A quantitative explanation for the different 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. And it is shown a transition in the variability of the first two modes of the HC during these stages, with the first mode exhibiting a larger explained variance in the decaying stage. The regime change in HC variability corresponds to underlying anomalous SST distributions, as confirmed by sensitive experiments. The differences in tropical 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. Quantitative assessment reveals stronger HC-SST response amplitudes during the decaying stage compared to the development stage. Employing the Kuo-Eliassen (KE) equation, diabatic heating anomalies during the decaying stage explain the difference in air-sea response intensity between the two stages. Diabatic heating variations are identified as the primary contributor to amplification or reduction of air-sea response intensity during the respective El Niño stages, providing insights into the different air-sea processes throughout the El Niño lifespan. 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.