How well do CMIP6 models simulate salinity barrier layers in the North Indian Ocean?

Previous studies have hypothesized that climatologically thick salinity-stratified Barrier Layers (BL) in the North Indian Ocean (NIO) could influence the upper ocean heat budget, sea surface temperature (SST) and monsoon. Here, we investigate the performance of state-of-the-art climate models from the Coupled Model Intercomparison Project phase 6 (CMIP6) in simulating the barrier layer thickness (BLT) in the NIO. CMIP6 models generally reproduce the main features of the BLT seasonal cycle and spatial distribution, but with a shallow November-February (NDJF) BLT bias in regions with thick observed BLT (eastern equatorial Indian Ocean [EEIO], Bay of Bengal [BoB] and southeastern Arabian Sea [SEAS]). CMIP6 models display an easterly equatorial zonal surface wind bias linked to dry rainfall and cold SST biases in the southern BoB, through the Bjerknes feedback loop. The easterly equatorial bias is also responsible for the shallow isothermal layer depth (ILD) and BLT bias in the EEIO. The underestimated rainfall over the BoB leads to higher sea surface salinity (SSS) and too deep mixed layer depth (MLD), resulting in the BoB BLT bias. The intensity of the easterly equatorial bias also contributes to the inter-model spread in BoB BLT bias, through the propagation of EEIO ILD signals into the coastal waveguide. Finally, the SEAS BLT bias is due to a too deep MLD, which is predominantly controlled by the high SSS related to attenuated monsoonal currents around India and a reduced inflow of BoB low-salinity water. The BL effect on the mixed layer entrainment cooling does not seem to operate in CMIP6 simulations. Rather, deep salinity-related MLD biases in the BoB result in a diminished cooling rate in response to winter negative surface heat fluxes, and hence alleviate cold BoB SST biases. This suggests that salinity effects alleviate the biases that develop through the positive Bejrknes feedback loop between BoB SST, BoB rainfall and equatorial wind stresses in CMIP6.