Identifying the Time Scale, Pattern, and Mechanisms Underlying North Atlantic Decadal Variability

Observations show considerable decadal to multidecadal variability in the surface climate over the North Atlantic Ocean. These low-frequency variations in North Atlantic sea surface temperature (SST) influence climate over adjacent continents. Understanding the physical mechanisms responsible for SST variations in the North Atlantic is important, because the existence of such slowly evolving signals in climate implies higher predictability. However, while much focus has been put on identifying the mechanisms responsible for Atlantic multidecadal variability, the origin and fingerprint of decadal variability remain unresolved.

Here we identify the time scale, pattern, and mechanisms underlying decadal SST variability in the North Atlantic by using low-frequency component analysis, a method to find patterns of low-frequency variability that has no a priori assumptions about the spatial or temporal structure of variability. Observations (HadISST) reveal a dominant pattern of SST variability with a time scale of 13-15 years and a tripolar structure – warming in the subpolar North Atlantic, and cooling in the Gulf Stream region and the Nordic Seas. This mode of decadal variability is also present in historical simulations in the Community Earth System Model (CESM) large ensemble and CMIP5 preindustrial control simulations. In the models, a warming of the subpolar North Atlantic is preceded by atmospheric circulation anomalies resembling a positive state of the North Atlantic Oscillation and a strengthening of the Gulf Stream/North Atlantic Current. The time scale of SST variability is consistent with the transit time of Rossby waves in the North Atlantic; both observed and simulated temperature anomalies are found to propagate from east to west across the North Atlantic in approximately half the period of variability.