A Possible Explanation of the Observed Persistence of Monthly Mean Circulation Anomalies

The level of month-to-month persistence of anomalies in the monthly mean atmospheric circulation was determined from a 29-year data set of Northern Hemisphere analyses of 500 mb height, surface pressure and 500–1000 mb thickness. A well-defined annual march is found, with greatest persistence from January to February and from July to August. The minima occur in spring and fall. Expressed in a linear correlation coefficient the largest persistence amounts to no more than 0.3. A qualitative explanation for the double peak in the annual march was sought in linear theory. The response of a stationary linear atmospheric model to the forcing of an anomalous heat source depends on the properties of the basic state around which the model is linearized. Model runs were made with climatological mean basic states corresponding to all 12 calendar months. In all runs the forcing was kept the same. As climatology changes least from January to February and from July to August, the model response to the constant forcing then is almost 100% persistent. The persistence is low from April to May and from October to November because in these months the basic state changes rather drastically. Although the maxima in persistence on the monthly time scale in the observed circulation are indeed found in summer and winter, the level of persistence is far below 100%. This can be interpreted as observational evidence of the very large forcing of the time-mean atmosphere by high-frequency transient eddies. The forcing associated with long-lived anomalies in external factors (oceans, snow, etc.) seems to control only a small part of the observed anomalies in the atmospheric circulation.