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Moisture Bursts over the Tropical Pacific Ocean

James P. McguirkTexas A&M University, College Station, TX 77843

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Aylmer H. ThompsonTexas A&M University, College Station, TX 77843

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Neil R. SmithTexas A&M University, College Station, TX 77843

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Abstract

“Moisture bursts” are bands of high clouds or middle and high clouds extending poleward and eastward from deep tropical locations into subtropical and middle latitudes. These events, synoptic in both temporal and spatial scales, are extremely common, particularly over the North Pacific Ocean.

We define moisture bursts objectively, in primarily geometric terms, to emphasize both their synoptic scale and their tropical-extratropical interaction. We apply this definition to infrared satellite imagery for four 6-month cool seasons (November–April) in the eastern North Pacific (160°E to the west coast of the Americas). The frequency of thew events is about ten bursts per month during normal cool seasons, distributed uniformly across the Pacific to the wen of 110°W; east of this longitude, few moisture bursts occur. Half of the bursts last 2 to 4 days, and no bunt lasted longer than 10 days.

Only 36 moisture bursts occurred during the 6-month El Niño cool season of 1982–83, with the location of occurrence shifted eastward. Few bursts occurred in the region of active tropical convection associated with the El Niño event.

Because moisture burst frequency decreases at times when the ITCZ strengthens, we hypothesize two modes in Hadley cell behavior: a strong zonally symmetric mode, and a weaker mode comprised of the statistical ensemble of a large number of transient moisture bursts. Through analysis of wind fields, zonal averages across moisture bursts are shown to resemble transient intensification of the mean meridional circulation in regions where the Hadley cell is typically weak.

Abstract

“Moisture bursts” are bands of high clouds or middle and high clouds extending poleward and eastward from deep tropical locations into subtropical and middle latitudes. These events, synoptic in both temporal and spatial scales, are extremely common, particularly over the North Pacific Ocean.

We define moisture bursts objectively, in primarily geometric terms, to emphasize both their synoptic scale and their tropical-extratropical interaction. We apply this definition to infrared satellite imagery for four 6-month cool seasons (November–April) in the eastern North Pacific (160°E to the west coast of the Americas). The frequency of thew events is about ten bursts per month during normal cool seasons, distributed uniformly across the Pacific to the wen of 110°W; east of this longitude, few moisture bursts occur. Half of the bursts last 2 to 4 days, and no bunt lasted longer than 10 days.

Only 36 moisture bursts occurred during the 6-month El Niño cool season of 1982–83, with the location of occurrence shifted eastward. Few bursts occurred in the region of active tropical convection associated with the El Niño event.

Because moisture burst frequency decreases at times when the ITCZ strengthens, we hypothesize two modes in Hadley cell behavior: a strong zonally symmetric mode, and a weaker mode comprised of the statistical ensemble of a large number of transient moisture bursts. Through analysis of wind fields, zonal averages across moisture bursts are shown to resemble transient intensification of the mean meridional circulation in regions where the Hadley cell is typically weak.

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