Global surface and subsurface geostrophic currents from multi-mission satellite altimetry and hydrographic data, 1996-2011

Climate indices have been shown to be correlated with changes of absolute ocean current velocities. Yet there has been a lack of available estimates of accurate surface and subsurface current velocities with adequate data span to afford a detailed study. Here, we combined multiple mission satellite altimetry along-track sea surface heights (SSHs), the Gravity field and steady-state Ocean Circulation Explorer (GOCE) time-wise solution generated geoid model, and in situ hydrographic data, to estimate global surface and subsurface absolute geostrophic currents, 1996-2011. We used the profile approach to process satellite altimetry data, mitigating the negative impact of omission errors resulting from the spatial resolution discrepancies between the truncated GOCE geoid model and SSHs, on the estimation of the absolute dynamic topography (ADT), which was then combined with the relative dynamic topography derived from in situ hydrographic profiles to estimate near global mesoscale geostrophic current velocities at different depth layers. Results were validated by in situ moored current meter observations from the Tropical Atmosphere Ocean/TRIangle Trans-Ocean buoy Network (TAO/TRITON) and the Prediction and Research Moored Array in the Atlantic (PIRATA), showing the outperformance of profile approach over the conventional pointwise approach in determination of geostrophic currents. After validating the subsurface geostrophic currents with in situ observations from the Kuroshio Extension System Study (KESS) and Line-W projects, statistically significant correlation, between the multi-layer geostrophic current changes for Atlantic Meridional Overturning Circulation (AMOC) branches and the North Atlantic Oscillation (NAO) index, was found, which is in general agreement with other published studies.