Thursday, 27 January 2011: 11:15 AM
608 (Washington State Convention Center)
El Niño-Southern Oscillation (ENSO) is a dominant mode of natural oscillation of the equatorial coupled atmosphere-ocean system in the Pacific. The question of whether the equatorial Pacific responds to radiative heating in a La Niña-like (cold-ENSO) pattern or an El Niño-like (warm-ENSO) pattern is under debate in the context of global warming (see Vecchi et al.  ). It has been argued that because of the tight coupling of the atmospheric Walker circulation with the thermocline depth in the eastern equatorial Pacific Ocean, the response to a larger radiative heating may not necessarily be a warmer sea-surface temperature (SST). There are currently two competing theories, differing in the degree with which the atmosphere is coupled to the ocean. Clement et al.  presumed that the eastern Pacific SST is controlled by ocean cold water upwelling and therefore a basin-wide heating increases only the SST in the western Pacific. The resulting east-west temperature gradient strengthens the atmospheric Walker circulation, whose easterly flow near the surface induces stronger ocean upwelling in the eastern Pacific, thus a cold-ENSO-like response. On the other hand, Held and Soden  and Vecchi and Soden  suggested that tropical circulations, especially zonal overturning circulations (such as the Walker Circulation) would weaken in a warmer climate. The weakened surface easterlies lead to an El Niño (warm-ENSO)-like SST, of a warm tongue in the eastern Pacific. Held and Soden  pointed out that this is a robust response of the current crop of coupled atmosphere-ocean general circulation models: As the SST warms, convection actually decreases, because the lower tropospheric water vapor increases faster than the global mean precipitation. Xie et al.  suggested that the warming pattern should be less El Niño-like because of the strengthened southeasterlies south of the equator, due to hemispheric asymmetry in land-sea area (Liu et al.  ). Observational evidence is ambiguous. Vecchi et al.  showed that the trend in 1880-2005 has a cold-ENSO-like pattern in one dataset (HadISST) but a warm-ENSO-like pattern with asymmetry in another (ERSST). Karnauskas et al.  found the zonal SST gradient strengthened in boreal fall but weakened in spring. We hope to reconcile these disparate results in the present study.
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