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. Modern coupled general circulation models producesystematic biases in the tropical Atlantic that hamper the reliability oflong-range predictions. This study focuses on a common springtime westerlywind bias in the equatorial Atlantic in seasonal hindcasts from two coupledmodels – ECMWF System 4 and EC-Earth v2.3 – and in hindcasts also based onSystem 4, but with prescribed sea-surface temperatures. The development of the equatorial westerly bias in early April is marked bya rapid transition from a wintertime easterly, cold tongue bias to aspringtime westerly bias regime that displays a marked double intertropicalconvergence zone (ITCZ). The transition is a seasonal feature of the modelclimatology (independent of initialisation date) and is associated with aseasonal increase in rainfall where a second branch of the ITCZ is producedsouth of the Equator. Excess off-equatorial convergence redirects the tradewinds away from the Equator. Based on arguments of temporal coincidence, theresults of our analysis contrast with those from previous work, and allegedcauses hereto identified as the likely cause of the equatorial westerly bias inother models must be discarded. Quite in general, we find no evidence ofremote influences on the development of the springtime equatorial bias inthe Atlantic in the IFS-based models. Limited evidence however is presentedthat supports the hypothesis of an incorrect representation of themeridional equatorward flow in the marine boundary layer of the southernAtlantic as a contributing factor. Erroneous dynamical constraints on theflow upstream of the Equator may generate convergence and associatedrainfall south of the Equator. This directs attention to the representationof the properties of the subtropical boundary layer as a potential sourcefor the double ITCZ bias.

A double ITCZ phenomenology of wind errors in the equatorial Atlantic in seasonal forecasts with ECMWF models