Mixed Rossby–Gravity Waves Triggered by Lateral Forcing

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  • 1 Program in Atmospheric and Oceanic Sciences, University Of Colorado, Boulder, Colorado Centro de Ciencias de to Atmosfera, Universidad Nacional Autonoma de Mexico, Mexico D.F.
  • | 2 Department of Atmospheric Sciences, University of California, Los Angeles, Los Angeles, California
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Abstract

The mechanisms associated with the excitation of mixed Rossby-gravity waves (MRGWs) in the upper troposphere are studied using wind and outgoing longwave radiation (OLR) data from 1979 to 1991. The largest anomalies in meridional wind associated with MRGWs at 200 mb generally appear in the Northern Hemisphere summer–fall periods and they are pronounced in the central/eastern Pacific where equatorial westerlies form. The OLR field in the intertropical convergence zone shows a spectral peak with time and space scales similar to those of MRGWs at 200 mb. However, tropical convective activity does not show a clear contrast between years of strong and weak 200-mb MRGW activity.

During the Northern Hemisphere summer, weak easterlies or westerlies often form over the equatorial central/eastern Pacific allowing disturbances to propagate from the Southern Hemisphere midlatitudes into the deep Tropics. Some of these disturbances that possess spatial and temporal scales similar to those of the observed MRGWs (zonal wavenumber 4–6 and period 5–7 days) appear to project onto MRGWs. MRGWs are then intensified when the flow associated with them, and that with extratropical disturbances, are favorably superposed. The extratropical disturbances propagating into the Tropics possess a baroclinic vertical structure in the midlatitude troposphere. As they approach the Tropics, the disturbances appear to confine themselves to the upper troposphere under the effect of the “easterly dome.”

Abstract

The mechanisms associated with the excitation of mixed Rossby-gravity waves (MRGWs) in the upper troposphere are studied using wind and outgoing longwave radiation (OLR) data from 1979 to 1991. The largest anomalies in meridional wind associated with MRGWs at 200 mb generally appear in the Northern Hemisphere summer–fall periods and they are pronounced in the central/eastern Pacific where equatorial westerlies form. The OLR field in the intertropical convergence zone shows a spectral peak with time and space scales similar to those of MRGWs at 200 mb. However, tropical convective activity does not show a clear contrast between years of strong and weak 200-mb MRGW activity.

During the Northern Hemisphere summer, weak easterlies or westerlies often form over the equatorial central/eastern Pacific allowing disturbances to propagate from the Southern Hemisphere midlatitudes into the deep Tropics. Some of these disturbances that possess spatial and temporal scales similar to those of the observed MRGWs (zonal wavenumber 4–6 and period 5–7 days) appear to project onto MRGWs. MRGWs are then intensified when the flow associated with them, and that with extratropical disturbances, are favorably superposed. The extratropical disturbances propagating into the Tropics possess a baroclinic vertical structure in the midlatitude troposphere. As they approach the Tropics, the disturbances appear to confine themselves to the upper troposphere under the effect of the “easterly dome.”

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