Subannual, seasonal and interannual variability of the North Atlantic meridional overturning circulation

Joël J.-M. Hirschi, Peter D. Killworth, Jeffrey R. Blundell

National Oceanography Centre, Southampton, United Kingdom

Abstract

An eddy-permitting numerical ocean model is used to investigate the variability of the meridional overturning circulation (MOC). Both wind stress and fluctuations of the sea water density contribute to MOC changes on subannual and seasonal timescales whereas the interannual variability mainly reflects changes in the density field. Even on subannual and seasonal timescales a significant fraction of the total MOC variability is due to changes of the density field in the upper 1000 m of the ocean. These changes reflect perturbations of the isopycnal structure which travel westward as Rossby waves. Due to a temporally changing phase difference between the eastern and western boundaries the Rossby waves affect the MOC by modifying the basin-wide east-west density gradient. Both the numerical model used in this study and calculations based on Rossby-wave theory suggest that this effect can account for an MOC variability of several Sv (1Sv = 10^6$ m^3/s). These results have implications for the interpretation of variability signals inferred from hydrographic sections and might contribute to the understanding of the results obtained from the RAPID monitoring array deployed at 26°N in the North Atlantic.