Southward Shift of Westerly Anomalies Around El Niño Decay Related to High-Frequency Variabilities

Abstract

The southward shift of anomalous westerlies from the equator to the south off-equatorial areas plays a curtail role in demising El Niño, and was attributed to seasonal changes in the large-scale environments in previous studies. Given that the southward shift exhibits a distinct spectral peak at the sub-seasonal timescale, we propose that it could also be caused by the seasonal meridional movement of high-frequency variabilities in the western tropical Pacific, such as the tropical cyclones (TCs) and Madden Julian Oscillation (MJO). It is found that the TCs and MJOs contribute to approximately 39% and 20% of the southward shift from October in El Niño years to the following March, respectively, both of which are significant at the 95% confidence level. Our findings supplement dynamics regarding to El Niño decay, and imply the necessity of improving seasonal forecast of high-frequency variabilities for a better prediction of El Niño.