The Impact of Climate Change on Submesoscale Instabilities in the Surface Ocean

Abstract:

Future global temperature rise is projected to influence mixed layer depths with the potential for significant changes in ocean dynamics at the submesoscale. The implications of such physical changes on ocean heat fluxes and biogeochemical cycling are unclear. Here we present results from an analysis of a submesoscale-resolving regional model, for a relatively quiescent, open-ocean region in the North Atlantic. Warming conditions produce a clear reduction in the depth of the wintertime mixed layer and a reduction in submesoscale activity. Conditions that are unstable at the submesoscale are identified using the balanced Richardson number, and results highlight how projected changes may impact the type, frequency and seasonal cycle of submesoscale instabilities. A scaling for the restratifying effect of baroclinic instabilities in the mixed layer (MLI) captures much of the domain-averaged seasonal and future changes in vertical buoyancy flux, where spatial variations in lateral buoyancy gradients are shown to dominate the spatial variations in this scaling. The relative contributions from frontogenesis and MLI to the seasonal cycle of lateral buoyancy gradients will also be presented. Model output is consistent with with available observations for the study region. Particular focus is on the projected changes in the vertical component of velocity and the implications for mixing in the surface ocean, ocean heat fluxes and biogeochemical cycles.