Global Contributions of Mesoscale Ocean Dynamics to Meridional Heat Transport

In previous studies of the effects of ocean eddies on meridional heat transport (MHT), definitions of the eddy contribution to MHT vary considerably. If the eddy heat transport is defined only based on deviations from a temporal or spatial mean, the contributions to MHT of large basin-scale gyres are conflated with those of mesoscale eddies. Here we quantify the contribution of mesoscale ocean dynamics to MHT globally, using spatial filters that distinguish between mesoscale, gyre-scale, and overturning (zonal-mean) processes. This method is applied to 32 years of output from an eddy-permitting forced ocean model simulation to study both time-mean and interannually-varying contributions to MHT. The mesoscale MHT contribution is generally found to be small relative to the overturning contribution, but with significant spatial variations that impact regional ocean heat content. The largest mesoscale contribution to time-mean MHT at any latitude is at 40°S, where ~0.6 PW is transported poleward mostly in the Agulhas Return Current and Brazil-Malvinas Confluence regions. In the Atlantic and Indo-Pacific basins, mesoscale contributions to MHT are associated with three types of processes: (1) western boundary current (WBC) recirculations, (2) eddies near WBC extensions, and (3) tropical instability waves. WBC recirculations and eddies account for nearly all of the mesoscale MHT contributions at 30°-45°N latitude, while tropical instability waves and WBC recirculations account for elevated mesoscale MHT equatorward of 15° latitude. Despite high levels of mesoscale eddy activity in some subtropical basins (15°-30° latitude), the impacts of these eddies on basin-integrated MHT appear to be negligible. Further analyses of mesoscale MHT variability show the dependence of mesoscale fluxes on meridional temperature gradients and current geometry, informing future observational campaigns and lateral mixing parameterizations for coarse-resolution ocean models.