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THE WEATHER AND CIRCULATION OF JULY 1967

Unusually Cool East of the Divide

ROBERT R. DICKSON

Abstract

No Abstract Available.

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THE WEATHER AND CIRCULATION OF MAY 1965

Severe Storms in Mid-Nation and Continued Drought in the Northeast

ROBERT R. DICKSON

Abstract

No Abstract Available.

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THE WEATHER AND CIRCULATION OF OCTOBER 1965

Long-Wave Progression and Temperature Reversal

ROBERT R. DICKSON

Abstract

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THE WEATHER AND CIRCULATION OF DECEMBER 1965

Including a Discussion of the Record Dry Year in the Northeast

ROBERT R. DICKSON

Abstract

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THE WEATHER AND CIRCULATION OF JUNE 1964

Reversal of a Long-standing Trend Over the Pacific

ROBERT R. DICKSON

Abstract

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THE WEATHER AND CIRCULATION OF MARCH 1968

A Warm Month With Increasing Westerlies

ROBERT R. DICKSON

Abstract

No Abstract Available.

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THE WEATHER AND CIRCULATION OF JUNE 1963

Interplay Between Blocking and Drought in the United States

ROBERT R. DICKSON

Abstract

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ROBERT R. DICKSON

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.

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Robert R. Dickson
and
Robert E. Livezey

Abstract

It is known that the Southern Oscillation Index (SOI) and the mean sea surface temperature off the Peru Coast are highly coherent and that variations of the latter are dominated by infrequent warming episodes. The present study examines the relative contribution of these warming episodes to the covariance of statistically significant correlations between the fall SOI and winter mean 700 mb heights in the Northern Hemisphere. The degree of dominance of the warming episode years in this context is evaluated by Monte Carlo methods.

It was found that, for the 30-year period studied, data pairs following tropical east Pacific warming events contributed disproportionately to major correlation maxima in much of the Northern Hemisphere. Such covariance concentrations, however, were found to be fairly likely outcomes (probability > 9%) if groups of years are chosen at random from the appropriate covariance arrays. Thus, we conclude that the influence of the fall SOI upon the subsequent winter mean 700 mb height distribution is a rather pervasive one, not limited to tropical east Pacific warming situations.

In contrast to other areas, correlation maxima in the North American sector received disproportionately small covariance contributions from the warming episode years. In northwest Canada, the contribution of those years was small and opposite in sign to the total covariance.

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Robert R. Dickson
and
Jerome Namias

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 extreme cold over the southeastern United States and the associated enhanced baroclinicity at the Atlantic seaboard, the zone of peak winter storm frequency is drawn far to the southwest of normal, with a corresponding decrease in cyclonic activity in the Iceland-Greenland area.

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