A quantitative explanation for the large impacts of El Niño during its decaying stage

This study examines the comparative atmospheric circulation and tropical sea surface temperature (SST) relationships during the developing and decaying stages of El Niño from a meridional structure standpoint. Results indicate a transition in the variability of the first two modes of the Hadley Circulation (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. Quantitative assessment reveals the HC-SST response amplitudes are approximately two times stronger during the decaying stage compared to the developing 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.