Strengthening of the Equatorial Pacific Upper-Ocean Circulation Over the Past Three Decades

Thirty years (1993-2022) of concurrent satellite and in-situ observations show a long-term strengthening of the equatorial Pacific upper-ocean circulation. Enhanced southeasterly and cross-equatorial winds have caused an annual mean, basin-wide acceleration of the equatorial westward near-surface currents by similar to 20% and an acceleration of poleward flow north (south) of the equator by similar to 60% (similar to 20%). Additional moored velocity data reveal a deepening of the Equatorial Undercurrent (EUC) core at 170 degrees W and significant shoaling at 140 degrees W and 110 degrees W, but no significant changes in EUC core velocity. The strongest subsurface zonal velocity trends are observed above the EUC core in the central to eastern Pacific and occur before and after the seasonal maximum of EUC core velocity, causing enhanced upper-ocean vertical current shear. Consistent with trends of the 20 degrees C isotherm depth along the equatorial Pacific, a significant basin-wide steepening of the equatorial thermocline is observed. Both the accelerating equatorial current system and the enhanced thermocline slope are consistent with an observed steepening of the zonal sea surface height gradient due to increased wind-driven westward mass transport at the surface. During February-March, both surface and subsurface currents along the equator show eastward velocity trends, in contrast to westward near-surface current trends during the remainder of the year. The trend reversal is attributed to both a long-term shift in equatorial Kelvin wave activity and to the impact of strong interannual variability due to El Ni & ntilde;o Southern Oscillation and other modes of natural variability on decadal to multidecadal time scales.