On the effect of the Arctic Oscillation on the Northern hemisphere anticyclones

The effect of the Arctic Oscillation on the weather of the Northern hemisphere has in many studies been examined as part of the relation between the low frequency and the high frequency component of the atmospheric circulation. The current work focuses on the positively signed atmospheric anomalies ie. anticyclones that appear over a range of scales and frequencies and examines their response to interannual variations of the Arctic Oscillation.

Height, temperature and other fields from the ECMWF Reanalysis (ERA-40) dataset are spectrally decomposed at 6 hourly intervals into three bands of scales representative of contributions to anticyclonic development from a) planetary scales, b) intermediate scales characteristic of baroclinic waves-blocking highs, and c) meso-scales, respectively. In each band, anticyclone centers are identified as maxima in the band-passed fields. The analysis is performed on several levels spanning the atmosphere from the surface to the tropopause. A sophisticated model is then used to track the anticyclones through their lifetime and to generate estimates of their properties, like lifetimes and growth rates, and also density distributions of anticyclo-genesis and lysis events. Average distributions for the 40 years of the dataset are first evaluated and the analysis is subsequently repeated for the subsets of years characterised by positive/negative AO index.

The results indicate significant sensitivity of anticyclonic activity in all three bands of scales to opposing polarity of the AO, manifested by variations in the frequency, intensity and the location of anticyclonic development. For the planetary scale anticyclones the variations are most pronounced in the North American-Atlantic-Eurasian sector. For a change from positive to negative AO index we identify equatorwards displacement in the positions of the Azores and the North American Highs and arcticwise displacement of the Siberian High, all these displacements pointing to a large scale transfer of mass occuring from the Eastern towards the Western hemisphere in association with the transition from positive to negative AO phase. In the intermediate scale band an intensification is identified, for negative AO, of the anticyclonic activity associated with the Bering blocks in the Pacific and the Greenland highs in the Atlantic. Further, a northwards shift of the Greenland center is evident for positive AO ; this allows the penetration of the Atlantic storm track into the Arctic ocean which in turn contributes to the lowering of the MSLP in the Arctic region for positive AO. In the meso-scale band we observe, for positive AO, an increase of anticyclones locally along the continental margins surrounding the Western Arctic ocean, and also a retrogression of the anticyclonic activity from Southern Greenland westwards towards the Baffin bay in a way that favours, as does the shift in the intermediate scales, the arcticwise extension of the Atlantic storm track. Finally, the sensitivity of the anticyclonic activity to changing AO phase is briefly examined for the pre-1980 and compared to that of the post-1980 period in view of a number of studies that point to the abrupt changes that occured in the thermal properties of the Pacific ocean in the late 70s.