Why Do Modelled and Observed Surface Wind Stress Climatologies Differ in the Trade Wind Regions?

Global climate models (GCMs) exhibit stronger easterly zonal surface wind stress and near surface winds in the Northern Hemisphere (NH) trade winds than observationally constrained reanalyses or other observational products. A comparison, between models and reanalyses, of the processes that contribute to the zonal mean, vertically integrated balance of momentum, reveals that this wind stress discrepancy cannot be explained by either the resolved dynamics or parameterized tendencies that are common to each. Rather, a substantial residual exists in the momentum balance of the reanalyses, pointing toward a role for the analysis increments. Indeed, they are found to systematically weaken the NH near surface easterlies in winter, thereby reducing the surface wind stress. Further analysis of the spatial structure and seasonality of these increments, demonstrates that they act to weaken the near surface flow over much of the low latitude oceans in both summer and winter. This suggests an erroneous /missing process in GCMs that constitutes a missing drag on the low level zonal flow over oceans. A likely candidate process is the formulation of the vertical transfer of momentum between the ocean and atmosphere. If this is the case, then it would imply that observation based surface stress products, which rely on similar drag formulations to GCMs, are underestimating the strength of the easterly surface wind stress and that the missing drag in free running models is leading to their overly strong easterly trades.