Venezuelan Rain Systems and the General Circulation of the Summer Tropics II: Relations between Low and High Latitudes

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  • 1 Natonal Center for Atmospheric Research, Boulder, Colo. 80307
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Abstract

The question, Why are there discrete synoptic weather systems in the tropics without a basic current that can become baroclinically unstable, is considered. Release of kinetic energy through vertical circulations and maintenance of the surface flow against friction by mean values indicate that existence of a simple general circulation cell should be possible. But, in the descending branch the requisite poleward temperature gradient cannot be maintained with radiation cooling. An additional mechanism for heat removal must be sought.

Extratropical troughs extending deep into the tropics may provide such a mechanism. At first their role in setting off tropical rain areas is investigated dynamically. It is found that a large imbalance of forces develops in the northerly current on the western side of the troughs which causes the equator-bound air to turn sharply clockwise. Absolute vorticity approaches zero and intense high-tropospheric divergence is initiated acting as a siphon on the lower layers.

Then the troughs are investigated for their capability of exchanging heat, mass and angular momentum between the tropics and higher latitudes during summer. Two or three discrete troughs around the hemisphere suffice to accomplish these functions mainly through the presence of cyclonic flow trajectories west and anticyclonic trajectories east of the trough axis. Thus, the entire exchange across the latitude circles can be seen as a unified process.

Abstract

The question, Why are there discrete synoptic weather systems in the tropics without a basic current that can become baroclinically unstable, is considered. Release of kinetic energy through vertical circulations and maintenance of the surface flow against friction by mean values indicate that existence of a simple general circulation cell should be possible. But, in the descending branch the requisite poleward temperature gradient cannot be maintained with radiation cooling. An additional mechanism for heat removal must be sought.

Extratropical troughs extending deep into the tropics may provide such a mechanism. At first their role in setting off tropical rain areas is investigated dynamically. It is found that a large imbalance of forces develops in the northerly current on the western side of the troughs which causes the equator-bound air to turn sharply clockwise. Absolute vorticity approaches zero and intense high-tropospheric divergence is initiated acting as a siphon on the lower layers.

Then the troughs are investigated for their capability of exchanging heat, mass and angular momentum between the tropics and higher latitudes during summer. Two or three discrete troughs around the hemisphere suffice to accomplish these functions mainly through the presence of cyclonic flow trajectories west and anticyclonic trajectories east of the trough axis. Thus, the entire exchange across the latitude circles can be seen as a unified process.

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