Mechanisms ssocitates with Subsurface Temperature Bias and Warm Water Trap in The tropical Indian Ocean in a Coupled Model

Persisting subsurface ocean heat content can severely affect the long-term regional climate. Strong Indian Ocean subsurface temperature biases, usually found in coupled models, have severe implications on seasonal monsoon forecasts. The National Centers for Environmental Prediction (NCEP) Climate Forecasting System version 2 (CFSv2) coupled model, which is used for seasonal forecast in several countries including India, displays stron warm (cold) subsurface ( surface) temperature bias in the Tropical Indian Ocean (TIO), with deeper than observed mixed layer depth and thermocline. Heat budget analysis revealed that horizontal advective processes play meagre role in basin-wide subsurface warming. Weak upper (200m) ocean stratification and strong vertical shear in horizontal currents are mainly responsible for the subsurface warm bias. The contributions from vertical advection further support this. In the model, maximum warm bias is trapped between 150m and 200m depth as a result of excess mixing and weaker stability. On the other hand about 0.3 deg.c to 0.4 deg.c of negative SST bias is explained by heat flux bias while rest in explained through ocean mixing. This study enforces the need for accurate representation of vertical mixing and the processes responsible for current bias to simulate TIO ventilation more accurately.