Storm Track Variations As Seen in Radiosonde Observations and Reanalysis Data

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Abstract
The interannual variations in the Northern Hemisphere storm tracks during 1949–99 based on unassimilated radiosonde data are examined and compared to similarly derived quantities using the NCEP–NCAR reanalysis at sonde times and locations. This is done with the motivation of determining the extent to which the storm track variations in reanalysis data are real. Emphasis is placed on assessing previous findings, based on NCEP–NCAR reanalysis data, that both storm tracks intensified from the 1960s to the 1990s with much of the intensification occurring during the early 1970s, and that the Atlantic and Pacific storm tracks are significantly correlated. Sonde data suggest that the Atlantic storm track intensified during the 1960s to 1990s, but the intensification was weaker than the reanalysis suggests. The larger trend in reanalysis is due to an overall decrease in biases with time. In the Pacific storm track entrance and exit regions, sonde data show notable decadal timescale oscillations, similar to the reanalysis, but no significant overall positive trend. Sonde data does show a positive trend over Canada, consistent with a Pacific storm track intensification and northeastward shift, but lack of data over the storm track peak prevents drawing any strong conclusions. The biases in the reanalysis are found to have a strong spatial pattern, with the largest biases being over the Pacific entrance region (Japan). The correlation between the Atlantic and Pacific storm tracks in sonde data (which exists mainly over the storm track entrance and exit regions) is not as significant as in the reanalysis, with differences being mostly due to the decadal timescale variations of the storm tracks rather than their year-to-year variations.