International audienceBarrier 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

A B S T R A C T Many coupled general circulation models (CGCMs) tend to overestimate the salinity in the Atlantic warm pool or the Northwestern Tropical Atlantic (NWTA) and underestimate the surface salinity in the subtropical salinity maxima region. Most of these models also suffer from a sea-surface temperature (SST) bias in the NWTA region, leading to suggestions that the upper ocean salinity stratification may need to be improved in order to improve the barrier layer (BL) simulations and thus the SST through BL-SST-intertropical convergence zone feedbacks. In the present study, we use a CGCM to perform a set of idealised numerical experiments to test and understand the sensitivity of the BL and consequently SST in the NWTA region to freshwater flux and hence the upper ocean salinity stratification. We find that the BL of the NWTA is sensitive to upper ocean salinity changes in the Amazon river discharge region and the subtropical salinity maxima region. The BL phenomenon is further manifested by the formation of winter temperature inversions in our model simulations, the maximum magnitude of inversions being about 0.2 8C. The atmospheric response causes a statistically significant reduction of mean precipitation and SST in the equatorial Atlantic region and helps improve the respective biases by 10Á15%. In the region of improved BL simulation, the SST change is positive and in the right direction of bias correction, albeit weak.

A new ocean mixed layer model (OMLM) was embedded into an ocean general circulation model (OGCM) with the aim of providing an OGCM that is ideal for application to a climate model by predicting the sea surface temperature (SST) more accurately. The results from the new OMLM showed a significant improvement in the prediction of SST compared to the cases of constant vertical mixing and the vertical mixing scheme by Pacanowski and Philander. More accurate prediction of the SST from the new OMLM reduces the magnitude of the restoring term in the surface heat flux and thus provides a simulated ocean that can be coupled to the atmospheric general circulation model more naturally. The new OMLM was also shown to improve various other features of the OGCM such as the mixed layer depth and the equatorial circulation.