Small-scale structures in the north-west Atlantic sub-tropical front

Abstract

Mixing in the ocean thermocline is thought to occur in part by random superposition of internal waves, leading to occasional episodes of shear instability1. Statistical models of the resulting mixing have been formulated2,3 which are roughly consistent with the few available direct estimates of average mixing rates4–6. However, clear evidence of the direct association between individual mixing patches and wave instability has been lacking7. Here we report the use of a new fine- and microstructure profiler during the Frontal Air–Sea Interaction Experiment (FASINEX)8 to investigate mixing processes in the sub-tropical front of the north-west Atlantic in great detail. The novel combination of fine-scale and microstructure sensors on one vehicle provided sufficient resol-ution to reveal quantitative evidence of shear instability in the thermocline. The strongest sub-surface mixing was found in association with small-vertical-scale internal waves that may have been influenced by the velocity and density structure of the front.