Strengthening of the equatorial Pacific upper-ocean circulation over the
past three decades
Abstract
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 ~20% and
an acceleration of poleward flow north (south) of the equator by
~60% (~20%). Additional moored
velocity data reveal a deepening of the EUC core at 170°W and
significant shoaling at 140°W and 110°W, but no significant changes in
EUC core velocity. The strongest subsurface zonal velocity trends are
observed above the EUC core 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°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 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 Niño Southern
Oscillation and other modes of natural variability on decadal to
multidecadal time scales.