Contrasting Historical Trends in Equatorial Indian Ocean Zonal Sea Surface Temperature Gradient in CMIP6 Models

The zonal sea surface temperature (SST) gradient in the tropical Indian Ocean (TIO) has been assessed using 50 climate models. Among these, 38 models exhibit an east-west negative gradient trend, indicating an intensified warming pattern in the Western Equatorial Indian Ocean (WEIO). This strong inter-model spread in representing the zonal SST gradient in the TIO mainly arises from the large variability of SST trends in the eastern Indian Ocean. The multi-model mean shows a westward SST gradient trend, which is approximately four-fold higher than the observed zonal gradient trend. However, models such as E3SM-1-1 and NESM3 realistically represent SST trends in both the eastern and western equatorial Indian Ocean regions, thereby capturing SST gradients close to observation. To investigate gradient variability and the underlying mechanisms, we categorised models into two groups, each comprising five models. The first group, comprising CESM2-FV2, EC-Earth3-Veg-LR, EC-Earth3-Veg, CAS-ESM2.0, and CIESM, demonstrates pronounced negative SST gradient trends. Conversely, the second group, consisting of CESM2-WACCM-FV2, CESM2, CESM2-WACCM, CMCC-CM2-SR5, and MIROC6, exhibits relatively subdued positive gradients, attributable to the slower warming of the WEIO. The inconsistent warming pattern formation, associated with eastward (westward) intensification of SST trends in positive (negative) gradient models, leads to larger gradient magnitudes compared to observations. The wind-evaporation-SST (WES) feedback plays a predominant role in shaping the SST warming pattern in both groups of models, while the mean state SST bias has a secondary role. The Bjerknes feedback is weak in positive zonal SST gradient models, whereas both Bjerknes and WES feedbacks act to enhance the zonal SST gradient in models with negative gradient trends. This study underscores the dominant role of air-sea interaction processes in forming SST warming patterns and highlights the unrealistic zonal SST gradient in the equatorial Indian Ocean.