Why do modelled and observed surface wind stress climatologies differ in the trade wind regions?

Isla Simpson, National Center for Atmospheric Research, Climate and Global Dynamics Laboratory, Boulder, CO, United States, Julio T Bacmeister, NSF National Center for Atmospheric Research, Boulder, United States, Irina Sandu, European Center for Medium-Range Weather Forecasts, Bonn, Germany and Mark J Rodwell, ECMWF, Reading, United Kingdom