A SHALLOW SUBTROPICAL SUBDUCTING WESTWARD PROPAGATING EDDY (SWESTY)

Swesty is a shallow subtropical subducting westward propagating eddy t hat was observed to move across the eastern basin of the North Atlanti c at a latitude near 25 degrees N. The eddy was first detected by the movement and temperature of a drogued Argos buoy near 23 degrees W, 26 degrees N in February 1993. By December 1993, the drogued buoy had mo ved 1000 km westward to 33 degrees W and a winter eddy survey was made from RRS Charles Darwin. The eddy was about 100 km in diameter and ha d a centre water mass core at a depth of 190 m with characteristic tem perature 19.9 degrees C and salinity 37.06 psu, but was best resolved by properties related to vertical gradient structure (e.g. Brunt-Vaisa la frequency, temperature perturbation structure). The eddy was partic ularly 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. Max imum azimuthal currents were ca. 16 cm s(-1) at a depth of 175 m and t he eddy azimuthal transport was only 3 Sv. However, the potential vort icity in the eddy core was 1.5 x 10(-11) rad ms(-1), comparable with o ther deep anticyclonic lenses and lower than background values by a fa ctor of ten. The eddy appeared slightly elliptical with a detectable s ea surface cooling (ca. 0.2 degrees C) over its centre. Three Argos bu oys 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 wes tward at ca. 100 km month(-1) and about half this westward rate was at tributed to self-propagation. During this ca. 1650 km continuously dro gued journey, the eddy only slipped southward by about one degree of l atitude and so was perhaps resisting the southward component of flow o f the Subtropical Gyre. The buoy results suggested that the central ro tation rate increased with time but the eddy became smaller. The initi al rotation period of the central core tvas ca. 8 d, giving a correspo nding normalized relative vorticity of -0.3. About 400 d later, the pe riod 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 ve locity were derived from the buoy positions. Kinetic energy levels in the eddy (ca. 73 cm(2) s(-2)) were markedly elevated with respect to b ackground levels (ca. 29 cm(2) s(-2)). The core properties of the eddy indicated a winter formation near 27 degrees N, 22 degrees W and hydr ographic sections showed that the eddy would be sinking at a rate of a bout 40 m a(-1). By the second year, the subducted water parcel, the e ddy core, was sufficiently deep below the surface of the ocean to esca pe further effects of winter mixing.