Seasonality of the SST-precipitation relationship over the tropical North Atlantic and possible mechanisms

Based on the monthly Sea Surface Temperature (SST) data from the Met Office Hadley Centre, the Global Precipitation Climatology Project monthly precipitation data, and the historical simulations from Coupled Model Intercomparison Project Phase 6 (CMIP6) climate models, the present work investigates the seasonality of the SST-precipitation relationship over the tropical North Atlantic and possible role of ENSO (El Niño-Southern Oscillation). It is found that the relationship of SST anomalies in the tropical North Atlantic with local precipitation exhibits a remarkable seasonality. During spring and summer, there is a significant positive correlation between SST and precipitation in this region, indicating a strong local ocean-atmosphere coupling. In contrast, in autumn and winter, the ocean-atmosphere coupling weakens significantly, and almost no significant precipitation response to SST is detected. Further analysis reveals that this seasonality is mainly associated with the seasonal cycle of the background SST and local SST variability in the tropical North Atlantic. Despite cooler background SST in spring, the strong SST variability during this season makes SST easy to exceed the convection threshold and thus induces precipitation anomalies. In summer, the warm background SST favors the enhanced local ocean-atmosphere coupling. The relatively weak SST variability in autumn weakens the local precipitation response, despite a relatively warm SST background. The cooler background SST in winter results in a weak ocean-atmosphere coupling. ENSO has a significant influence on spring SST and precipitation anomalies in the tropical North Atlantic. As a result, the strong local SST anomalies in spring are more likely to actively trigger local convective responses under ENSO forcing. However, in other seasons, the impact of ENSO on SST anomaly in the tropical North Atlantic is relatively small, and thus there is almost no difference in local ocean-atmosphere coupling in the tropical North Atlantic with or without ENSO SST forcing. These findings emphasize the critical role of spring and summer tropical North Atlantic SST anomalies in local convection and associated climate impacts, which is important for short-term climate prediction related to the tropical North Atlantic SST.