THE F H FLOAT - MEASURING STRETCHING VORTICITY DIRECTLY/

By changing the volume of an isopycnal float by a small, prescribed am ount, one can force it to rise or sink to neighboring isopycnal surfac es. The pressure difference between these is a simple measurement of s tratification, DELTArho/DELTAz. This technique is adapted to the RAFOS float technology to measure stretching vorticity changes along the tr ajectories of floats in Gulf Stream meanders. The measurement uncertai nty of the depth of an isopycnal is only a few meters, but the uncerta inty of the layer thickness estimate, when measured over about 200 m, is about 7 m due to differential advection of small scale density stru cture. This includes all periods between the 8 x day-1 sampling rate o f the prototype float and 1 day. The records from two floats are discu ssed in detail. As the floats up- or downwell and are expelled from th e Gulf Stream, order 10-20% changes in stretching vorticity are observ ed. Particularly along the cyclonic edge of the current the changes ca n be quite abrupt corresponding to 1-2 x 10(-6) s-1 horizontal diverge nces. The changes in curvature vorticity, estimated from the float tra jectories, are, by and large, weaker and tend to evolve more gradually . On the assumption that potential vorticity is conserved, changes in shear can be estimated. For the two cases discussed, one shows suggest ive similarity with what might be expected using a ''frozen field'' mo del of downstream velocity. The other retains strong negative shear af ter expulsion from the stream, which may be indicative of an anticyclo nic circulation just south of a meander crest.