Seasons, Singularities, and Climatic Changes over the Midlatitudes of the Northern Hemisphere during 1899–1969

Abstract

The annual regime of atmospheric circulation over the midlatitudes of the Northern Hemisphere for the years 1899?1969 was determined by factor analysis of daily frequencies of Dzerdzeevskii's Northern Hemisphere extratropical latitude circulation types. This analysis found seasons separated by abrupt circulation transitions. Other singularities including Indian summer and the January thaw were identified in the hemispheric circulation. In the mean, summer circulation occurred during 17 June to 31 August, autumn 1 September to 22 October, winter 23 October to 8 March, and spring 9 March to 16 June. Indian summer episodes occurred in late September, mid-October, and mid-November. The January thaw singularity occurred most often during 18?25 January. Maxima in meridional circulation occurred in spring during April and May. The timing of singularities and length of seasons differed among three climatic episodes within the 1899?1969 period of analysis. The warm episode, 1920?52, had the longest summer season, prolonged Indian summers, and a lack of meridional circulation in winter and spring. The cold episode, 1899?1919, had the longest winter season, infrequent Indian summers, and relatively high frequencies of meridional flow in winter and spring. The cool episode, 1953?69, lacked a true summer season and had high frequencies of meridional circulation all year.