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An Examination of the Frequency and Mean Conditions Surrounding Frontal Incursions into the Gulf of Mexico and Caribbean Sea

Geoffrey J. DiMegoDepartment of Atmospheric Science, State University of New York at Albany, Albany, N.Y. 12222

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Lance F. BosartDepartment of Atmospheric Science, State University of New York at Albany, Albany, N.Y. 12222

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G. William EndersenDepartment of Atmospheric Science, State University of New York at Albany, Albany, N.Y. 12222

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Abstract

Maps of mean monthly frequency and duration of frontal incursions into the Gulf of Mexico and Caribbean Sea are presented for the 1965–72 period. The transition from the low-frequency regime of summer to the high-frequency regime of winter is quite sharp in the fall, occurring between September and October. A more gradual decrease in activity occurs in spring. During the cooler months, relative maxima in frequency exist over the western Gulf of Mexico and east of Florida, while an arch-shaped region of maximum duration extends northeastward from the Yucatan Peninsula into the central Gulf and then southeastward along the north coast of the Greater Antilles with a second maximum in the central Caribbean. The frequency and degree of penetration of cold fronts are directly related to topographic features and the position, strength and amplitude of the mid-latitude circulation.

Time-sections centered around the time of frontal passage are used to present mean data for three regions. Tropical stations experience veering winds, rising temperatures, falling pressures, and increasing moisture content in the 1000–700 mb layer as fronts approach the area in winter. The stability of the atmosphere decreases and the trade-wind inversion is lifted and weakens in intensity. After the passage of the front, cold advection, subsidence and ridging produce an abrupt reversal of all these trends. A composite-case study shows that the depth of cold air to the rear of the front decreases southward and is accompanied by the development of a low-level anticyclone over the Gulf coast. Ahead of the front, the combination of an inverted trough in the central Caribbean and the subtropical anticyclone in the Atlantic produces a return flow of warm, moist tropical air into mid-latitudes.

Abstract

Maps of mean monthly frequency and duration of frontal incursions into the Gulf of Mexico and Caribbean Sea are presented for the 1965–72 period. The transition from the low-frequency regime of summer to the high-frequency regime of winter is quite sharp in the fall, occurring between September and October. A more gradual decrease in activity occurs in spring. During the cooler months, relative maxima in frequency exist over the western Gulf of Mexico and east of Florida, while an arch-shaped region of maximum duration extends northeastward from the Yucatan Peninsula into the central Gulf and then southeastward along the north coast of the Greater Antilles with a second maximum in the central Caribbean. The frequency and degree of penetration of cold fronts are directly related to topographic features and the position, strength and amplitude of the mid-latitude circulation.

Time-sections centered around the time of frontal passage are used to present mean data for three regions. Tropical stations experience veering winds, rising temperatures, falling pressures, and increasing moisture content in the 1000–700 mb layer as fronts approach the area in winter. The stability of the atmosphere decreases and the trade-wind inversion is lifted and weakens in intensity. After the passage of the front, cold advection, subsidence and ridging produce an abrupt reversal of all these trends. A composite-case study shows that the depth of cold air to the rear of the front decreases southward and is accompanied by the development of a low-level anticyclone over the Gulf coast. Ahead of the front, the combination of an inverted trough in the central Caribbean and the subtropical anticyclone in the Atlantic produces a return flow of warm, moist tropical air into mid-latitudes.

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