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

Abstract

Observed oceanic and atmospheric anomalies in the tropical Pacific are analyzed to understand the symmetric and asymmetric characteristics between El Nino and La Nina evolutions. It is noted that the evolutions are largely symmetric for El Nino and La Nina 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 Nino is stronger than the recharge counterpart associated with La Nina. The symmetric and asymmetric evolution for different phases of El Nino-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 eastern equatorial Pacific is different between El Nino and La Nina 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 to negative, meanwhile the off-equator areas are occupied by positive SSHAs. Such changes on the equator and in the off-equatorial regions make the concavity of the meridional SSHA favorable for growth and persistence of westward surface zonal current anomaly in the equatorial Pacific, then for the ENSO phase transition. During the decay of a cold event, the slow decrease of the negative SSHA on the equator combined with the disappearance of the negative SSHA in the off-equatorial regions makes the meridional concavity reverse easily and is unfavorable for the steady growth of the eastward surface zonal current anomaly and for the phase transition. Thus, the asymmetric evolution of ENSO is associated with both the atmosphere and ocean anomalies on the equator and off the equator.