Cross-Equatorial Response to Middle-Latitude Forcing in a Zonally Varying Basic State

Peter J. Webster Division of Atmospheric Physics, C.S.I.R.O., Aspendale, Victoria Australia

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James R. Holton Department of Atmospheric Sciences, University of Washington, Seattle 98195

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Abstract

A nonlinear model based on the shallow water equations is used to study cross-equatorial propagation of forced waves in the presence of a longitudinally varying time-mean basic-state zonal wind field. It is found that global-scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes. However, if the longitudinally-asymmetric basic state includes a “duet” in which the zonal winds are westerly, waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally-symmetric mean zonal wind remains easterly in the equatorial region. The amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct. The existence and annual variations of a westerly duct region in the upper troposphere in the eastern Pacific appear to account for some features of the low-frequency variability in the Northern Hemisphere.

Abstract

A nonlinear model based on the shallow water equations is used to study cross-equatorial propagation of forced waves in the presence of a longitudinally varying time-mean basic-state zonal wind field. It is found that global-scale planetary waves are unable to propagate past a critical latitude where the mean zonal wind speed vanishes. However, if the longitudinally-asymmetric basic state includes a “duet” in which the zonal winds are westerly, waves of zonal scale less than the zonal scale of the westerly duct may propagate from one hemisphere to the other even though the zonally-symmetric mean zonal wind remains easterly in the equatorial region. The amplitude of the response in one hemisphere to forcing in the opposite hemisphere increases strongly with the magnitude of the westerlies in the equatorial duct. The existence and annual variations of a westerly duct region in the upper troposphere in the eastern Pacific appear to account for some features of the low-frequency variability in the Northern Hemisphere.

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