Origin of variability in Northern Hemisphere winter blocking on interannual to decadal timescales

Variability of midlatitude blocking in the boreal winter Northern Hemisphere is investigated for the period 1960/1961 to 2001/2002 by means of relaxation experiments with the model of the European Centre for Medium-Range Weather Forecasts. It is shown that there is pronounced interannual and decadal variability in blocking, especially over the Eurasian continent, consistent with previous studies. The relaxation experiments show that realistic variability in the tropics can account for a significant part of observed interannual blocking variability but also that about half of the observed variability can only be explained by extratropical tropospheric variability. On the quasi-decadal timescale, extratropical sea surface temperature and sea ice, in addition to tropical variability, play a more important role. The stratosphere, which has been shown to influence interannual variability of the North Atlantic Oscillation in previous studies, has no significant influence on blocking according to our analysis.