Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data

The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the period from 1958 to 1999, u sing software which provides improved accuracy in cyclone identification in comparison to numerical tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North America and the Arctic. Analysis of Northern Hemisphere cyclones shows sec ular and decadal-scale changes in cyclone frequency, intensity, lifetime an d deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity and deepening during the 42-year period, alt hough the eastern Pacific and continental North America demonstrate opposit e tendencies in most cyclone characteristics. There is an increase of the n umber of cyclones in the Arctic and in the western Pacific and a downward t endency over the Gulf Stream and subpolar Pacific. Decadal scale variabilit y in cyclone activity over the Atlantic and Pacific exhibits south-north di pole-like patterns. Atlantic and Pacific cyclone activity associated with t he NAO and PNA is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s, associate d with considerable changes in European storm tracks. The PNA is largely li nked to the eastern Pacific cyclone frequencies, and controls cyclone activ ity over the Gulf region and the North American coast during the last two d ecades. Assessment of the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerica l procedures are not negligible.