MAJOR MODES OF VARIABILITY OF WINTER CYCLONE FREQUENCY IN THE ATLANTIC SECTOR

M. Yu. Bardin

Major variability modes of the geographic distribution of the frequency of surface cyclones in the North Atlantic in January are investigated. The modes are expressed by the first four empirical orthogonal functions (EOFs) of frequency. Considered: EOF shapes; cyclone frequency patterns at high positive and negative EOF scores (principal components); and corresponding fields of the mean circulation and synoptic-scale disturbances in the middle troposphere. The leading variability mode (explains 15% of the total variance) is shown to describe fluctuations of cyclone tracks about the Newfoundland—Iceland axis. Distribution functions of the third and fourth principal components, which represent fluctuations of the total cyclone frequency over the North Atlantic and its western part, respectively, appear to be multimodal. It is shown that major features of variations of the distribution of synoptic disturbances in the troposphere generally correspond to the variability of the surface cyclone frequency: both are associated with certain features of the mean circulation.