Annual cycle of modes of low-frequency circulation variability

The modes of low-frequency variability of atmospheric circulation (teleconnections) have been studied mostly in winter. Of the four modes dominating over the North Atlantic – European sector (North Atlantic Oscillation (NAO), and East Atlantic (EA) and two Eurasian (EU1, EU2) patterns), only the NAO has been investigated in other seasons. The objective of this study is to describe the development of the North Atlantic – Europen circulation modes throughout the year, that is, to characterize their annual cycle.

The modes are identified in 500 hPa heights in the Northern Extratropics (north of 20°N inclusive) for period 1948-2016. The data source is the NCEP/NCAR reanalysis. The analysis tool is rotated principal component analysis (using covariance matrix and varimax rotation). The key ingredience of our methodology is that the analysis is based on sliding seasons 90 days long, which are moved with a step of 5 days. That is, the entire year is covered by 73 (=365:5) sliding seasons. The number of rotated components (that is, of detected modes) varies from 9 in winter to 13 in summer.

The four standard modes listed above are detected unambiguously from December to March. They change their position and spatial structure in spring, when also other modes appear. Specifically, the centres of the NAO shift westward in spring and northward in summer. The EA pattern loses its zonal character in the warm half year; its southern centre shifts towards Europe and weakens. The behaviour of the Eurasian modes is more complex. Since they describe a moving wave, being in quadrature one with another, their geographical position is to some extent random and their attribution in successive sliding seasons is hardly possible in spring and autumn. The shape of the modes stabilizes in May and remains fairly stable until October when the circulation returns to the winter character. The results document the changes in circulation action centres and their connectivity and show the obvious asymmetry of annual course of modes. One point correlation maps are employed to support the physical realism of the modes detected by principal component analysis.