Testing the trade wind charging mechanism and its influence on ENSO variability.
Abstract
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 and substantially increase subsurface heat content along the equator. Previous studies suggest that this "trade-wind charging" (TWC) mechanism provides a direct and robust pathway between extratropical atmospheric circulations and El Niño Southern Oscillation (ENSO). We conducted a set of ensemble coupled-model experiments to examine the ENSO response to TWC-favorable wind-stress (+TWC). The anomalous forcing were imposed on ocean in thermally and mechanically decoupled mode through November-April, then climatological forcing is applied until the end of July. In 1 st of August we remove all constrain and let model to freely evolve in a fully-coupled configuration through following winter. The same experiments were repeated with negative TWC forcing (-TWC). The +TWC (-TWC) wind-stress anomalies consistently charged (discharge) the equatorial Pacific in boreal spring and generated significant positive (negative) subsurface temperature anomalies. Under climatological forcing, the subsurface temperature anomalies propagated eastward and upward along the zonally-tilted equatorial thermocline, and emerged as like-signed SST anomalies in the eastern Pacific creating favorable conditions upon which coupled feedbacks can act. During the fully-coupled stage, the surface anomalies in the +TWC forced runs were amplified by coupled air-sea feedbacks and on average lead to an El Niño event. In contrast, while the -TWC forcing resulted in negative eastern Pacific SST anomalies, but these didn't develop into La Niña-type cooling, emphasizing the asymmetric-nature of ENSO's evolution. The sensitivity experiments highlight that TWC mechanism provide adequate fuel for ENSO development, but the spark (model internal variability) played a large role in determining whether the subsequent ENSO events develop and grow.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMOS52B..07C
- Keywords:
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- 4215 Climate and interannual variability;
- OCEANOGRAPHY: GENERAL;
- 4231 Equatorial oceanography;
- OCEANOGRAPHY: GENERAL;
- 4522 ENSO;
- OCEANOGRAPHY: PHYSICAL;
- 4922 El Nino;
- PALEOCEANOGRAPHY