Abstract
The geographical and seasonal distributions of cyclogenesis are studied for the North American sector for the period 1958–77. The primary zone of cyclogenesis is located between 35 and 40°N with this zone providing a maximum in all seasons except summer when a broad maximum exists between 45 and 55°N. Of the individual months, March provides a maximum and September a minimum. The most active areas are: 1) the East Coast, including the Gulf of Mexico coast in winter, 2) Colorado and Great Basin, 3) Alberta and Northwest Territories. A pronounced maximum exists in the Colorado–Great Basin area in March, while at the same time a minimum is present in Alberta–Northwest Territories. As summer approaches the Colorado activity declines markedly while activity in the Alberta–Northwest Territories region increases somewhat. Of the various regions, the greatest concentration of cyclogenesis occurs in the Colorado region. This is attributed to the sharp eastward decline in the terrain in this region and to the fact that the polar jet stream is present at this latitude through much of the year.
The frequency of North American cyclogenesis exhibits a statistically significant decline between 1958 and 1977. The secular trend is examined for each of the four midseason months. The largest downward trend is in January and July, months for which Zishka and Smith (1980) also noted a significant decline. Analysis of the North American data show that the principal contribution to the decline is provided by the region north of 40°N. While a significant decline is present in the Alberta–Northwest Territories activity, no trend is found in the Colorado activity. Although a downward trend is present in the frequency of North American cyclogenesis, data from the entire extratropics of the Northern Hemisphere do not show a trend, implying that the decrease in North American activity has been compensated for by increases elsewhere.
