Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the Stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream’s warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (70 to 64 W). However, on two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly-stratified water nearly saturated in oxygen that appears to have been subducted from the surface mixed layer north of the Stream. The intrusion was identified on three of the sections in profiles with a nearly identical temperature-salinity relation. From the western- to easternmost sections where the intrusion was observed, the depth of the intrusion’s salinity minimum descended by ~90 m in the 71 hours it took to complete this part of the survey. This apparent subduction occurred on the downstream side of a eander crest where the cross-stream velocity was confluent, frontogenetic, and hence suited for driving a frontal vertical circulation. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion panning from 10-40 m day-1, a range of values consistent with the intrusion’s observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. Hydrographic evidence of subduction of weakly stratified surface waters was observed on the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.
This work has been submitted for publication.
