A Dynamical Model of the Intertropical Convergence Zone

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Abstract
A tropical-wave, zonal-flow interaction model is used to test the hypothesis that through the CISK process the “critical-latitude mechanism” (mass convergence in the tropical planetary boundary layer tends to concentrate around the latitude where the Coriolis frequency equals the wave frequency) is responsible for the development of the ITCZ. The wave, zonal-flow interaction model is formulated as a. multi-level numerical model from the ground (sea surface) to the top of the boundary layer, which is set at 5.5 km. The primitive equations aroused for the interior dynamics of the boundary layer. These are coupled with the vorticity equation applied at the top through a CISK parameterization. The model is then integrated in time, using observed wave scales. The results show that for the case of asymmetric mode waves (pressure asymmetric about the equator), an ITCZ consisting of both the mean and perturbation components is developed 1°−3° north of a critical latitude which corresponds to the maximum Doppler-shifted frequency of the waves. This northward displacement is attributed to the latitude dependence of the heating efficiency of the boundary layer pumping. The development of the ITCZ does not take place if the waves are symmetric about the equator. In both cases, the existence of an “equatorial region,” characterized by zonal mean and perturbation wind structures different from those of the mid-latitude Ekman layer, is found. This is in accord with observations of time-mean radiosonde data at some stations near the equator. These results thus indicate that the critical-latitude mechanism may be important for the development of the ITCZ when cross-equatorial perturbation flow takes place.

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