A census of eddies observed in North Atlantic SOFAR float data

Abstract

SOFAR floats that looped in discrete eddies were studied in order to map and describe the distribution and characteristics of eddies in the North Atlantic. One hundred eighteen individual looping float trajectories (loopers) were identified, each consisting of two or more consecutive loops. Each looper was interpreted to be in a discrete eddy, and its characteristics were estimated from the float trajectory. The highest percentage of loopers occured at 700m in the Newfoundland Basin, where roughly half of the float data were in loopers, mostly cyclones. In the Gulf Stream region, approximately 20% of the float data recorded at 700m were in loopers, again mostly cyclones. Overall, 21% of 700m data and 6% of 2000m data were in loopers. The fastest swirl speeds, >40cm s −1, were in cyclones south of the Gulf Stream (most of which were in Gulf Stream rings), but numerous swift ∼35cm s −1 anticyclones were found there too. Swirl velocity decreased with depth, to roughly half as swift at 2000m as at 700m for three eddies in the Sargasso Sea measured simultaneously with floats at these two depths. In the western North Atlantic, the average swirl velocity of cyclones and anticyclones was the same. Translation velocity of eddies was generally westward to southwestward at a few cm s −1. The mean translation velocity of 39 eddies in the Sargasso Sea was u bar = −2.8±0.4 cm s −1, v bar = −0.4±0.4 cm s −1 . Many of these were located just south of the Gulf Stream in the region of its recirculation and were probably advected by a westward current there. Near the Gulf Stream and along its extension in the Newfoundland Basin, eddies were often advected downstream with speeds up to 15–20cm s −1 and eddy trajectories were often complicated. South of 30°N and near the western boundary, 700m eddies were advected northwestward and 1300m and 2000m eddies southeastward by boundary currents there. Numerous energetic anticyclones were observed south of the Gulf Stream; one was tracked for 430 days and its properties well measured. The formation of these eddies has not been documented, but they are inferred to have formed near and by the Gulf Stream and to consist of a thick layer of 18°C water lying above a depression in the thermocline. Analogous anticyclones were observed in the Newfoundland Basin seaward of the Gulf Stream extension there.