Tropical Indian Ocean Mixed Layer Bias in CMIP6 CGCMs Primarily Attributed tothe AGCM Surface Wind Bias

The relatively weak sea surface temperature bias in the tropical Indian Ocean (TIO) simulated in the coupled
general circulation model (CGCM) from the recently released CMIP6 has been found to be important in model simulations
of regional and global climate. However, the cause of the bias is debated because the bias is strongly model
dependent and shows marked seasonality. In this study, we separate the bias in CGCMs into bias arising from oceanic
GCMs (OGCMs) and bias that is independent of OGCMs using a set of CMIP6 and OMIP6 models. We found that
OGCMs contribute little to mixed layer bias in the CGCMs. The OGCM-independent bias exhibits a large-scale cold
mixed layer bias in the TIO throughout the year, with an unexpectedly high degree of model consistency. By conducting a
set of OGCM experiments, we show that the OGCM-independent mixed layer bias is caused mainly by surface wind bias
in the utilized CGCMs. About 89% of surface wind bias in the CGCMs is due to the inability of atmospheric GCMs
(AGCMs), whereas atmosphere–ocean coupling in the CGCMs has only a minor influence on surface wind bias. The bias
in surface wind is also found to be the cause of subsurface temperature bias besides the ocean dynamics such as vertical
mixing and vertical shear in currents. Our results indicate that correcting TIO mixed layer bias in CGCMs requires improvement
in the capability of AGCM in simulating the climatological surface winds. The results improve our understanding of the cause of the bias in the Indian
Ocean and show that our method of bias separation is effective for attributing the source of bias to different proposed
mechanisms.