Geophysical Research Letters, 27, 3857-3860, 2000.

Intrusions into the Tropical Upper Troposphere.

Darryn W. Waugh
Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, MD

Lorenzo M. Polvani
Department of Applied Physics and Applied Mathematics, Columbia University, New York

Abstract

Regions of upper tropospheric equatorial westerly winds, typically observed over the Pacific and Atlantic Oceans during northern fall to spring, are important for extratropical-tropical interactions. This paper focuses on one feature of these ``westerly ducts'' that has received relatively little attention to date: the occurrence of Rossby wave breaking events that transport tongues of extratropical air deep into the tropics, mix tropical and subtropical air, and can affect deep convection. A climatology of these ``intrusion'' events formed from 20 years of meteorological analyses shows a strong dependence on the basic-state flow. Notably, intrusion events are found to occur almost exclusively within westerly ducts, with more events in the presence of stronger equatorial westerlies. It is also found that there is strong interannual variability in the frequency of Pacific events, with fewer events during the warm phases of ENSO (consistent with the changes in the basic flow). Since these intrusion events laterally mix trace constituents and have been linked to tropical convection, their spatial and temporal variability may cause related variability in the distribution of trace constituents and tropical convection.



Example of stratospheric intrusion into tropical upper troposphere during January 26 to February 1, 1997.



Histograms showing the number of intrusion events per (a) month, (b) longitude, and (c, d) year (NDJFM winter) for Pacific and Atlantic events, respectively. The solid (unfilled) bars show northern (southern) hemisphere events. Curves correspond to climatological equatorial zonal winds at 350 K.

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