Seasonal locking of the ENSO asymmetry and its influence on the seasonal cycle of the tropical eastern Pacific sea surface temperature

Using the monthly extended reconstructed sea surface temperature (SST) dataset version 3 (ERSST.v3) spanning 1880-2007 and SODA ocean assimilation data spanning 1958-2001, we investigate the seasonality of the asymmetry of the El Nino-Southern Oscillation (ENSO) and its possible impacts on the tropical eastern Pacific annual cycle. Like the amplitude of ENSO, the skewness of ENSO (i.e., asymmetry of ENSO) is locked to the seasonal cycle, showing large amplitude during the winter and small amplitude during the spring except May. Furthermore, the seasonality of the asymmetry of ENSO is changing decade by decade, which is strong during 1930s, 1950s, and 1990s and weak during 1900s, 1910s, 1940s, and 1970s. These decadal changes are significantly correlated to those in the amplitude of the annual and semi-annual cycles of tropical eastern Pacific SST suggesting that the changes in the seasonality of the asymmetry of ENSO may modify the amplitude of the annual and semi-annual cycles of the tropical eastern Pacific SST via a nonlinear process. Using the coupled general circulation model simulations, we also showed similar results to the aforementioned observed features, which overcame some deficiencies due to relatively short reliable records of the observation. (C) 2008 Elsevier B.V. All rights reserved.