A review of North Atlantic modes of natural variability and their driving mechanisms

This paper reviews three modes of natural variability that have been identified in the North Atlantic Ocean, namely, the North Atlantic Oscillation (NAO), the Atlantic Multidecadal Oscillation (AMO) and the Atlantic Meridional Mode (AMM). This manuscript focuses on the multidecadal fluctuations of these three modes. A range of different mechanisms to initiate phase reversals in these modes on multidecadal timescales has been suggested previously. We propose a systematic grouping of these mechanisms into three types that involve, respectively, (1) the dependency of the Atlantic thermohaline circulation (THC) on salinity, (2) the sensitivity of the THC to changes in ocean heat transport and (3) the dependency of the NAO to changes in the Atlantic meridional temperature gradient. Some new density data is also provided, demonstrating physical links between the THC and the AMO.