748

Multiple indices of Northern Hemisphere Cyclone Activity, Winters 1949-1999

Paciorek, C.J., J.S. Risbey, V. Ventura, R.D.Rosen

Abstract:

This study uses the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis to compare six indices of cyclone activity or forcing for the winters of 1949-1999, over the region $20^{\circ}$-$70^{\circ}$ N. The indices are Eady growth rate and temperature variance, both at 500-hPa, the surface meridional temperature gradient (MTG), the 95th percentile of near-surface wind speed, and counts of cyclones and intense cyclones. The study analyzes the relationships among indices and examines time trends in storm activity. The use of multiple indices allows a comprehensive examination of different aspects of storm activity and forcing and an assessment of the robustness of the results to various definitions of a cyclone index. Results are reported at the regional level and at the full spatial resolution of the NCEP-NCAR reanalysis grid, the latter providing important detail obscured in the regional averages.

The Eady growth rate, temperature variance, and extreme wind indices are reasonably well correlated over the two major storm track regions of the Northern Hemisphere as well as over northern North America and Eurasia, but weakly correlated elsewhere. These indices show moderately strong correlations with the two cyclone count indices over much of the storm tracks when the count index is offset $7.5^{\circ}$ to the north of the other index. Correlations between MTG and all other indices are weak.

The Atlantic, Pacific, and Eurasian regions show either no trend or a decrease in the total number of cyclones; however all show an increase in the number of intense cyclones. The Eady growth rate, temperature variance, and wind indices increase in these regions, except for Eady growth rate in the Atlantic region and wind in Eurasia. Linear regressions at individual grid points show that these three indices increase significantly over the storm tracks and over parts of Eurasia. The two cyclone count indices also increase over the storm tracks, although the increases are not significant. The wind and intense cyclone indices provide some evidence for an increase in impacts from cyclones, primarily over the oceans. The results share features of predictions from some global climate models with enhanced CO₂, such as an increase in intense cyclones and downstream shift in the storm tracks, but they provide little support for other predictions, such as a poleward shift in the storm tracks.