Influence of boreal spring sea surface temperature anomalies over the tropical South Atlantic on the Meiyu onset

Abstract

This study investigates the impact of boreal spring tropical South Atlantic surface sea temperature anomalies (TSA-SSTA) on the anticyclone over the western North Pacific (WNPAC) and the Meiyu onset date (MOD) based on reanalysis data and numerical experiments. The results indicate an intimate linkage between the MOD and TSA-SSTA, in which warmer spring TSA-SSTA are associated with an earlier MOD and vice versa, and the underlying mechanism is identified. Warm TSA-SSTA can trigger a Gill-type response and anomalous equatorial Walker circulation, which leads to anomalous upward motion and latent heating over the Maritime Continent. This anomalous condition over the Maritime Continent strengthens local Hadley circulation accordingly accompanied by anomalous descending motion over the western North Pacific. This descending motion reduces the local rainfall and enhances the equatorward northerly wind at low level. Further analysis reveals that local Sverdrup positive feedback between the anomalous diabatic cooling owing to reduced rainfall and the lower-level equatorward northerly wind is critical for sustaining the well-developed anomalous WNPAC. The abundant water vapor transport embedded in the northwestern flank of the anomalous WNPAC eventually favors an earlier MOD. Atmospheric conditions corresponding to cold TSA-SSTA produce the opposite effect. The spring TSA-SSTA can therefore prominently communicate with the subsequent East Asian MOD via the aforementioned mechanism, and the spring TSA-SSTA can be interpreted as a precursor signal of the East Asian MOD.