资源环境科技发展态势分析平台

La Nina years tend to provide increased Indian summer monsoon (ISM) rainfall. However, observations show 6-8% reduction in ISM rainfall during post-1980 La Ninas relative to pre-1980. Using a suite of atmospheric general circulation model experiments, we replicate this observed phenomenon and attribute it to a combination of weakening La Nina events themselves plus strongly warming tropical Indian Ocean. We demonstrate that half of the ISM rainfall reduction during post-1980 La Ninas can be attributed to changes in the spatial pattern and intensity of La Nina within the tropical Pacific Ocean. Warmer eastern-equatorial Pacific Ocean temperatures during post-1980 La Ninas weaken the Walker circulation, resulting in large-scale anomalous subsidence over the Indian subcontinent, thereby inhibiting the deep convection that drives ISM rainfall. Further, we demonstrate the declining central ISM rainfall during La Nina years with increasing tropical Indian Ocean warming, which has several serious concerns for regional water resources and stability.