Asymmetric evolution of El Niño and La Niña: the recharge/discharge processes and role of the off-equatorial sea surface height anomaly Climate Dynamics

Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the sym-metric and asymmetric characteristics between El Niño and La Niña evolutions. It is noted that the evolutions are largely symmetric for El Niño and La Niña prior to and in their mature phase, but become asymmetric afterwards. It is further demonstrated that this asymmetry is due to the fact that, on average, the discharge process associated with El Niño is stronger than the recharge counterpart associated with La Niña. The symmetric and asymmetric evolution for different phases of El Niño-Southern Oscillation (ENSO) is consistent with the associated recharge and discharge processes that are linked with the evolution of anomalous ocean surface current and gradient of sea surface height anomalies (SSHAs). It is suggested that the evolution of the meridional gradient of SSHAs in the central and east-ern equatorial Pacific is different between El Niño and La Niña years due to relatively different rates of the SSHA changes on and off the equator. During the decay of a warm event, the equatorial SSHA quickly switches from positive * Zeng-Zhen Hu [e-mail] 1 Climate Prediction Center, NCEP/NWS/NOAA, 5830 University Research Court, College Park, MD 20740, USA 2 Department of Atmospheric, Oceanic, and Earth Sciences, Center for Ocean-Land- Atmosphere Studies, George Mason Grant no. NA09OAR4310058