Asymmetry in the duration of El Niño and La Niña

El Niño and La Niña are not a simple mirror image but display significant differences in their spatial structure and seasonal evolution. 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 associated atmospheric deep convection anomalies are displaced eastward during El Niño compared to La Niña due to the nonlinear atmospheric response to SST. In addition to these well-known features, our analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Niño and La Niña. Most El Niños and La Niñas develop in late boreal spring-summer, when the climatological cold tongue is intensifying, and peak near the end of the calendar year. After the mature phase, El Niños tend to decay rapidly by next summer but many La Niñas persist through the following year and often re-intensify in winter. Throughout the analysis period, this asymmetric feature is evident for strong events, in which NINO3.4 SST anomalies exceed one standard deviation in December. 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.