Observations of the Velocity and Vorticity Structure of Gulf Stream Meanders

Abstract

In September?October 1988 and in April 1989 two 4-week cruises were organized to study the structure and dynamics of Gulf Stream meanders. The first one focused on an anticyclonic crest, and the second one on a cyclonic trough. One objective of this program was to conduct a high-resolution study of the upper-ocean velocity and vorticity field using a CTD and a continuously profiling acoustic Doppler current profiler (ADCP). The cross-stream sections of velocity exhibit the typical pattern of a single velocity maximum with a narrow zone of cyclonic shear and a broad region of anticyclonic shear. The cyclonic velocity shear is larger and extends to greater depths in a crest than in a trough and can exceed 120% of the Coriolis parameter (f) at depths as great as 300 m. The vertically integrated transport to 250 m appears to be more symmetric in troughs than in crests. The computed sea level difference across the stream is about 0.46 m greater in the trough than the crest after seasonal correction and ?normalization? to the same surface transport. The surface velocity vectors are divergent upstream and convergent downstream of a crest, consistent with upwelling and shingle formation, and entrainment/downwelling, respectively. By combining the CTD observations of density with the velocity data, the cross-stream structure of the potential vorticity field and its components can be mapped. It is found that shear vorticity greatly enhances ?(f/?)d?/dz on the cyclonic side and weakens it to a distinct minimum in the center of the current at the crest. Although the database is not so extensive for the trough, the evidence suggests that the opposite is true: cyclonic shear is weakened and anticyclonic shear is strengthened resulting in a less asymmetric velocity and transport distribution. There was considerable ?marbling? of the potential vorticity field by both the stratification and the shear vorticity field, but there is no evidence to suggest that the two fields are correlated in such a way as to reduce the marbling to less than what the component fields individually contribute.