Horizontal and Vertical Structure of Cross-Equatorial Wave Propagation

Observational evidence of inter-hemispheric wave propagation through the equatorial upper tropospheric mean westerlies in the Eastern Pacific Ocean and Eastern Atlantic Ocean is found in nine years (1980-81 to 1988-89) of European Center for Medium Range Weather Forecasting analyses during boreal winter. Use of time mean, standard deviation and one-point correlation fields of potential vorticity on isentropic surfaces (IPV), shows that waves associated with local fluctuations with periods between 6 to 30 days propagate from the Northern Hemisphere extratropics, cross the equator and continue into the Southern Hemisphere extratropics. This inter-hemispheric wave propagation is stronger in the Eastern Pacific than in the Eastern Atlantic and does not occur where the time mean wind is easterly. The horizontal structure of the waves at upper levels appears to change little during the course the inter -hemispheric propagation. However, in the lower troposphere where the mean zonal wind is easterly, the lower portions of the extra- tropical 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. 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 the two-dimensional perspectives of the "westerly duct" and "easterly dome" be combined to yield a three-dimensional perspective: a "westerly gap" in an impenetrable ridge composed of the tropical easterlies. In general, there is not a consistently strong correlation between the strength of the westerlies and the IPV standard deviation on intra-annual time scales. However there is a good correlation between these quantities on inter-annual time scales. An out of phase relationship is found between the activity in the Eastern Pacific and Eastern Atlantic during the different phases of El-Nino/Southern Oscillation (ENSO) with stronger activity occurring in the Eastern Pacific during La-Nina periods, but shifting to the Eastern Atlantic during El-Nino periods. A case study in the Eastern Pacific is included. The signal in the daily IPV maps is easily visible and many aspects of the behavior in this case are consistent with those observed in the one-point correlation results, which adds support to the statistical results. The daily potential temperature values are included on the vertical cross-sections and there is the suggestion that the signal in this component of the IPV decreases as the disturbances enter the tropics. (Abstract shortened by UMI.).