The effects of mesoscale ocean-atmosphere coupling on the large-scale ocean circulation

Andrew McC. Hogg¹, William K. Dewar², Pavel Berloff ^3,4, Sergey Kravtsov⁵ and David K. Hutchinson¹

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Department of Oceanography, Florida State University, Tallahassee, USA
3 Woods Hole Oceanographic Institution, USA
4 Cambridge University, UK
5 University of Wisconsin, Milwaukee, USA

Snapshot of wind stress curl (or Ekman pumping) in colour, with contours of streamfunction superimposed. The streamfunction shows a western boundary current separating from the boundary, and forming a meandering zonal jet in the interior. This jet is accompanied by strong sea surface temperature gradients, which interact with the atmospheric boundary layer to produce large values of wind stress curl over the core of the jet. This extra forcing, somewhat paradoxically, acts to reduce the circulation by destabilizing the mean flow.

Recent satellite measurements of wind stress at the ocean-atmosphere interface have pointed to large variations in stress on very fine scales. These fine scales are set by ocean mesoscale dynamics, and the variations in stress occur due to coupled interaction between the ocean and atmosphere. Given that wind stress drives the ocean circulation, there is a realistic possibility for coupled feedback acting to alter ocean currents.

We model this ocean-atmosphere interaction using high-resolution ocean model, coupled to a dynamic atmospheric mixed layer. The goal is to answer the question: Do small-scale variations in stress alter the large-scale ocean circulation?

The model results show that, despite the small spatial scale of the forcing anomalies, mesoscale coupling reduces the strength of the large-scale ocean circulation by approximately 30-40%. This result is due to the highest transient wind stress curl (or Ekman pumping) anomalies (see Fig. 1) destabilising the flow in a dynamically sensitive region close to the western boundary current separation.

These results indicate the complexity involved in migrating ocean models to eddy-resolving scales. The next generation of ocean models need to resolve atmospheric boundary layer processes at the same resolution as the ocean model.