Variability of the North Atlantic Cyclone Activity in Winter Analyzed from NCEP–NCAR Reanalysis Data

Principal component analysis is applied to the cyclone density over the North Atlantic in winter analyzed with an objective cyclone identification and tracking algorithm by using the 6-h National Centers for Environmental Prediction reanalysis data from 1958 to 1998. Regressions of the cyclone density, deepening rate, moving speed, and central pressure gradient with the first principal component show that the cyclone activity over the northern North Atlantic exhibits a significant intensifying trend along with a decadal timescale oscillation in winter during the past 40 yr. All these variables vary consistently with larger (smaller) cyclone density corresponding to stronger (weaker) cyclone intensity, faster (slower) moving speed, and stronger (weaker) deepening rate. Analysis shows that the variations of the cyclone activity over the North Atlantic are closely related to the changes of large-scale baroclinicity at the lower troposphere and the North Atlantic oscillation. The relationships with the change of the North Atlantic SST are also discussed.