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An Improved Model of the Equatorial Troposphere and Its Coupling with the Stratosphere

Jun-Ichi YanoCenter for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, Massachusetts

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Kerry EmanuelCenter for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, Massachusetts

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Abstract

The Wind-Induced Sea–Air Heat Exchange (WISHE) Model of the 30–60-day oscillation developed by Emanuel is improved by adding downdrafts to the representation of convection and by coupling the troposphere to a passive stratosphere into which equatorial waves may propagate. The downdrafts are associated with a precipitation efficiency that is less than unity; this means that not all of the adiabatic cooling due to ascent in a wave disturbance can be countered by condensation heating, and the wave therefore “feels” a stable stratification as in the work of Neelin et al. As in the latter's model, growth rates of eastward-propagating Kelvin-like modes asymptote to a constant at large zonal wavenumber.

The presence of the stratosphere is shown to have a profound effect on the unstable tropospheric modes. As the upward group velocity is larger for smaller zonal wavelengths, short waves in the troposphere are strongly damped and the most unstable mode shifts to low wavenumbers.

Abstract

The Wind-Induced Sea–Air Heat Exchange (WISHE) Model of the 30–60-day oscillation developed by Emanuel is improved by adding downdrafts to the representation of convection and by coupling the troposphere to a passive stratosphere into which equatorial waves may propagate. The downdrafts are associated with a precipitation efficiency that is less than unity; this means that not all of the adiabatic cooling due to ascent in a wave disturbance can be countered by condensation heating, and the wave therefore “feels” a stable stratification as in the work of Neelin et al. As in the latter's model, growth rates of eastward-propagating Kelvin-like modes asymptote to a constant at large zonal wavenumber.

The presence of the stratosphere is shown to have a profound effect on the unstable tropospheric modes. As the upward group velocity is larger for smaller zonal wavelengths, short waves in the troposphere are strongly damped and the most unstable mode shifts to low wavenumbers.

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