Reconciling Roles of External Forcing and Internal Variability in Indian Ocean Decadal Variability Since 1920

On decadal time scales, Indian Ocean sea surface temperatures (SSTs) exhibit coherent basin-wide changes, but their origins are not well understood. Here we analyze observations and model simulations from Coupled Model Intercomparison Project Phase 6 and Community Earth System Model Version 1 to quantify the roles of external forcing and internal climate variability in causing Indian Ocean decadal SST variations. Results show that both external forcing and internal variability since 1920 have contributed to the observed decadal variations in linearly detrended Indian Ocean SSTs, and they exhibit an out-of-phase relationship since the 1950s. The internally-generated variations arise from remote influences from the tropical Pacific and possible contributions from internal local processes, while the influence from the Atlantic Multidecadal Oscillation is opposite to that of the Interdecadal Pacific Oscillation. Decadal SST changes caused by nonlinear variations in greenhouse gases and aerosols are roughly out-of-phase with the internal variability, thus dampening observed SST variations since the 1950s.