Influence of the north Atlantic SST tripole on northwest African rainfall

[1] The sea-surface temperature (SST) tripole, with warm anomalies off the east coast of the United States and cold anomalies north of 40 degreesN and south of 25 degreesN, is the leading mode of interannual variability in wintertime North Atlantic SST. Its influence on northwest African rainfall is investigated by using a large-ensemble of GCM simulations. Firstly the modeled basin-scale rainfall impact is displayed, and the results suggest: in early-mid winter ( November - January), a positive SST tripole causes a reduced rainfall extending from the tropical North Atlantic northeastward to Mediterranean while a negative SST causes a south-north increased rainfall across the central Atlantic from the subtropics to the midlatitude. In late winter ( February - April) a positive SST tripole causes a reduced rainfall in the central Atlantic from the subtropics to the midlatitude while a negative SST tripole induces a zonal increased rainfall from the subtropics to Mediterranean. The asymmetry and seasonal dependence of the SST influence on the basin-scale rainfall is consistent with the nonlinear response of the large-scale atmospheric circulation. Under the large-scale impact background, northwest Africa regional rainfall response is also nonlinear and seasonally dependent. In early-mid winter a positive SST tripole causes reduced rainfall, while a negative SST has little effect. In late winter a negative SST tripole induces increased rainfall, while a positive tripole has little effect. A similarly large-scale asymmetric association between SST and rainfall-circulation exists in observations in late winter, while the observed seasonal dependence of this association is relatively weak. Also, a similar SST tripole association with the regional rainfall over the northwest coast of Africa exists in observations.