Hydrographic features in the Tropical Indian Ocean: Insights from coupled and uncoupled models from the CMIP6 group

Coupled Model Intercomparison Project (CMIP) and Ocean Model Intercomparison Project phase 2 (OMIP2) models from the 6th phase of the CMIP group were used in the current study to represent the annual mean biases of hydrographic features. The OMIP2 models are ocean-only simulations, while the CMIP models are coupled ocean-atmosphere-land-sea ice simulations. These models are assessed against the observations in the Tropical Indian Ocean (TIO). This study identified that many of the models from both CMIP and OMIP2 exhibited cold and warm temperature biases at the surface (0-100m) and subsurface (100-300m) on an annual scale, respectively. Overall, the CMIP models were observed to have larger biases than the OMIP2 models. Also, strong positive biases of salinity were identified in the south-eastern Arabian Sea (AS) and the western Bay of Bengal than in other regions of TIO. In addition, a deeper thermocline was identified in the northern AS and Seychelles-Chagos Thermocline Ridge region in CMIP and OMIP2 models compared to observations, which was predominant in the CMIP models than in the OMIP2 models. This deeper thermocline is associated with subsurface warm temperature biases. Brunt-Väisälä frequency revealed weaker stratification from surface to 100m with a peak at 80m. Further, vertical shear currents revealed strong shear bias at the top 40m, that can result in vertical mixing, which is chiefly accountable for the biases of temperatures and salinities. The heat and salt transport analysis at different straits in the TIO suggested positive northward and negative southward transport. Positive transport occurred during the post-monsoon season, while negative transport occurred during other seasons. SST-based upwelling index analysis revealed strong upwelling signals during summer months in all individual models for all regions. However, strengthened upwelling has been identified in the CMIP models than in OMIP2 models due to strong winds over the upwelling regions. A strong negative correlation has been identified between surface temperature and windspeed in CMIP models over most of the TIO, suggesting that strong surface wind speeds lead to vertical mixing, which in turn causes further surface cooling.