The physical structure and life history of cyclonic Gulf Stream ring, Allen

Abstract

A cyclonic Gulf Stream ring, Allen, was followed over its life from September 1976 to April 1977 in the region north of Bermuda. Conductivity, temperature, and depth; expendable bathythermograph; and velocity profile measurements were made in Allen, and over the last 5 months of its life, satellite buoys were used to track continuously its movement. The measurements indicate that in December 1976 Allen split into two rings, a large one, Allen, and a small one, Arthur. Arthur moved rapidly eastward and coalesced with the Gulf Stream near the New England seamounts. Allen moved in a large clockwise loop; at the end of February 1977 it became attached to the Gulf Stream and reformed into a modified ring, smaller in size and faster in rotation . At the end of April 1977 the modified ring coalesced with the Gulf Stream and disappeared as it was advected downstream in the stream. The principal results of this study are that (I) the New England Seamount chain was a major influence in the genesis of Allen and on the trajectories of nearby rings; (2) while a free eddy, months after its formation, Allen evolved into a bi modal or peanut-shaped structure; (3) the bimodal structure ultimately bifurcated, spawning a new isolated eddy, denoted as Arthur, and a modified remnant, Allen; (4) the velocity field of Allen involved the whole water column, with bottom velocities of 10-15 em s- 1; (5) the barotropic velocity at the center of Allen (6 cm s-1 to NNW) was about equal to its translation velocity (4 cm s-1 to NW); (6) especially energetic inertial motions were seen at the center of Allen, and these may play a role in enhancing the stirring of water properties; (7) Allen survived several close encounters or entrainments with the Gulf Stream, proving that such encounters can be nonfatal to a ring; (8) the encounters appear to result in injections (exchanges) of water (momentum, heat, etc.) into the rings at an estimate rate of 106 m3 s-1 per ring; and (9) the behavior of Allen and Arthur was in contrast to the results of some other studies which have shown that rings generally drift slowly and passively southwestward.