Dynamic Causes of ENSO Decay and Its Asymmetry

Abstract

El Niño and La Niña exhibit asymmetric evolution characteristics during their decay phases. The decay speed of El Niño is significantly greater than that of La Niña. This study systematically and quantitatively investigates the relative contributions of the equatorial western Pacific (WP) and central-eastern Pacific (CEP) wind stress anomalies to ENSO decay and its asymmetry through data analysis, numerical experiments, and dynamic and thermodynamic diagnoses. It is demonstrated that the sea surface temperature anomalies (SSTAs) forced by the wind stress anomalies in the equatorial CEP play a dominant role in ENSO decay and contribute to ENSO decay asymmetry, while the forcing by the equatorial WP wind stress anomalies has a small contribution. Diagnoses of the oceanic mixed layer heat budget indicate that anomalous zonal advection term and vertical advection term forced by the wind stress anomalies in the equatorial CEP are the most important dynamic terms contributed to ENSO decay. Both terms in El Niño decay phase are much larger than in La Niña decay phase, resulting in a larger decay speed in El Niño than in La Niña. The contributions of these two terms do not depend on the equatorial WP wind field, confirming that the equatorial WP wind stress anomalies do not act as a pivotal part in ENSO asymmetric decay. Moreover, it is demonstrated that within the equatorial CEP, dominant contribution comes from the wind stress anomalies in the equatorial central Pacific, in which those in the equatorial southern central Pacific play a major role.