The annual cycle of surface eddy kinetic energy and its influence on eddy momentum fluxes as inferred from altimeter data

The annual cycle of surface eddy kinetic energy (EKE) and its influence on eddy momentum fluxes are investigated using an updated record of satellite altimeter data. It is found that there is a phase difference between the annual cycles of EKE in the western boundary current regions and EKE in the interior of the subtropical gyres, suggesting that different mechanisms may be at work in different parts of the subtropical gyres. The annual cycles of EKE averaged in the two hemispheres are found to be of similar magnitude but in opposite phase. As a result, the globally-averaged EKE shows little seasonal variability. The longer record of altimeter data used in this study has brought out a clearer and simpler picture of eddy momentum fluxes in the Gulf Stream and Kuroshio Extensions: eddies in both regions systematically flux westerly momentum into the western boundary current jets. Considerable seasonal variations in eddy momentum fluxes are found in the western boundary current regions, which potentially play an important role in modulating the strength of the western boundary currents and their associated recirculation gyres on the seasonal time scale.