Inhomogeneous warming of the Tropical Indian Ocean in the CMIP5 model simulations during 1900–2005 and associated mechanisms

Abstract

The characteristics and causes of inhomogeneous warming of the Tropical Indian Ocean (TIO) sea surface temperature during 1900–2005 are investigated based on observations and 16 Coupled Model Intercomparison Project phase 5 (CMIP5) models. Over the TIO, the observed warming trend has more than doubled since 1965, which is well simulated by the CMIP5 historical runs. However, as to spatial warming pattern, observations manifest a double-peak pattern during 1900–1940 and a non-uniform Indian Ocean Mode (IOBM)-like pattern during 1965–2005, which is not captured by the CMIP5 historical runs. Herein, an optimal detection analysis is employed, which indicates that the double-peak warming pattern can be explained well by a combination of Greenhouse Gas (GHG) and natural forcing, and the non-uniform IOBM-like pattern is mostly attributable to anthropogenic forcing. Further, a mixed-layer heat budget analysis shows that atmospheric and oceanic processes, especially latent heat flux from atmospheric forcing part associated with GHG forcing, are beneficial for the warming patterns formation. Our study supports the claim that intrinsic ocean–atmosphere interaction within the TIO is the key mechanism for maintaining the TIO warming. From the model perspective, during 1900–1940, the weak anti-symmetric atmospheric circulation with easterly (northwesterly) anomalies north (south) of the equator helps to sustain the double-peak warming pattern. During 1965–2005, the intensified anti-symmetric wind pattern is in favor of the non-uniform IOBM-like warming pattern.