Tropical North Atlantic response to ENSO: sensitivity to model spatial resolution
AbstractIn this study the ENSO teleconnection with the Tropical North Atlantic (TNA) sea surface temperatures (SSTs) in boreal spring is analyzed in ocean-atmosphere coupled global circulation models. To assess the role played by horizontal resolution of models on this teleconnection, we used a multi-model dataset which is the first to combine models with both low and high resolution. The TNA response to ENSO projects onto the most significant SST mode of the tropical Atlantic at interannual timescales, the Atlantic meridional mode (AMM). Its evolution is primary driven by the wind-evaporation-SST (WES) feedback, which in turn is based on the development of an initial SST gradient. This study examines and quantifies the relative contribution of a dynamic-related (upwelling) and a thermodynamic-related (evaporation) process in triggering this gradient in the case of the ENSO-TNA teleconnection. While no major contribution is found with the evaporation, a consistent contribution from the coastal upwelling off north-west Africa is identified. This contribution is enhanced in high resolution models and highlights the close link between the upwelling in winter and the development of the AMM in spring. It is further shown that high resolution models present a thinner and more realistic ocean mixed layer within the upwelling area, which enhances the effect of surface winds on upwelling and SSTs. As a consequence high resolution models are more sensitive than low resolution models to surface wind errors, thereby do not ensuring an improved reliability nor predictability of the TNA SST response to ENSO.