Statistics of vertical vorticity, divergence, and strain in a developed submesoscale turbulence field

Abstract

A detailed view of upper ocean vorticity, divergence, and strain statistics was obtained by a two-vessel survey in the North Atlantic Mode Water region in winter 2012. Synchronous Acoustic Doppler Current Profiler sampling provided the first in situ estimates of the full velocity gradient tensor at O(1 km) scale without the usual mix of spatial and temporal aliasing. The observed vorticity distribution in the mixed layer was markedly asymmetric (skewness 2.5), with sparse strands of strong cyclonic vorticity embedded in a weak, predominantly anticyclonic background. Skewness of the vorticity distribution decreased linearly with depth, disappearing completely in the pycnocline. Statistics of divergence and strain rate generally followed the normal and χ distributions, respectively. These observations confirm a high-resolution numerical model prediction for the structure of the active submesoscale turbulence field in this area. Key Points Vorticity, divergence, and strain of submesoscale turbulence observed directly Distribution of vorticity in the upper ocean is highly asymmetric in winter Observed statistics of active submesoscale turbulence confirm model predictions ©2013. American Geophysical Union. All Rights Reserved.