![]() The resulting zonal gradient of sea surface temperature (SST), in turn, drives the equatorial easterly winds that gather water vapor to fuel intense atmospheric deep convection over the western Pacific warm pool. The equatorial upwelling forced by these easterly winds induces stronger surface cooling in the eastern Pacific because of the shallower thermocline. In the equatorial Pacific, prevailing easterly winds accumulate warm surface water to the west, lifting the thermocline in the east. Seasonally stratified composite analysis suggests that the eastward displacement of atmospheric deep convection anomalies during El Niño enables surface winds in the western equatorial Pacific to be more affected by remote forcing from the Indian Ocean, which acts to terminate the Pacific events. Throughout the analysis period, this asymmetric feature is evident for strong events in which Niño-3.4 SST anomalies exceed one standard deviation in December. After the mature phase, El Niños tend to decay rapidly by next summer, but many La Niñas persist through the following year and often reintensify in the subsequent winter. ![]() Most El Niños and La Niñas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. The associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña because of the nonlinear atmospheric response to SST. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Niño compared to La Niña, resulting in positive skewness of interannual SST variations. The net easterly surface wind anomaly over the far western Pacific during La Niña is hypothesized to prolong the event.Įl Niño and La Niña are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. The lack of balance of the surface wind anomalies over the far western Pacific during La Niña results from the westward shift of the Pacific precipitation anomaly because of the nonlinear dependence of atmospheric deep convection on the absolute SST. In the far western Pacific, the easterly (westerly) surface wind anomaly induced by the atmospheric Kelvin wave response to tropospheric heating (cooling) over the Indian Ocean balances (fails to balance) the westerly (easterly) surface wind anomaly forced by precipitation anomalies over the Pacific during El Niño (La Niña). Red (blue) arrows indicate surface wind anomalies forced by Pacific SST anomalies during El Niño (La Niña) black arrows indicate surface wind anomalies forced by Indian Ocean SST anomalies. Pink (blue) shaded ellipses depict positive (negative) SST anomalies over the Pacific and Indian Ocean during El Niño (La Niña). Schematic diagram illustrating the hypothesized cause of the asymmetric wind response in the far western equatorial Pacific between (top) El Niño and (bottom) La Niña during the mature phase.
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