North Atlantic Atmospheric Blocking and Atlantic Multidecadal Oscillation in CESM1 Large Ensemble Simulations

Atmospheric blocking is an unusual weather phenomenon that is often associated with severe weather events such as heat waves, cold spells, and droughts. Improved understanding of the long-term variability of atmospheric blocking has thus important societal implication, but its mechanisms are not well understood. Multidecadal variability of winter (DJFM) atmospheric blocking in the North Atlantic, especially its relationship with multi-decadal oceanic variability represented by the Atlantic Multidecadal Oscillation (AMO), is examined using observational datasets and the Community Earth System Model version1 Large Ensemble (CESM1LE) simulations. The CESM1LE has 30 ensemble members from 1920 to 2005 forced with the identical historical radiative forcing but slightly different initial conditions. Therefore, the internal climate variability in the 30 simulations do not necessarily exhibit identical temporal evolutions, while the externally driven variability due to the radiative forcing are likely to be coherent. The mean spatial patterns of the number of blocking days in the North Atlantic are examined in the 20th Century Reanalysis (20CR) and the 30 member CESM1LE simulations. In addition, the AMO index is examined in the Hadley Centre Sea Ice and Sea Surface Temperature data set version 1 (HadISST) and the CESM1LE. In the observations, the two primary maxima of atmospheric blocking occurrence are found over the Greenland and the British Isles. CESM1LE underestimates the mean number of blocking days in these two locations, but the time-scale of variability in each region is comparable to that in the observations. CESM1LE also shows a reasonable AMO with similar amplitude and periodicity to the observations. In the observations, preliminary results show some correlation between the blocking in the North Atlantic and the AMO on decadal time-scales when the AMO leads the blocking by 4 to 8 years. This suggests that atmospheric blocking and the associated extreme weather variability in the North Atlantic might be modulated by the multi-decadal oceanic variability associated with the AMO. In CESM1LE, the ensemble mean of AMO index and the number of blocking days show a robust increasing and decreasing trends, respectively, which is likely driven by the anthropogenic radiative forcing.