The Three-Dimensional Secondary Circulation in Developing Gulf Stream Meanders

Abstract

From June 1988 until August 1990 an array of 12 inverted echo sounders (IES), 12 additional IES with pressure gauges to measure the abyssal pressure signal, and 12 tall current meter moorings instrumented at depths 400 m, 700 m, 1000 m, and 3500 m was deployed in the Gulf Stream near 688W. The current meter array spanned an area of approximately 260 km by 200 km with nominal horizontal instrument spacing of 56 km in the cross-stream direction and 65 km in the downstream direction. Six events were observed during the two year deployment during which a steep Gulf Stream meander trough and an abyssal cyclone developed concurrently. Two of the meander troughs eventually pinched off to form cold core rings. As the abyssal flows spun up they became nearly barotropic with strong [ O(30 cm s21)] swirl speeds that advected the main thermocline and helped steepen the meanders. Although the geostrophic barotropic flow played a large role in the development of the system, ageostrophic flow was also required. In this study the three-dimensional mesoscale ageostrophic flow within the main thermocline is diagnosed during two steep meander trough events and one steep meander crest event. It is shown that in addition to the alongstream horizontal ageostrophic flow associated with the curvature of the main jet (cyclostrophic speeds greater than 25 cm s21 are not unusual on trough axes) there is significant horizontal cross-stream ageostrophic flow. Cross-stream ageostrophic flow as high as 18 cm s 21 is diagnosed. Vertical momentum advection is also shown to be an important contributor to cross-stream ageostrophic flow, with values as high as 7 cm s 21 diagnosed. The patterns of the ageostrophic flow are shown to be tied to the phasing between the upper-level meander and the abyssal vortex. Both rotational and divergent patterns in the ageostrophic flow are evident. For parcels on main thermocline isopycnals, the ageostrophic flow provides an asymmetric cross-frontal flow relative to the symmetric cross-frontal flow associated with the barotropic vortices.