Storm Track Variations As Seen in Radiosonde Observations and Reanalysis Data

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.

The 1960s Drought and the Subsequent Shift to a Wetter Climate in the Catskill Mountains Region of the New York City Watershed*

“…The strengthening in the 20CR is also evident but much weaker. This is very interesting as it provides yet further evidence that the strengthening is real as the 20CR uses only SLP data and excludes the radiosonde data analyzed by Harnik and Chang (2003). Harnik and Chang (2003) were unable to provide a physical explanation for the strengthening but do note that its timing is similar to the negative to positive NAO transition already discussed.…”

“…The storm track strengthened in the early 1970s across the North Pacific Ocean, North America, and the North Atlantic Ocean with maximum strengthening from the western Pacific to the eastern Atlantic. The strengthening was later more closely examined by Harnik and Chang (2003), who found that it can also be seen in radiosonde data albeit in a much weaker form than in the reanalysis. They concluded that it is probably genuine and not an artifact of the observing and reporting systems.…”

Sensitivity of US air quality to mid-latitude cyclone frequency and implications of 1980–2006 climate change

“…Various metrics can be used to diagnose mid-latitude cyclone activity, including eddy kinetic energy (Hu et al, 2004), temporal variability of sea-level pressure, temperature or meridional wind (Harnik and Chang, 2003), the meridional temperature gradient, and the Eady growth rate (Paciorek et al, 2002). Some studies have used the probability distribution of sea-level pressure as a diagnostic of cyclone frequency (Murazaki and Hess, 2006;Lin et al, 2008;Racherla and Adams, 2008).…”

Emission inventories and projections

“…It has been suggested that decreasing mid-latitude cyclone frequency over the 1980-2006 period may have offset by half the ozone air quality gains in the northeastern U.S. from reductions in anthropogenic emissions . This decrease in mid-latitude cyclone frequency has been associated with an increase at high-latitudes or a poleward shift of the storm track [McCabe et al, 2001] and a positive trend in the Northern Annular Mode or Arctic Oscillation, and NAO [Harnik and Chang, 2003] during the 1970s through the 1990s. The poleward movement of the storm track is consistent with the widening of the tropical belt and the Hadley cell as estimated from multiple observational datasets over the past 2-3 decades [Seidel et al, 2008].…”