Climatology and Changes of Extra-Tropical Storm Tracks and Cyclone Activity: Comparison of ERA-40 with NCEP/NCAR Reanalysis for 1958-2001

In this study, a cyclone detection/tracking algorithm was used to identify cyclones from two gridded 6-hourly mean sea level pressure datasets: the ERA-40 reanalysis and the NCEP/NCAR Reanalysis (NNR) for 1958-2001. The cyclone activity climatology and changes inferred from the two reanalyses are intercompared. The cyclone climatologies and trends are found to be in very good agreement with each other in the northern oceanic areas, as well as in northern Europe and eastern North America. However, significant differences were identified over some land areas, especially the mid-latitudes of Eurasia. Both data sets exhibit problems (with sudden changes or apparent outliers) in mid-latitude Eurasia for years 1963-68. Differences between ERA-40 and NNR are more extensive in the southern hemisphere than in the northern hemisphere. In particular, the ERA-40 shows significantly fewer cyclones over the Antarctic but more cyclones over the southern oceans, especially in the warm season.

The most notable changes in cyclone activity were found to be associated with the numbers and tracks of deep cyclones (central pressure < 990 hPa). In agreement with previous studies, changes during the past half century are characterized by a significant increase in the number of deep cyclones over the North Pacific in JFM and AMJ, and over the North Atlantic in OND and JFM. In particular, the North Atlantic storm track has shifted slightly northward in winter (JFM). Changes in the austral extra-tropics are characterized by more frequent deep cyclones along the Antarctic coastal areas, and fewer deep cyclones to the north of this region (around 60S).