The unique 2009-2010 El Niño event: A fast phase transition of warm pool El Niño to La Niña
Abstract
The latest El Niño event in 2009-2010, which can be classified as warm pool El Niño, also known as central Pacific El Niño, holds a unique ground among other warm pool events in that it marks the strongest warming signal in the central Pacific but rapidly decays to strong La Niña in the following year. Generally, warm pool El Niño is known to exhibit moderate SST and subsurface anomalies, which last for a short period of time in a spatially restricted region. It turns out that the discharge process plays a minor role in its fast transition mechanism as does in other warm pool events. Also the meridional oceanic current in the equatorial Pacific reveals that the inflow is only significant at the shallow surface layer for the cold tongue cases, or eastern Pacific El Niño, while no significant basin-wide discharge is evident in the warm pool and 2009-2010 cases. On the other hand, the strong eastward-propagating cold anomaly at the subsurface level is found to be a key factor that distinguishes the 2009-2010 El Niño from other warm pool events. Two possible mechanisms are suggested for the anomaly that resulted in the fast phase transition of the 2009-2010 event: 1) The anomalously warm Indian Ocean (IO) induces a surface easterly anomaly over the western edge of the Pacific, which generates the forced negative Kelvin waves. Although the warm SST anomaly is pronounced in the central Pacific, the substantial warming is also found in the IO sector for the 2009-2010 case. The warm-IO sea surface temperature (SST) produces easterly-wind stress over Indonesia and the western edge of the Pacific, which generates the upwelling Kelvin wave that rapidly terminates El Niño to result in the La Niña state. In general warm pool cases, on the other hand, the equatorial wind anomaly over the western Pacific is slightly westerly, and none of them accompanied clear easterly wind anomaly over the western Pacific. 2) The record-breaking high sea surface temperature in the central Pacific excites a strong Rossby response that is to be reflected as an upwelling Kelvin wave at the western boundary. The westerly anomaly is confined to the west of dateline during its developing phase like other warm pool cases, but expands to the central Pacific during its mature phase where the wind stress forcing is more effective in producing oceanic disturbances. This westerly-wind stress forcing is to excite a positive oceanic Rossby wave which should be reflected as upwelling Kelvin components at the western boundary. The two mechanisms proposed above emphasize the relative importance of the thermocline feedback. The strong subsurface cold anomaly then propagates eastward in the transition period and results in an unusually fast phase transition of the 2009-2010 warm pool El Niño event.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMOS21A1579K
- Keywords:
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- 1620 GLOBAL CHANGE / Climate dynamics;
- 1635 GLOBAL CHANGE / Oceans;
- 4522 OCEANOGRAPHY: PHYSICAL / ENSO