Spatial and temporal evolution of a Gulf Stream crest‐warm core ring interaction

Abstract

An intensive, near‐synoptic hydrographic study of the reabsorption of a warm core ring into the Gulf Stream at a meander crest was conducted near 70°W in fall 1988. Using a mixture of measurement techniques and statistical optimal interpolation to map the velocity and density fields, the study documents how the normally robust cross‐stream structure of the Gulf Stream breaks down and the process by which a high pressure ridge between the stream and the ring develops. Examination of the layer potential vorticity (LPV) structure on two specific volume anomaly (δ) surfaces (approximately 18°C and 10°C) showed different characteristics of cross‐stream LPV structure. On the shallower surface the cross‐stream LPV structure was dominated by the cross‐stream gradient of layer thickness; on the deeper surface the much weaker cross‐stream LPV structure reflected the structure of the relative vorticity field. During the 10‐day period of ringstream contact, considerable water was exchanged, rupturing the ring and eventually resulting in the ring's reentrainment by the Gulf Stream. Time‐varying transports as large as 30 Sv were exchanged to the north and south between the surface and 5°C. The Gulf Stream bifurcates in such a way that the anticyclonic (southern) side continues through the meander crest uninterrupted, while the cyclonic side is diverted to the north to replace the low potential vorticity waters of the ring with waters of high potential vorticity. Contact between the ring and the stream is established first at the surface and progresses downward with increasing time. The decreased time needed to remove the ring's vorticity in the lower layers is consistent with the geometrical picture of the Gulf Stream colliding with a bowl‐shaped ring. The flushing time for the lens of δ(18°C) water in the ring is on the order of 5 days, slightly longer than the 2–3 days for the relative vorticity of the δ(10°C) surface.