Evidence for nonlinearity in observed stratospheric circulation changes

The leading mode of variability of the lower atmosphere circulation in the Northern Hemisphere is a largely zonally symmetric mode known as the Arctic Oscillation. We calculate Arctic Oscillation (AO) indices on a range of le vels from 1000 to 10 hPa by means of a principal component analysis of Nati onal Centers for Environmental Prediction daily geopotential height anomali es. We find the apparent downward propagation of anomalies noted by other a uthors to be statistically significant compared to a red noise model. By ex amining histograms of these indices for each month, we note that the distri bution of the index is generally close to Gaussian in the troposphere. In t he stratosphere, however, the index is negatively skewed in the winter and positively skewed in the spring. We conclude that the positive skewness in April results from the coexistence of distinct summer and winter circulatio n states, and by examining polar stratospheric temperatures, we conclude th at the negative skewness in January may be due to the radiatively determine d limit on the vortex strength. This radiative limit responds relatively sl owly to anthropogenic forcing, whereas changes in planetary wave forcing co uld have a much faster impact on the number of warm events. This suggests a hypothesis that the vortex strength may respond nonlinearly to anthropogen ic forcing, which is supported by an observed change in the shape of the hi stograms of 20-200 hPa AO indices in January over the past 40 years.