Interannual variability of observed surface divergent meridional eddy heat fluxes in the Kuroshio Extension and Gulf Stream

Variability in meridional ocean heat transport is fundamental to global climate change. Eddies make up a large contribution of the poleward transport of heat within the ocean interior. Most attempts to estimate the global distribution of meridional eddy heat fluxes (MEHFs) have been model-based. Here we use remote sensing observations of sea surface height (SSH) and sea surface temperature (SST) to estimate surface MEHFs from 1993-2017. The reconstructed MEHFs show that Western Boundary Currents (WBCs), such as the Kuroshio Extension (KE) and Gulf Stream (GS) in the northern hemisphere, significantly contribute to poleward transport because of strong regional variability in SSH and SST. Prevalent meanders and eddies induced by instabilities allow cross-frontal transport of heat. However, the meandering features in these currents make the MEHFs unclear where prominent rotational wave-like structures appear in the fluxes and do not contribute to the total integrated flux. A Helmholtz decomposition is performed on the global estimates of eddy heat flux to remove this non-dynamic phenomenon and to extract long-term variability of divergent eddy heat fluxes. The dynamical divergent eddy heat flux is largest near the jet separation from coast in both the KE and GS. The baroclinic conversion rates associated with these dynamical fluxes are positive and down-gradient, suggesting a conversion of mean to eddy potential energy. The long-term (25 years) observations of global surface divergent eddy heat fluxes make it possible to study times scales of variability across different scales in the WBCs from mesoscale to decadal. Differences between summer time and winter time indicates seasonality of eddy heat flux with stronger fluxes during the wintertime. Results of interannual and longer temporal periods show that the surface divergent MEHFs in the KE and GS have been increasing in amplitude over the past decade.