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This study investigates biases of the climatological mean state of the northern Arabian Sea (NAS) in 31 coupled ocean-atmosphere models. The focus is to understand the cause of the large biases in the depth of the 20 degrees C isotherm (Delta D20) that occur in many of them. Other prominent biases are the depth (h(PG)) and temperature (T-PG) of Persian Gulf water (PGW) and the wintertime mixed-layer thickness (MLT) along the northern boundary.

For models that lack a Persian Gulf (group 1), Delta D20 is determined by the wintertime MLT bias (Delta MLT) through the formation of an Arabian Sea high-salinity water mass (ASHSW) that is too deep. For models with a Persian Gulf (group 2), if h(PG) > MLT (group 2B), PGW remains mostly trapped to the western boundary and, again, Delta MLT directly controls DD20. If h(PG) less than or similar to MLT (group 2A), PGW spreads into the NAS and impacts Delta D20 because T-PG > 20 degrees C; nevertheless Delta MLTstill influences Delta D20 indirectly through its impact on h(PG).

The thick wintertime mixed layer is driven primarily by surface cooling Q during the fall. Nevertheless, variations in Delta MLT among the models are more strongly linked to biases in the density stratification (jump) across the bottom of the mixed layer than toQbiases. The jump is in turn determined primarily by sea surface salinity biases (Delta SSS) advected into the NAS by the West India Coastal Current, and the source of DSSS is the rainfall deficit associated with the models' weak summer monsoon. Ultimately, then, Delta D20 is linked to this deficit.