Testing the trade wind charging mechanism and its influence on ENSO variability

During the positive phase of the North Pacific Oscillation, westerly wind-stress anomalies can extend from the subtropical North Pacific to the western-equatorial Pacific and substantially increase subsurface heat content along the equator. This “trade-wind charging” (TWC) mechanism provides a direct pathway between the extratropical atmospheric-circulations and El Niño Southern Oscillation (ENSO) onset. As previous ocean-only model studies lack ocean-atmospheric coupling which is vital for ENSO initiation, it is crucial to examine whether TWC-induced heat content anomalies can develop into mature ENSO events in a coupled-model setting.

A set of ensemble coupled-model experiments, imposing positive TWC (+TWC) and negative TWC (-TWC) wind-stress forcing, is used to examine the ENSO response to TWC. Anomalous wind-stress are imposed on the ocean from November-April, overlaid on climatological forcing which is applied through July, and then model evolves in a fully-coupled configuration. The +TWC (-TWC) wind anomalies charge (discharge) the equatorial Pacific in spring and generate positive (negative) subsurface temperature anomalies. These subsurface temperature anomalies propagate eastward and upward along the equatorial thermocline and emerge as like-signed sea surface temperature (SST) anomalies in the eastern-equatorial Pacific, creating favorable conditions upon which coupled air-sea feedback can act. During the fully-coupled stage, SST anomalies in +TWC forced runs are amplified by coupled feedback and lead to El Niño events. In contrast, while -TWC forcing results in negative eastern-equatorial Pacific SST anomalies, the model doesn’t consistently develop La Niña. Our results show that the TWC mechanism provides adequate fuel (equatorial heat content) for ENSO development, but the spark (model internal variability) also plays a major role in determining whether an ENSO event will flourish.