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James R. Angel
and
Scott A. Isard

Abstract

Case studies have shown that the Great Lakes can intensify and alter the speed of passing cyclones in winter by contributing latent and sensible heat to the storms. However, the influence of the Great Lakes on cyclones has not been systematically examined using an extensive dataset. In this research, a National Climate Data Center dataset for the period 1965–90 was used to examine the rate of movement and change in mean sea level pressure of 583 cyclones as they passed over the Great Lakes.

The Great Lakes had a strong effect on the passing cyclones during the ice-free/unstable season from September through November. As cyclones approached the lakes during this season, they accelerated. Once in the Great Lakes region, their rate of intensification increased (the change in pressure tendency at the center of the cyclone was negative). The acceleration into the region was less for cyclones during the ice-cover/unstable season, and rates of intensification for these cyclones did not change within the region. Cyclones that traversed the Great Lakes region during the stable season from May through July exhibited essentially the same behavior as those in the ice-free/unstable season.

The authors’ results for the unstable seasons (ice free and ice cover) are consistent with previous modeling case studies of the influence of the Great Lakes on passing cyclones. Because the lakes are generally cooler than the overriding air during spring and summer, a satisfactory explanation for the influence of the Great Lakes on cyclones during the stable season is not apparent.

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Scott A. Isard
,
James R. Angel
, and
Geoffrey T. VanDyke

Abstract

The zones of origin for all cyclones that traversed the Great Lakes region from 1899 to 1996 are analyzed using a digital daily record of central pressure and location for individual cyclones. Plots of latitude of formation show that Great Lakes cyclones form (or reform) east of the Rocky Mountains at all latitudes between 25° and 65°N. In winter, about the same number of cyclones originate to the northwest as to the southwest of the Great Lakes region. In spring, the southwest zone is dominant. The number of summertime cyclones is greatly reduced, with the west zone of origin most active, while the fall plot displays a transition between the summer and winter distributions. The proportion of strong Great Lakes cyclones that originate in the southwest zone is greater than for all cyclones; however, the seasonal shifts in the latitudinal distributions of origin in the two datasets are similar.

An analysis of differences in frequencies by zone of origin for Great Lakes cyclones during months characterized by positive and negative Pacific–North American (PNA) index patterns reveals a statistically significant relationship between the midtropospheric flow pattern and cyclogenesis. The results indicate that the number of cyclones per month for the positive (PNA index > 0.5) and negative (PNA index < −0.5) categories are approximately equal and that the combined frequencies for positive and negative PNA pattern categories for the northwest, west, and southwest zones of origin are similar. The study supports the intuitive assertion that more Great Lakes cyclones originate from the northwest during months characterized by positive PNA index values than the negative pattern while more cyclones from the west and southwest are associated with the negative PNA index pattern than the positive one.

Approximately 20% of the cyclones that traversed the Great Lakes from 1899 to 1996 originated in the region. The most noteworthy and puzzling finding of the study is that cyclogenesis over the lakes as a proportion of cyclone presence in the region is highest in the summer months. This result corresponds with the finding that cyclones traversing the Great Lakes region in May–July accelerate as they approach the region and increase their rates of deepening over the lakes.

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