Intra-annual Changes of the Circulation Variability Modes Dominating over the Northern Hemisphere Extratropics

The Pacific/North American pattern, the North Atlantic Oscillation or the West Pacific oscillation belong to the main atmospheric teleconnections in the Northern Extratropics. Their existence in winter season has been documented by several authors in different pressure levels since 1980s using either one-point correlation maps or principal component analysis (PCA). These and approximately further seven modes create the main drivers of surface climate variability on monthly and seasonal scale.

The leading modes in Z500 level were detected in three different reanalysis (NCEP/NCAR, JRA-55 and ERA-40) and high similarity across reanalysis was demonstrated on AMS 2018. In further research the longest reanalysis (NCEP/NCAR, period 1948-2016) was used for the detection of an annual cycle of modes. Instead of common monthly means, the mean anomalies for sliding months of the 31-days length shifting within the calendar year with a 1-day step were computed and reduced with mean seasonal variability (for whole area) and latitude in each grid-point. Circulation variability modes were then detected for sliding seasons (three following 31-days sliding months; the first and the third with a half weight) with a step of 5-days using the PCA. Generally, from 11 to 13 components were rotated (varimax) except the cold part of the year when the number of components was between 9 and 11 (optimum number for each sliding season was estimated based on the graph of eigenvalues and existence of known modes).

The poster displays intraanuual changes of the main teleconnections explaining the large amount of variability and determining weather over the North America: the North Atlantic Oscillation (NAO), the West Pacific oscillation (WPO), the East Pacific pattern (EP), Pacific/North America pattern (PNA), the North Pacific oscillation (NP) and the Transient Northern Hemisphere pattern (TNH) (using the same names as Barnston and Livezey, 1987).

The modes can be simply detected from December to March. The increase of incoming solar radiation in spring causes a rebuilding of atmospheric circulation during April. The shape of modes stabilize in May; new modes develop and some winter modes disappear. Zonal modes change the position of its centres westward while meridional modes remain pronounced only over the oceans and their action centres over continents fade or wear off. Summer character of modes persists until November which suggests that autumn changes of the atmospheric circulation are slow and their mechanism is completely different from spring. The results show the obvious asymmetry of annual course of modes.

Barnston, A. G., Livezey, R. E., 1987: Classification, seasonality and persistence of low-frequency atmospheric circulation patterns. Mon. Wea. Rev., 115, 1083-1125