Human-caused expansion of the Indo-Pacific warm pool

The Indo-Pacific Warm Pool (IPWP) is Earth’s largest region of warm sea surface temperatures (SSTs) and is fundamental to global atmospheric circulation and hydrological cycle. Therefore attributing changes in the IPWP has important implications for changes in monsoonal rainfall and  tropical storms, which have profound socioeconomic consequences. During the past 60 years, the IPWP has warmed by 0.3 degree and expanded about 32% compared to the climatology. Previous studies have considered mechanisms for the basin-scale ocean warming, but not the causes of the observed IPWP expansion, in which expansion in the Indian Ocean has far exceeded that in the Pacific Ocean. In this study, we indentify anthropogenic and natural contributions to the observed IPWP changes since the 1950s by comparing observations with multiple climate model simulations using an optimal fingerprinting technique. Results indicate that greenhouse gas forcing is the dominant cause of the observed increases in IPWP intensity and size, whereas natural fluctuations associated with the Pacific Decadal Oscillation (PDO) have played a smaller yet significant role, contributing to the IPWP growth (about 12-18%) during the analysis period. It is further demonstrated that the shape and impact of human-induced IPWP growth could be asymmetric between the Indian and Pacific basins, the causes of which remain uncertain.