1. Leibniz-Institut für Meereswissenschaften, Düsternbrooker Weg 20, 24105 Kiel, Germany
2. Department of Oceanography, University of Cape Town, 7700 Rondebosch, South Africa

Correspondence to: A. Biastoch¹ Correspondence and requests for materials should be addressed to A.B. (e-mail: abiastoch@ifm-geomar.de).

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 26.5° N, and MOVE, at latitude 16° 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 = 10⁶ m³ s^-1) on decadal timescales in the subtropics⁶. Such variability has been attributed to variations of deep water formation in the sub-arctic Atlantic, particularly the renewal rate of Labrador Sea Water⁷. 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.

Link to letter: http://www.nature.com/nature/journal/v456/n7221/abs/nature07426.html