Barrier layers and tropical Atlantic SST biases in coupled GCMs

Barrier layers (BLs) tend to suppress entrainment cooling of the ocean surface mixed layer. Consequently, model biases in BL formation may result in SST biases. Coupled General Circulation Models (GCMs) capture the major observed BL regions in the tropical Atlantic, although with considerable biases in the BL characteristics. Thick BLs form in the Northwestern Tropical Atlantic (NwTA) during boreal fall and winter. The effect of BL biases in the NwTA on the cold SST bias in this region is likely small. On the contrary, the models simulate spurious BLs in the Southeastern Equatorial Atlantic (SeEA), which contribute significantly to the warm SST bias in this region. These spurious BLs originate partly from a fresh surface bias associated with a southeastward displaced ITCZ and partly from a warm subsurface bias associated with the underestimated equatorial trades during boreal spring and summer. It is hypothesized that a positive BL—SST—ITCZ feedback mechanism exists by which the BL and SST biases in the SeEA are maintained. An implication is that the upper ocean salinity stratification is a significant link in the chain of cause and effect by which precipitation and wind stress biases already present in uncoupled atmospheric models are amplified in coupled models.