Response of the Hadley Circulation to Convective Forcing in the ITCZ

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
  • | 2 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado
  • | 3 Naval Environmental Prediction Research Facility, Monterey, California
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Abstract

Through the use of a zonal balance model we investigate the response of the mean meridional circulation to a specified diabatic forcing for both resting and nonresting zonal flows. The use of a potential latitude coordinate and transformed meridional circulation components results in a simplified meridional circulation equation in which the variable coefficients are the normalized potential vorticity and inertial stability. Solutions of this equation illustrate how latent heat release away from the equator forces a winter hemisphere Hadley cell that is more intense than the summer hemisphere cell. This asymmetric response is due primarily to the anisotropy associated with the spatial variation of the inertial stability field. Despite the sensitivity of the meridional circulation to the location and breadth of the forcing, the low latitude thermodynamic response is for the most part insensitive as long as the total latent heat release remains the same.

Numerical solutions of the zonal balance model result in evolving zonal wind and temperature fields that modify the potential vorticity and inertial stability fields. In the vicinity of the ITCZ, the potential vorticity becomes large in the lower troposphere and small in the upper troposphere which, in addition to modifying the response of the meridional circulation, generates the necessary dynamical conditions for wave instability. Since the inertial stability is only slightly modified, however, the basic anisotropy in the response of the meridional circulation remains. At the same time, the evolving zonal wind and temperature fields result in an increasing dynamic efficiency of latent heat release, which leads to an accelerated growth of zonal kinetic energy, especially when the ITCZ is located poleward of 10 degrees latitude.

Abstract

Through the use of a zonal balance model we investigate the response of the mean meridional circulation to a specified diabatic forcing for both resting and nonresting zonal flows. The use of a potential latitude coordinate and transformed meridional circulation components results in a simplified meridional circulation equation in which the variable coefficients are the normalized potential vorticity and inertial stability. Solutions of this equation illustrate how latent heat release away from the equator forces a winter hemisphere Hadley cell that is more intense than the summer hemisphere cell. This asymmetric response is due primarily to the anisotropy associated with the spatial variation of the inertial stability field. Despite the sensitivity of the meridional circulation to the location and breadth of the forcing, the low latitude thermodynamic response is for the most part insensitive as long as the total latent heat release remains the same.

Numerical solutions of the zonal balance model result in evolving zonal wind and temperature fields that modify the potential vorticity and inertial stability fields. In the vicinity of the ITCZ, the potential vorticity becomes large in the lower troposphere and small in the upper troposphere which, in addition to modifying the response of the meridional circulation, generates the necessary dynamical conditions for wave instability. Since the inertial stability is only slightly modified, however, the basic anisotropy in the response of the meridional circulation remains. At the same time, the evolving zonal wind and temperature fields result in an increasing dynamic efficiency of latent heat release, which leads to an accelerated growth of zonal kinetic energy, especially when the ITCZ is located poleward of 10 degrees latitude.

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