SOME MEASUREMENTS OF CURRENT BY SHALLOW DROGUES IN THE FLORIDA CURRENT

Abstract

Several shallow parachute drogues were cast adrift in the region of the speed axis of the Florida Current and tracked for 27 hr by a Decca Hi‐Fix navigation system in a test run to explore the advantages of combining Lagrangian‐ and Eulerian‐type data. Their initial separations were of the order of 50–100 m, 200–500 m, and 2–4 km, but all turned cyclonically and then anticyclonically to trace out a meander with amplitude of some 3 km and a wavelength of some 150 km. Bathythermograph casts along the path revealed a thermal field marked by a cool column at the cyclonic bend.In the absence of friction and downstream acceleration, the geostrophic departure is computed at −27 cm/sec at the cyclonic and 16 cm/sec at the anticyclonic turn. The implied difference in the cross‐stream slope of the sea surface is consistent with the presence of the cool column. For stationary, homogeneous conditions, the turbulence level is estimated to be 1%, with a corresponding coefficient of lateral kinematic eddy viscosity of 8.5 × 104 cm2/sec.The drogue data were combined with Richardson dropsonde data collected by Nova University personnel a day before, for a synoptic view. The presence of the cool column, detected along the drogue path, may then be interpreted as a consequence of upstream horizontal divergence, detected by the dropsondes. The magnitude of the horizontal divergence is of the order of 10−5/sec. All these features are consistent with a proposed meandering model based on the theorem of conservation of potential vorticity. Some alternative interpretations are reviewed.