The Impact of Tropical Atlantic Freshwater Fluxes on the North Atlantic Meridional Overturning Circulation

Abstract

Abstract The influence of ENSO-related changes in the Atlantic-to-Pacific freshwater budget on the North Atlantic meridional overturning is examined using the University of Victoria (UVic) Earth System Climate Model. The initial analysis of freshwater fluxes in the 50-yr NCEP–NCAR (NCEP50) reanalysis product and Global Precipitation Climatology Project (GPCP) dataset reveals that the transport of water vapor out of the tropical Atlantic drainage basin is enhanced during El Niño phases and reduced during La Niña phases; a one standard deviation in the Southern Oscillation index alters the tropical Atlantic freshwater balance by about 0.09 Sv (Sv ≡ 106 m3 s−1). A weaker link with ENSO is found in the 40-yr ECMWF Re-Analysis (ERA-40), although its usefulness is severely limited by a strong and spurious trend in tropical precipitation. Model results suggest that tropical Atlantic salinity anomalies generated with the frequency and amplitude of ENSO tend not to impact deep-water formation as they are diluted en route to the North Atlantic. Lower frequency, decadal time-scale anomalies, however, do have an impact, albeit weak, on the rate of North Atlantic Deep Water formation. In addition, and contrary to earlier results, it is found that even a shift of the tropical Atlantic freshwater balance toward permanent El Niño conditions only slightly mitigates the transient reduction of North Atlantic Deep Water formation associated with the increase of anthropogenic greenhouse gases. Taken together, the results suggest that the poleward propagation of salinity anomalies from the tropical Atlantic, associated with changes in ENSO, should not be considered a significant mechanism for the variability of the North Atlantic meridional overturning in the present and foreseeable future climate.