, while similar to each other, both show unrealistic features. it is argued that the POGO method is preferable to the shipboard ADCP for a deep-water hydrographic experiment.