Robert R. Dickson

North American Influences on the Circulation and Climate of the North Atlantic Sector

Abstract During the post-war period the pressure field at Greenland has been characterized by long-sustained winter regimes of alternating high and low pressure, with important effects on the winter climate of Europe. Although these alternations of pressure anomaly at Greenland may be shown to be associated with periods when the pattern of long waves in the upper westerlies showed a general reversal over much of the Northern Hemisphere, it is also suggested that within this hemispheric pattern of change, contemporary variations of winter climate along the Atlantic seaboard of North America have exerted an important influence on the pressure field at Greenland and, through teleconnections, elsewhere (e.g., the North Atlantic and Europe). Comparing months of extreme winter warmth and cold over the southeastern United States it is shown that changes in the strength of the baroclinic field at the coast are associated with major changes in the distribution of winter storms. More specifically, during winters of...

WEATHER NOTE: TORNADOES OF JANUARY 21, 1959—A FEATURE OF A WEATHER SINGULARITY?

Abstract The tornadoes of January 21, 1959, in Tennessee and neighboring States are considered in relation to the New England January thaw singularity described by Wahl [1]. This singularity, in the form of a warm spell, is shown to occur on the average on January 20–22 at Nashville. It is shown that at the time of the singularity there coexist on the average in the Tennessee area certain conditions favorable for the formation of severe storms. These include a tongue of warm, moist air at the surface, a wind shift from southerlies during the warm period to cool northwesterlies immediately afterward, a 500-mb. trough to the west with southwesterly winds and contour inflection point over the Tennessee area, and the presence of a jet stream aloft. Review of past records reveals that tornadoes in the Arkansas, Alabama, Mississippi, Tennessee, and Kentucky area have occurred more often during the time of the singularity, January 20–22, than at any other time of the month.

A Preliminary Analysis of Factors Affecting the Frequency of August Southeastern worth Pacific Tropical Storms and Hurricanes Since the Advent of Satellite Observations

Abstract Satellite-augmented observations of tropical storm and hurricane frequencies in the southeastern North Pacific during recent Augusts (1966–1974) were compared to various environmental factors. On a regional scale, a relatively strong mean 700 mb ridge from the Gulf of Mexico to Baja California was found to accompany high storm frequency. The linear correlation coefficient between storm frequency and a measure of the strength of this ridge amounted to 0.77. On a larger scale, the 700 mb Subtropical Westerlies Index (20°N to 35°N) for west longitudes from 0° to 180° had a somewhat stronger relation to storm frequency (r=0.86). Average August sea surface temperature in the vicinity of storm formation was poorly correlated with storm frequency. This suggests that the unfailingly warm August water temperatures in this area— always exceeding 27°F—were not a limiting factor in storm development.