Asymmetric Impacts of El Niño Development and Decay Stages on the Hadley Circulation

The impact of El Niño on the Hadley Circulation (HC) has been a topic of previous studies, but the results have been inconclusive. We study how El Niño affects the HC during different stages of its cycle. In development years, the HC anomaly shows an equatorial quasi‐symmetric pattern, while in decay years, it shows an asymmetric pattern. This resolves previous discrepancies among studies about El Niño's impacts on the HC. The differences in tropical sea surface temperature (SST) during different stages of El Niño cause differences in SST meridional gradients, which determine the location of convergence. This explains why the HC anomalies have different spatial structures during El Niño development and decay years. Our results show that the meridional distribution of SST during different El Niño stages has significantly distinct impacts on meridional circulation and clarify the differences in El Niño's effects on climate. Plain Language Summary El Niño is a powerful source of year‐to‐year climate variability, with significant impacts on the Hadley Circulation (HC), one of the most important large‐scale atmospheric circulations affecting precipitation and drought in the tropics and subtropics. It has been reported that the spatial distribution and magnitude of sea surface temperature (SST) anomalies under El Niño events can influence the long‐term variability of HC. However, the impact is still inconclusive. Detailed research on how El Niño events modulate the HC is necessary. We investigate the impacts of the meridional gradient of anomalous SST during El Niño development and decay stages on the spatial structure of the HC. The results indicate that the distinct effects of different stages on the HC are mainly due to their associated SST meridional structures. The results explain why the deduced influences of El Niño on the HC in previous work were different. When considering the impacts of El Niño, the stage of the event must be considered. Furthermore, the physical mechanisms that significantly affect the HC in different stages are explained through theory and data analysis. These results and mechanisms help further our understanding of the climate impacts of El Niño and the variability of the HC. Key Points The discrepancy in previous studies regarding the impacts of El Niño on the HC is caused by the stage shift of El Niño El Niño development and decay stages have distinct impacts on HC anomalies due to their associated spatial distribution of SST anom