26th Conference on Hurricanes and Tropical Meteorology

Wednesday, 5 May 2004
ITCZ breakdown in three dimensional flows
Richelieu Room (Deauville Beach Resort)
Chia-chi Wang, University of California, Irvine, CA; and G. Magnusdottir
Poster PDF (2.3 MB)
The Intertropical Convergence Zone (ITCZ) rarely stays the same. A well defined ITCZ undulates and eventually breaks apart. This process is referred to as ITCZ breakdown. The typical timescale of ITCZ breakdown is less than two weeks. It happens most frequently in the tropical East Pacific and may be one of the possible mechanisms of pooling vorticity so that tropical cyclogensis can take place. The suggested mechanism is the following: diabatic heating induces a positive (negative) potential vorticity (PV) anomaly below (above) the heating (eg. Hoskins et al. 1985, Hoskins 1991). Timescale for producing PV anomalies is a few days which is very efficient. Then, the meridional PV gradient changes sign on the poleward (equatorward) side of the heating in the lower (upper) troposphere. Thus, the flow satisfies the necessary conditions for combined barotropic/baroclinic instability. Nieto Ferreira and Schubert (1997, hereafter NFS) found this to be a valid mechanism in a barotropic model with no background flow. Here we extend their work to three dimensions and also consider the effects of background flow.

Reading spectral primitive equation (PE) model is employed. The imposed heating has the same horizontal and temporal structure as in NFS. The vertical structure is designed by two parabolas connected at 600 hPa where the heating has its maximum in the vertical. Three experiments are considered. An experiment with no background flow, an experiment where the heating is imposed in an idealized trade-wind background flow, and thirdly, an experiment with the heating imposed in a climatological background flow.

We found that the process of ITCZ breakdown remains rather barotropic in the baroclinic model. The upper tropospheric PV anomaly is very weak and its lifetime is short. The result of the second experiment shows that the evolution of ITCZ breakdown is significantly accelerated by the idealized trade-wind background flow. It is of interest to see a vortex break off from the western end of the PV strip first. Then, the remaining part of the ITCZ continues breaking in sequence from the west and produces more disturbances. The same behavior was observed in an ITCZ breakdown event that occurred during 23 - 31 Aug. 2000 in the tropical East Pacific. The modeling result suggests that the background flow plays an important role. In the third experiment, the wind field near the eastern end of the PV strip has been influenced by the background flow. On day 8, the cross-equatorial flow heavily distorts the eastern half of the PV strip and greatly weakens the development of the cyclonic flow of the easternmost disturbance. The lifetime of the produced disturbances tends to be still shorter when climatological flow is present. This result may explain why the produced disturbances in the real atmosphere are not as readily observed as one might expect.

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