Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation

Predicting the evolution of climate over decadal timescales requires a quantitative understanding of the dynamics that govern the meridional overturning circulation (MOC)(1). Comprehensive ocean measurement programmes aiming to monitor MOC variations have been established in the subtropical North Atlantic(2,3) ( RAPID, at latitude 26.56 degrees N, and MOVE, at latitude 16 degrees N) and show strong variability on intraseasonal to interannual timescales. Observational evidence of longer- term changes in MOC transport remains scarce, owing to infrequent sampling of transoceanic sections over past decades(4,5). Inferences based on long- term sea surface temperature records, however, supported by model simulations, suggest a variability with an amplitude of +/- 1.5-3 Sv ( 1 Sv = 10(6) m(3) s(-1)) on decadal timescales in the subtropics(6). Such variability has been attributed to variations of deep water formation in the sub- arctic Atlantic, particularly the renewal rate of Labrador Sea Water(7). Here we present results from a model simulation that suggest an additional influence on decadal MOC variability having a Southern Hemisphere origin: dynamic signals originating in the Agulhas leakage region at the southern tip of Africa. These contribute a MOC signal in the tropical and subtropical North Atlantic that is of the same order of magnitude as the northern source. A complete rationalization of observed MOC changes therefore also requires consideration of signals arriving from the south.