Abstract
Two regimes are found in the interannual variability of the large-scale stratospheric flow in the Northern Hemisphere cold season. The regimes are identified by studying the probability distribution of the leading principal component of the geopotential height, which explains approximately 50% of the variance. The probability distribution has a bimodal structure with two clearly separated peaks corresponding to two circulation regimes. The two regimes are characterized by a strong and a weak vortex, respectively, and they therefore resemble the two phases of the stratospheric part of the Arctic Oscillation. While the upper troposphere and the lower stratosphere are colder in the strong vortex regime than in the weak vortex regime, the lower troposphere is warmer—in particular over the continents. An abrupt regime shift took place in the last half of the 1970s in favor of the more zonal regime. The shift is manifested by a substantial change in the frequencies of the two regimes. Strong statistical significance for the two separate regimes is obtained by a Monte Carlo approach. The regimes and the regime shift are found in two different datasets, reducing the possibility that the results are due to inhomogeneities in the data.
The results support the nonlinear dynamical perspective on climate change suggested by T. N. Palmer. According to this idea the response to a weak forcing would be seen mainly in a change of the frequencies of the climate regimes, while the spatial structure of the regimes would be relatively insensitive to the forcing.
Corresponding author address: B. Christiansen, Danish Meteorological Institute, Climate Division, Lyngbyvej 100, DK-2100 Copenhagen Ø, Denmark. Email: boc@dmi.dk
