The Influence of Idealized Zonally-Varying Boundary Conditions on Zonally-Asymmetric ITCZ Migrations

One of the most prominent features of the Earth’s climate and large-scale atmospheric circulation is the Intertropical Convergence Zone (ITCZ), a zonally-elongated band of deep convective clouds and intense precipitation at low latitudes. Controls on its mean position and possible migrations on different timescales remain an active area of research. An emerging theoretical framework links the ITCZ to the atmospheric energy balance and transport. While this framework has been well studied and is well understood in the context of zonally symmetric dynamics, the Earth’s atmosphere exhibits significant zonal asymmetry. Observations for instance show that a large fraction of the global cross-equatorial energy transport is effected by the Somali jet (SJ), in conjunction with the development of the large-scale Asian monsoon during Northern Hemisphere (NH) summer. The relationship between these zonally-asymmetric transports and zonally localized ITCZs, such as those associated with monsoons, remains relative unexplored.

In this study, we want to investigate this relationship and explore to what extent it can be understood within the energetic framework. To do this, we perform experiments with a GCM with idealized physics, in which zonal asymmetry is introduced through a zonally asymmetric but hemispherically symmetric continent over an otherwise uniform aquaplanet. The simplified model physics and lower boundary allow for conceptual progress in the absence of poorly understood and constrained feedbacks. We examine the atmospheric energy balance in the oceanic and continental sectors separately and explore its skills in predicting the ITCZ position and structure. We find that even in this very idealized setting, the energetic constraint does not provide a good predictor of the sector mean ITCZ when zonal energy fluxes and the rotational component of the energy transport are large. More local thermodynamic predictors, such as the maximum in lower-level moist static energy, perform better in these cases. Relationships between the atmospheric energy balance and local thermodynamic predictors are investigated.