Recent studies, based largely on analyses of reanalysis datasets, suggest that the Northern Hemisphere winter storm track activity has increased significantly during the second half of the twentieth century. In this study, an attempt is made to validate this increasing trend using surface ship observations. GCM simulations have also been examined in order to understand the observed changes.

Mean sea level pressure observations made by ships, archived as part of the reanalysis project conducted by the National Center for Environmental Prediction/ National Center for Atmospheric Research, have been analyzed. Consistent with previous studies, the observational errors of ship pressure observations are found to be very large during the late 1960s and early 1970s. Without correcting for observational errors, the storm track activity over the Pacific, computed based on ship observations, is found to be decreasing with time, while the upward trend in the Atlantic is much smaller than that found in the reanalysis data. Even after corrections have been made to account for secular changes in observational error statistics, the ship-based trend in the Pacific is still found to be much smaller than that found in the reanalysis, while over the Atlantic, the corrected ship-based trend is consistent with that found in the reanalysis.

An ensemble of 10 GCM simulations with SST distribution prescribed for the period 1950-2000 has been examined. The model response to SST forcing does not show any sign of significant increase in storm track activity, suggesting that the observed increase in storm track activity is probably not forced by SST changes alone. The 500 winters of GCM simulations are used to construct a linear model based on canonical correlation analysis (CCA), using monthly mean distribution of MSLP anomalies as predictor to hindcast monthly mean MSLP variance. The Atlantic storm track in the CCA model hindcast is well correlated with the storm track in the reanalysis in both interannual and decadal time scales, with the hindcast trend being 82% of that found in the reanalysis. Over the Pacific, the CCA hindcast does not perform as well, and the hindcast trend is only 32% of that found in the reanalysis. Currently, more GCM simulations archived as part of the IPCC assessment process are being analyzed to examine whether the increase in storm track activity may be related to global warming.

The results of this study suggest that the actual trend in Pacific storm track activity is probably only about 20-60% of that found in the reanalysis, while over the Atlantic, the actual trend is likely to be about 70-80% of that found in the reanalysis. Two new storm track indices which should contain less bias in the secular trends have been defined based mainly on ship observations.