Trade Wind Charging of the Equatorial Pacific As a Driver for ENSO Variability

Abstract:

The El Niño/Southern Oscillation (ENSO) imposes changes to regional climates across the globe, with significant physical, biological, and socio-economic impacts. Previous evidence suggests that extratropical North Pacific sea-level pressure (SLP) variations--associated with the southern lobe of the North Pacific Oscillation (NPO)--and accompanying central Pacific trade wind changes can drive ENSO variability by inducing subsurface heat content changes across the equatorial Pacific. Here we present the first process-based confirmation of this "trade wind charging" (TWC) hypothesis for initiating changes in the state of the equatorial Pacific. Both observationally-constrained ocean data and high-resolution numerical model simulations corroborate that NPO-related trade wind variations modify the subsurface equatorial Pacific heat content in a manner that is consistent with wind-stress induced vertically-integrated, meridional mass transport into and out of the equatorial Pacific—i.e. a trade wind charging (and discharging) of the equatorial Pacific. Further, analysis of the numerical and observational results indicates that these NPO-induced trade wind variations and related heat content changes are not part of the ENSO cycle itself but instead represent a separate forcing mechanism of that cycle, initiating the onset of ENSO events which reach maturity approximately 12 months later.