Barotropic Aspects of ITCZ Breakdown

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Abstract
In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a... Abstract In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a...

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