Wintertime observations of Subtropical Mode Water formation within the Gulf Stream

Abstract

We study the structure of Subtropical Mode Water (STMW) within the eastward‐flowing Gulf Stream as it forms during strong winter cooling. Shipboard observations using SeaSoar and ADCP reveal that while active mixing by gravitational instabilities is common, large vertical and lateral shears of the Gulf Stream play a central role in determination of the modes of active mixing. Evidence is presented that low static stability and large vertical shear can combine to cause slantwise convection/symmetric instabilities, while the large anticyclonic shears to the south of the Gulf Stream core can cause low absolute vorticity and precondition the Ertel potential vorticity to be small and more susceptible to instabilities. The area of active mixing driven by surface forcing in the presences of shear occupies a swath 50–90 km wide immediately south of the Gulf Stream core at the northern edge of the Sargasso Sea.