Strengthened ENSO Amplitude Contributed to Regime Shift in the Hadley Circulation

Understanding the variability in the Hadley circulation (HC) changes is crucial for understanding ocean-atmosphere interactions. In this study, the variability in the boreal winter HC in the last 4 decades is explored using multiple reanalyzes and model simulations. The results show that regime shift occurred in the leading mode of HC variability. The primary mode of the recent HC is dominated by an equatorially symmetrical pattern, which was considered the second mode in previous studies. The regime shift in HC variability is mainly due to the El Nino-Southern Oscillation (ENSO), which explains both the spatiotemporal variation and formation of HC variability. Moreover, the abilities of the models to reproduce HC variability is subject to their ability to simulate ENSO variability, suggesting that the ENSO has become a more important modulator of the HC variability in recent decades, and additional research is warranted to evaluate future climate changes and potential effects on the HC. The Hadley circulation (HC) has significant regulatory impacts on tropical and extratropical interactions. As a thermal-dynamical circulation, the variability in the HC exhibits a substantial correlation with the underlying thermal conditions. However, strong interdecadal variations have been observed in the tropical sea surface temperature (SST) around 1976/1977, particularly in the SST related to the El Nino-Southern Oscillation (ENSO). The HC variability after 1977 is still unknown. We investigate the spatial-temporal variation in the boreal winter HC during the period of 1980-2020. The results show that the primary dominant mode (EOF1) of the HC presents an equatorially symmetrical structure with ascension around the equator. This pattern corresponds to the second dominant mode in preceding studies. The different result indicates that a regime shift in HC variability has occurred. We find that the formation of EOF1 is due to the enhanced variation in ENSO, which explains the spatial-temporal variations in EOF1. This result is further confirmed in model simulations, highlighting a more important role of the ENSO on the HC in recent decades. These results deepen our understanding of HC variability and emphasize the important climatic effects of the ENSO, which is of great interest because more severe ENSO events are projected for the future. The principal mode of boreal winter Hadley circulation (HC) during recent 4 decades shows a regime shift and is dominated by an equatorially symmetrical patternThe strengthened El Nino-Southern Oscillation (ENSO) explains the spatiotemporal variation and regime shift of the HC variabilityModel simulations indicate that their ability to reproduce the ENSO variability is a key factor in determining HC variability