Evaluation of Hydrographic Features in The Tropical Indian Ocean from CMIP and OMIP2 Models

Abstract

Coupled Model Intercomparison Project (CMIP) and Ocean Model Intercomparison Project phase 2 (OMIP2) models from the CMIP6 group were used in the current study to represent the annual mean biases of hydrographic features. 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 found that many models from both CMIP and OMIP2 exhibited cold temperature biases at the surface and warm biases in the subsurface on an annual scale, respectively. Overall, the CMIP models were observed to have larger biases than the OMIP2 models. Also, stronger saltier biases were identified in the south-eastern Arabian Sea (AS) and the western Bay of Bengal. In addition, a deeper thermocline was identified in CMIP models compared to OMIP2 and observations in the northern AS and the Seychelles-Chagos Thermocline Ridge. This deeper thermocline is associated with subsurface warm 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. Heat and salt transport analysis at different straits in the TIO suggested positive transport to the north (east) and negative transport to the south (west). Positive transport occurred during the post-monsoon season, while negative transport occurred during other seasons. SST-based upwelling index analysis revealed stronger upwelling signals in CMIP models than in OMIP2 during the summer months for all regions, primarily due to strong winds. 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.