Mechanisms of Enhanced Historical Indian Ocean Warming: Insights from the CESM1 Large-Ensemble Coupled Simulations

The tropical Indian Ocean (TIO) has experienced enhanced surface warming relative to the tropical mean during the past century, a phenomenon sometimes called TIO relative or enhanced warming. But its underlying mechanisms remain unclear. In this study, we investigate the physical mechanisms of the enhanced historical TIO warming using the Community Earth System Model version 1 (CESM1) large-ensemble (LE) coupled climate model simulations. We find that historical changes in radiative forcing can give rise to an enhanced TIO warming, while internal climate variability may also play a role. Using CESM1 LE all-but-one-forcing experiments, we further demonstrate that changes of biomass burning (BMB) aerosols act as the primary forcing factor, as compared to the changes of greenhouse gases or industrial aerosols, for the historical TIO relative warming. Although the BMB aerosol changes have little effect on global mean temperatures due to strong regional cancellation, they significantly influence the pattern of warming over the tropical oceans. In particular, the long-term reduction of BMB aerosols over the TIO, mainly emitted from the Indian subcontinent, leads to an increase of shortwave radiative flux reaching the surface and thus induces a TIO warming, further amplified by surface wind speed reduction. In contrast, the increase of BMB aerosols over South America and Africa causes a cooling of the tropical Pacific and Atlantic, respectively. The resultant TIO relative warming leads to prominent local oceanic and atmospheric changes, including a westward expanded Indo-Pacific warm pool and a fresher TIO due to enhanced rainfall, and global climate changes via atmospheric teleconnections.