1. Annual cycle of the circulation variability modes dominating over the Euro-atlantic sector
Lucie Pokorná1,2, Romana Beranová 2, Radan Huth1,2,3
1 Dept. of Physical Geography and Geoecology, Faculty of Science, Charles University, Prague, Czechia
2 Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czechia
3 Global Change Research Institute, Czech Academy of Sciences, Brno, Czechia
Motivation
Data
Methods
mean anomalies for 31-day sliding months (shifted with a step of 1 day) were reduced by the mean standard devia-tion for the whole area (to get a comparable strength of the action centers during the year), the reduction to latitude was applied
modes were detected for seasons composed from three subsequent sliding months (the first and the third with a half weight) shifted with a step of 5 days
principal component analysis (covariance matrix) was used, varimax rotation applied (9 to 14 components rotated)
Modes of circulation variability in 500 hPa fields were recognized to be the main drivers of the surface climate variability on monthly and seasonal scale. Their winter character was described by Rodgers in 1981 and confirmed by many authors on seasonal scale and by Barnston and Livezey (1987) on monthly scale. Some of the authors uncovered the possible character of modes (particularly on the NAO) also in other seasons but with no evidence of continuity.
In this research the circulation variability modes were detected within the calendar year with a step of 5 days. The modes dominating over east Atlantic and Europe are presented: the North Atlantic Oscillation (NAO), the East Atlantic pattern (EA), the Scandinavian pattern (SCAN) and the Euro-Asian pattern (EU).
NCEP/NCAR reanalysis with resolution 2.5 x 2.5 in the Northern Hemisphere extratropics (20°-85° north)
Anomalies of mean daily Z500
period
Figures
display annual cycle of four main circulation variability modes over the Euro-Atlantic sector. Loadings of modes are displayed in terms of dimensionless covariance with the 500 hPa heights. The contour interval is 0.2; positive (negative) values are indicated by red (blue) lines; zero line is not shown. Number in the right bottom corner of each loading indicates the importance (the order) of an individual mode and the number of rotated components. January is in the left upper corner, the sequence is clockwise. The date at the top right of each mode indicates a centre of sliding season (Jan 15 for season from November 30 to March 3, Feb 14 season from December 30 to April 2, etc.)
SCAN
NAO
The NAO in Z500 is located particularly in the North Atlantic ocean; the northern centre between 50°-85° north while the southern centre between 30° and 50 ° north and also over western USA and southern part of Europe but not Mediterranean. The northern centre shifts westward in March and both centres weaken in May. The southern centre influences Scandinavia in summer and expands to North America in September.
The SCAN mode has the main centre over Scandinavia and western Russia and centres of the opposite sign over the Mediterranean and over Mongolia. The Scandinavian centre shifts westward in March and northward in June. The Mediterranean centre fades and the Mongolian centre shifts westward during spring. The mode is replaced by two modes with slightly different position and variable intensity of centres in summer.
Conclusions
The existence of zonal modes (NAO and EA) was documented for the whole year; the position of centres slightly changes during year but the basic character of the modes remains unchanged.
Two meridional modes (SCAN and EU) exist from October to March. They are replaced by two modes after a rebuilding of atmospheric circulation during April. The position of their centres differs between the modes and the character of the modes is distinct from winter modes.
References
Barnston, A. G., Livezey, R. E., 1987, Mon. Wea. Rev., 115,
Wallace, J. M., Gutzler, D. S., 1981, Mon. Wea. Rev., 109, EA EU
The EA mode is a similar pattern to the NAO with centres shifted southward. The southern centre is fold down over Europe in SW- NE direction and extends from Africa to Belarus in winter but fades and shifts westward from May to July.
The EU mode has two main centres; one over Denmark and the British Isles and the other over Caspian sea and Russia in winter. A minor centre is situated over the North Atlantic. The EU mode is replaced by two modes from May to September; one with a strong Caspian centre and the other with a strong centre over the British Isles.
Acknowledgement
The research was supported by the Grant Agency of the Czech Academy of Sciences, project S