Effect of the Tropical Sea Surface Temperature on the Interannual Rainfall Variability and Its Mechanism over the Amazon in the Dry Season

Using the PLS (partial least square) regression method, the leading tropical Sea Surface Temperature (SST) modes that affect the interannual rainfall variability over the Amazon in the dry season (June–August, JJA) in 1979–2018 were analyzed. The first SST mode features a decaying La Niña-like cooling in the tropical eastern Pacific from the preceding rainy season (December–February, DJF) to the dry season (JJA), explaining 64% of the total variance of the rainfall. Anomalous cooling appears in the tropical eastern Pacific in DJF and develops in March–May (MAM). Meanwhile, the tropical Indian Ocean and North Atlantic also become cooler and the tropical south Atlantic becomes warmer. The cooling in the Pacific decreases in JJA, but the anomalous SST still exists in other oceans. Finally, all anomalous SSTs decay in September–November (SON). The second SST mode exhibits anomalous warming in the central Pacific from the preceding rainy season to the dry season, explaining 19% of the total variance. There is anomalous warming in the tropical central Pacific, south Atlantic, and the Indian Ocean, which exists from DJF to SON in the Pacific and Atlantic but decays during SON in the Indian Ocean. This suggests that the interannual rainfall variability over the Amazon in the dry season is related to the evolution of the tropical SST. The evolution of La Niña (Modoki El Niño) starting from the preceding DJF, the anomalous negative temperature gradient between the tropical North Atlantic and South Atlantic, and cooling (warming) in the tropical Indian Ocean in March–August all lead to increased rainfall over the Amazon. The two SST modes’ contribution is closely related to the rainfall index, with their correlation coefficient reaching 0.92. In addition, the contributions have experienced interdecadal changes with a considerable decline during 1979–2018. Moreover, this study investigated the mechanism of the leading SST modes affecting the interannual rainfall variability over the Amazon in the dry season, which indicates that the SST modes are critical in the anomalous circulation, moisture transport, and troposphere stability, resulting in rainfall changes. The first SST mode triggers a convergence in the lower troposphere and divergence in the upper troposphere in the northern Amazon, inducing an anomalous upward motion. The moist static energy budget also suggests considerable tropospheric instability results from the first SST mode. Besides, the first SST mode causes an anomalous vapor convergence. These all favor more rainfall in the northern Amazon. The second SST mode causes upward movements in southeastern Amazon and downward movements in the west. The moist static energy budget suggests that the troposphere becomes more stable in the middle Amazon and opposite in the southeast, inducing increasing rainfall in the eastern Amazon. Finally, the ensemble-averaged data of seven models from the Atmospheric Model Intercomparison Project (AMIP6) were used to verify the above conclusions. The results show that not only the SST modes but also the mechanism is highly consistent with previous studies. This indicates that the Amazon rainfall in the dry season is definitely highly correlated with the tropical SST.