A shallow subtropical subducting westward propagating eddy (Swesty)

Abstract

Swesty is a shallow subtropical subducting westward propagating eddy that was observed to move across the eastern basin of the North Atlantic at a latitude near 25° N. The eddy was first detected by the movement and temperature of a drogued Argos buoy near 23° W, 26° N in February 1993. By December 1993, the drogued buoy had moved 1000 km westward to 33° W and a winter eddy survey was made from RRS Charles Darwin. The eddy was about 100 km in diameter and had a centre water mass core at a depth of 190 m with characteristic temperature 19.9 °C and salinity 37.06 psu, but was best resolved by properties related to vertical gradient structure (e.g. Brunt-Vaisala frequency, temperature perturbation structure). The eddy was particularly flat with a height-to-width aspect ratio of only ca . 0.14% and a vertical decay scale of ca . 250 m for temperature displacements. Maximum azimuthal currents were ca . 16 cm s -1 at a depth of 175 m and the eddy azimuthal transport was only 3 Sv. However, the potential vorticity in the eddy core was 1.5 x 10 -11 rad ms -1 , comparable with other deep anticyclonic lenses and lower than background values by a factor of ten. The eddy appeared slightly elliptical with a detectable sea surface cooling ( ca . 0.2 °C) over its centre. Three Argos buoys with drogues set at a depth of 200 m were deployed near the centre of the eddy and one buoy continued looping westward for a further ca . 800 km. The drogued Argos buoy results showed that the eddy moved westward at ca . 100 km month -1 and about half this westward rate was attributed to self-propagation. During this ca . 1650 km continuously drogued journey, the eddy only slipped southward by about one degree of latitude and so was perhaps resisting the southward component of flow of the Subtropical Gyre. The buoy results suggested that the central rotation rate increased with time but the eddy became smaller. The initial rotation period of the central core was ca . 8 d, giving a corresponding normalized relative vorticity of —0.3. About 400 d later, the period of core rotation was ca . 5 d, with normalized relative vorticity of —0.5. Some of the increase in central rotation rate was attributed to subduction effects. Variance, spectra and filtered components of velocity were derived from the buoy positions. Kinetic energy levels in the eddy ( ca . 73 cm 2 s -2 ) were markedly elevated with respect to background levels ( ca . 29 cm 2 s -2 ). The core properties of the eddy indicated a winter formation near 27° N, 22° W and hydrographic sections showed that the eddy would be sinking at a rate of about 40 m a -1 . By the second year, the subducted water parcel, the eddy core, was sufficiently deep below the surface of the ocean to escape further effects of winter mixing.