Horizontal and Vertical Structure of Cross-Equatorial Wave Propagation

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 lowe...