Multimodel study of tropical Atlantic variability and change

Interannual variability associated with the zonal and the meridional mode in the tropical Atlantic is studied in nine coupled ocean–atmosphere models for twentieth century climate conditions (TC) and the SRES-A1B scenario for future greenhouse gas concentrations. For TC, the subtropical part of the meridional mode is reasonably well simulated, in contrast to the deep tropical part of the meridional mode and the zonal mode. A common model bias is that the onset of the meridional mode is preceded by the presence of a zonal mode in boreal fall that extends towards the western boundary of the Atlantic basin and which initiates a Wind-Evaporation-SST feedback. As a result of this, there is a spuriously strong interaction between the zonal and the meridional mode. The models that seem to best represent the meridional mode show a weakening for future climate conditions. Biases in the zonal mode are too strong to assess changes.