Possible Root Causes of the Surface Westerly Biases over the Equatorial Atlantic in Global Climate Models

Most global climate models (GCMs) suffer from biases of a reversed zonal gradient in sea surface temperature and weak surface easterlies (westerly biases) in the equatorial Atlantic. These biases have persisted through decades despite of considerable model improvements in other aspects. The errors exist in atmospheric alone models and are amplified by air-sea interactions in coupled models. It is not only a problem at the surface but extend to the mid-troposphere, with maximum biases near 850 hPa. Another manifestation of the westerly biases is a weakening of the atmospheric zonal overturning circulation over the equatorial Atlantic. We propose a hypothesis that there are two possible root causes for the surface westerly biases. The first is insufficient low-level diabatic heating over the Amazonia. The second is weak momentum flux between the boundary layer and the lower troposphere. The base of this hypothesis comes from diagnosing simulations of eight atmospheric GCMs. Severe westerly biases tend to occur when diabatic heating at low-levels (850 - 600 hPa) over the Amazonia is relatively weak for a given amount of precipitation. Over the equatorial Atlantic Ocean the zonal momentum balance in the boundary layer, approximated as a mixed layer, is dominated by three processes: the zonal pressure gradient (which is closely related to the vertical structure of diabatic heating over the Amazonia), surface drag, and momentum flux across the top of the mixed layer (entrainment). Inaccurate entrainment in models may result in westerly biases even if the distribution of precipitation and vertical profiles of diabatic heating over the Amazonia are well reproduced.