Ocean circulation underlies the Atlantic meridional mode

The Atlantic meridional mode (AMM) is characterized by north-south bands of alternate anomalies in surface-ocean temperatures, and winds from colder bands to the warmer, at a periodicity of 10-15 years. The AMM has been linked to variations in Atlantic hurricanes, global surface-air temperature, and climate variability over the Sahel, South American, North American, and European. Despite these far-reaching impacts, the role of ocean circulation remains uncertain, and the prevailing AMM theories are based on thermodynamic air-sea interactions. Here we we uncover ocean-circulation variability that is linked to the AMM using twentieth century observations. Specifically, sea level-derived index of ocean circulation variabilityleads the AMM pattern by several years, through the interactions of overturning and gyre circulations with Kelvin wave anomalies that propagate from the North Atlantic to the low latitudes and by the thermocline feedback in the Atlantic cold tongue region. The peak of the sea surface temperature variability in the tropical Atlantic in turn drives inter-hemispheric atmospheric teleconnections represented by negative NAO phase over the North Atlantic. These findings imply that, rather than a passive role postulated by the prevailing thermodynamic paradigm, ocean circulation plays an active role in AMM variability.