Skewness of Low-Frequency Fluctuations in the Tropospheric Circulation during the Northern Hemisphere Winter

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Abstract
Departures of lowpass filtered 500 mb height and sea-level pressure anomalies from a Gaussian distribution have been examined, based on Northern Hemisphere operational analyses for 30 winters. At each gridpoint we evaluated several indices that are measures of the differences between the amplitudes and frequencies of the persistent anomalies with positive and negative polarities, and differences in the temporal variability observed within extended periods of positive and negative anomalies. The spatial patterns for these indices are all quite similar, and they resemble the distribution of the moment coefficient of skewness documented by White. Positive skewness is observed to the north of the stormtrack latitudes and the negative to the south of them. The skewness pattern in the sea-level pressure field is weaker, particularly at high latitudes, and it exhibits a bias toward negative skewness. Anomalies with amplitudes larger than two standard deviations, which occur about 5% of the time, account for most of the skewness. A smaller number of “extreme events” with amplitudes larger than three standard deviations account for about half of the negative skewness at the lower latitudes.

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