On The Seasonality Of Arctic Oscillation With Its Structure And Mechanism

Using NCEP/NCAR reanalysis monthly dataset, the seasonal variability of Arctic Oscillation (AO) is studied with the emphasis on the temporal and spatial features of summertime AO. Meanwhile, the three-dimensional structure of AO is presented, thereby its mechanism being explained.The analysis results show that the seasonality of AO is significant in its strength and spatial structure. Wintertime AO exhibits the out-of-phase vacillation between the Arctic active center and the Atlantic-west Europe center and stronger signature of annular mode and equivalent barotropic structure than summertime AO. In contrast, the out-of-phase vacillation in summertime AO covers the Arctic region and mid-latitude Asian continent. Though the summertime AO exists on intraseasonal as well as interannual time scale, its Asian active center abates on the intraseasonal pattern, thus awarding it more conspicuous zonal symmetric structure.Interannually, summertime AO exhibits the zonal asymmetric and baroclinic features, and its seesaw mode is significant only in the lower troposphere. In the upper troposphere, there is no intact Asian active center. The troposphere structure of summertime AO is implicated with the horizontal heterogeneity of air temperature throughout the whole troposphere with its center over Asian continent, which results in the zonal asymmetry and baroclinic structure of the summertime AO (especially its Asian center). On the contrary, the correlation between wintertime AO and sea-land thermal contrast confines in the lower troposphere, hence its zonal asymmetry and baroclinity in the lower level. Therefore, the physical mechanics of AO also presents seasonal variability. While thermal forcing plays an important role in the process of summertime AO formation and maintenance, wintertime AO is subject to the dynamic process. This is probably the radical cause of the seasonal discrepancy of AO.In addition, the AO index released by American Climate Prediction Center (CPC AOI) fails to reflect the summer AO mode. In this paper, the time series of the leadingprincipal component of the summertime (June-September) surface level pressure anomaly field over the domain poleward of 20 N is defined as the summertime AO index.