ITCZ Breakdown and Its Upscale Impact on the Planetary-Scale Circulation over the Eastern Pacific

Date

2017

Authors

Yang, Qiu
Majda, A. J.
Khouider, Boualem

Journal Title

Journal ISSN

Volume Title

Publisher

Journal of the Atmospheric Sciences

Abstract

The eastern Pacific (EP) intertropical convergence zone (ITCZ) 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 multiscale 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 upscale impact of mesoscale disturbances. In an idealized scenario where the heating only varies on the mesoscale, key features of the ITCZ breakdown in the baroclinic modes are captured. The eddy flux divergence of zonal momentum is characterized by midlevel (low level) eastward (westward) momentum forcing at subtropical latitudes of the Northern Hemisphere and opposite-signed midlevel momentum forcing at low latitudes. Such upscale impact of mesoscale fluctuations tends to accelerate (decelerate) planetary-scale zonal jets in the middle (lower) troposphere. Compared with deep heating, shallow 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 where the heating also varies on the planetary scale, the most significant zonal velocity anomalies are confined in the diabatic heating region.

Description

Keywords

Intertropical convergence zone, Atmospheric circulation, Baroclinic flows, Deep convection, Meridional overturning circulation, Numerical analysis/modeling

Citation

Yang, Q., Majda, A. J., & Khouider, B. (2017). ITCZ Breakdown and Its Upscale Impact on the Planetary-Scale Circulation over the Eastern Pacific. Journal of the Atmospheric Sciences, 74(12), 4023-4045. https://doi.org/10.1175/JAS-D-17-0021.1