Seasonal-varying characteristics of tropical Pacific westerly wind bursts during El Niño due to annual cycle modulation.

Westerly wind bursts (WWBs), aperiodic burst-like atmospheric disturbances in the tropics, play an important role during the development stage of El Niño–Southern Oscillation (ENSO) events and in turn can be strongly modulated by ENSO itself. Previous work argued that WWBs exhibit a pronounced seasonality at the equator during an El Niño evolution, i.e., being more frequent and vigorous during the developing boreal autumn compared to the decaying spring. Here we show that this seasonal difference in WWB activity at the equator is a segmentary manifestation of a meridional migration of WWB activity, which is closely linked to the southward displacement of the El Niño-related deep convection from the developing autumn to decaying spring. The highest climatological SSTs over the western-central Pacific are located mainly to the north of the equator in boreal autumn and south of it during boreal spring; hence the equatorially quasi-symmetric El Niño-related SST anomalies tend to induce deep convection and generate more WWBs in the Northern Hemisphere during the developing autumn and more in the Southern Hemisphere during the decaying spring. Despite some differences in the zonal distribution and intensity, there appears a close resemblance between the seasonal-varying features of WWBs during eastern Pacific (EP) and central Pacific (CP) El Niño events regarding the north–south displacement, due to similar meridional shifts of deep convection anomalies. This study provides a more comprehensive picture of the spatiotemporal characteristics of WWBs during El Niño events and improves our understanding of the relationship between ENSO and high-frequency atmospheric variability. [ABSTRACT FROM AUTHOR]