Impact of eddies on ocean diapycnal mixing in Gulf Stream region

Abstract

Based on high-resolution, Array for Real-time Geostrophic Oceanography (Argo) profiles and Sea Level Anomaly (SLA) data, this study statistically analyzes and compares turbulent diapycnal mixing profiles inside and outside mesoscale eddies in the Gulf Stream region. The result indicates that average diapycnal diffusivity at 300–540 m depths in anticyclonic eddies reaches 4.0×10−5 m2 s−1. This is significantly higher than the 1.6×10−5 m2 s−1 outside eddies and 0.8×10−5 m2 s−1 in cyclonic eddies. Probabilities of diapycnal diffusivity greater than 10−4 m2 s−1 within anticyclonic and cyclonic eddies and outside eddies are 29%, 5% and 12%, respectively. However, magnitudes of average diapycnal diffusivity at 540–900 m depths in these three cases are of the same order, 10−5 m2 s−1. Twenty-four of a total 38 anticyclonic eddies had enhanced mixing in the ocean interior, and 22 were observed during or shortly after strong winds. The coincidence between enhanced mixing and strong wind stress indicates that more wind-induced, near-inertial wave energy propagates downward in anticyclonic eddies. The deeper part of 12 profiles (below 540 m) in anticyclonic eddies had vertical overturns with Thorpe scale exceeding 5 m, among which three profiles had overturns reaching 20 m. Enhanced mixing may have occurred in deep layers of some profiles, although it was not evident in average conditions.