Horizontal and Vertical Structure of Cross-Equatorial Wave Propagation

Robert A. Tomas Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado

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Peter J. Webster Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, Colorado

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Abstract

Observational evidence of interhemispheric wave propagation through the equatorial upper-tropospheric mean westerlies in the eastern Pacific Ocean is found in nine years (1980/81 to 1988/89) of European Centre for Medium-Range Weather Forecasts analyses during boreal winter. Using time mean, standard deviation, and one-point correlation fields of potential vorticity on isentropic surfaces (IPV), it is found that waves associated with local fluctuations with periods between 6 and 30 days propagate from the Northern Hemisphere extratropics, cross the equator, and continue into the Southern Hemisphere extratropics. This result is in agreement with hypotheses that claim regions of time-mean westerlies in the tropics act as “ducts” allowing extratropical Rossby waves to propagate into and through the tropics.

Horizontal structure of the waves appears to change little during the course of the interhemispheric propagation. However, in the lower troposphere where the mean zonal wind is easterly, the lower portion of the extratropical waves do not cross the equator but stop and appear to dissipate locally. The horizontal structure of the lower-tropospheric waves changes dramatically when the waves encounter mean easterlies. Vertical onepoint correlation analyses along the horizontal teleconnection path confirm the differential propagation with height when the waves reach the “easterly dome.”

Linear theory is used to interpret the propagation characteristics of the waves and the changes in their horizontal and vertical structure as they encounter various mean flow distributions. It is suggested that during E1 Niño years, strong interhemispheric wave propagation may be observed in the eastern Atlantic region owing to an increase in the strength of the upper-level time-mean westerlies in that location. It is also suggested that the reverse phenomenon of wave propagation from the Southern Hemisphere into the Northern Hemisphere will most likely be observed when the westerly duct is open and the Southern Hemisphere midlatitudes act as a source of strong Rossby wave activity, such as during boreal fall.

Abstract

Observational evidence of interhemispheric wave propagation through the equatorial upper-tropospheric mean westerlies in the eastern Pacific Ocean is found in nine years (1980/81 to 1988/89) of European Centre for Medium-Range Weather Forecasts analyses during boreal winter. Using time mean, standard deviation, and one-point correlation fields of potential vorticity on isentropic surfaces (IPV), it is found that waves associated with local fluctuations with periods between 6 and 30 days propagate from the Northern Hemisphere extratropics, cross the equator, and continue into the Southern Hemisphere extratropics. This result is in agreement with hypotheses that claim regions of time-mean westerlies in the tropics act as “ducts” allowing extratropical Rossby waves to propagate into and through the tropics.

Horizontal structure of the waves appears to change little during the course of the interhemispheric propagation. However, in the lower troposphere where the mean zonal wind is easterly, the lower portion of the extratropical waves do not cross the equator but stop and appear to dissipate locally. The horizontal structure of the lower-tropospheric waves changes dramatically when the waves encounter mean easterlies. Vertical onepoint correlation analyses along the horizontal teleconnection path confirm the differential propagation with height when the waves reach the “easterly dome.”

Linear theory is used to interpret the propagation characteristics of the waves and the changes in their horizontal and vertical structure as they encounter various mean flow distributions. It is suggested that during E1 Niño years, strong interhemispheric wave propagation may be observed in the eastern Atlantic region owing to an increase in the strength of the upper-level time-mean westerlies in that location. It is also suggested that the reverse phenomenon of wave propagation from the Southern Hemisphere into the Northern Hemisphere will most likely be observed when the westerly duct is open and the Southern Hemisphere midlatitudes act as a source of strong Rossby wave activity, such as during boreal fall.

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