Dr. Tao Zhang

NOAA-Earth System Research Laboratory (ESRL), USA

No. 319, Building 40, IAP

10:00, 16 July 2014

Abstract

　　The two phases of ENSO—El Nino and La Nina—are not mirror images of each other: the strongest El Nino is stronger than the strongest La Nina, a fact that has been referred as ENSO asymmetry. Understanding the asymmetry may hold the key to understand decadal variability.

　　In this study, the El Nino–La Nina asymmetry in 14 CMIP5 models is evaluated. The results show that an underestimate of ENSO asymmetry, a common problem noted in CMIP3 models, still remains there. The weaker ENSO asymmetry primarily results from a weaker SST warm anomaly over the eastern Pacific and a westward shift of the center of the anomaly. In contrast, SST anomalies for the La Nina phase are close to observations. Corresponding AMIP runs are analyzed and reveal that during the warm phase, precipitation anomalies are weaker over the eastern Pacific, and westerly wind anomalies are confined more to the west in most models. The time-mean zonal winds are stronger over the central and eastern Pacific for most models. Wind-forced ocean GCM experiments suggest that the stronger time-mean zonal winds and weaker asymmetry in the interannual anomalies of the zonal winds in AMIP models can both be a contributing factor to a weaker ENSO asymmetry, but the former appears to be more fundamental.

　　The study suggests part of atmospheric origin of the underestimate of ENSO asymmetry.