Impact of Multidecadal Variability in Atlantic SST on Winter Atmospheric Blocking
Recent studies have suggested coherent multidecadal variability exists between North Atlantic atmospheric blocking frequency and the Atlantic Multidecadal Variability (AMV). However, the role of AMV in modulating blocking variability on multidecadal times scales is not fully understood. Our study found the ocean variability associated with the AMV impacts the occurrence frequency of atmospheric blocking in the Atlantic sector and the North Atlantic Oscillation in wintertime based on multiple observational and reanalysis datasets for 1901-2010.
The impact of slowly varying ocean variability on the atmospheric blocking, an intra-seasonal weather phenomenon resulting in various extreme weather events, implies long-term predictability of the blocking frequency in seasonal to multi-year time scale. Our on-going follow-up research on the prediction aspect of this relationship already suggests statistically significant prediction skill can indeed be found from the state-of-the-art decadal prediction systems.
Our analyses found that more frequent blocking over Greenland and less over the Azores, which are associated with the negative NAO, occur following the warm phase of the AMV by several years. This lag is due to the slow evolution of the AMV SST anomalies, which is likely driven by ocean circulation. Following the warm phase of AMV, the warm SST anomalies emerge in the western subpolar gyre over 3-7 years. The ocean-atmosphere interaction over these 3-7 year periods is characterized by the damping of the warm SST anomalies by the surface heat flux anomalies, which in turn reduce the overall meridional gradient of the air temperature and thus weaken the meridional transient eddy heat flux in the lower troposphere. The anomalous transient eddy forcing then shifts the eddy-driven jet equatorward, resulting in enhanced Rossby wave breaking and blocking on the northern flank of the jet over Greenland. The opposite is true with the AMV cold phases but with much shorter lags, as the evolution of SST anomalies differs in the warm and cold phases.