ITCZ Breakdown and Its Upscale Impact on the Planetary-Scale

1 ABSTRACT 2 The Intertropical Convergence Zone (ITCZ) over the eastern Pacific is sometimes observed to break down into several vortices on the synoptic time scale. It is still a challenge for present-day numerical models to simulate the ITCZ breakdown in the baroclinic modes. Also, the upscale impact of the associated mesoscale fluctuations on the planetary-scale circulation is not well understood. Here a simplified multi-scale model for the modulation of the ITCZ is used to study these issues. A prescribed two-scale heating drives the planetary-scale circulation through both planetary-scale mean heating and eddy flux divergence of zonal momentum, where the latter represents the up-scale impact of mesoscale fluctuations. In an idealized scenario with zon-ally symmetric planetary-scale flow, both deep convective heating and shallow congestus heating are considered. First, several key features of the ITCZ breakdown in the baroclinic modes are captured in this multi-scale model. Secondly, the eddy flux divergence of zonal momentum is characterized by mid-level (low-level) eastward (westward) momentum forcing at high latitudes of the Northern Hemisphere and alternate mid-level momentum forcing at low latitudes. Such upscale impact of mesoscale fluctuations tends to accelerate (decelerate) planetary-scale zonal jets in the middle (lower) troposphere. Thirdly, compared with deep convective heating, shallow congestus heating induces stronger vorticity anomalies on the mesoscale and more significant eddy flux divergence of zonal momentum and acceleration/deceleration effects on the planetary-scale mean flow. In a more realistic scenario with zon-ally varying planetary-scale flow, the most significant zonal velocity anomalies are confined in the diabatic heating region.