The Influence of Idealized Topography on Zonally Asymmetric ITCZ Migrations

The South Asian monsoon (SAM), the largest scale monsoon in the Earth's atmosphere, is characterized by an energetically-direct overturning circulation with ascending branch, and associated convergence zone, displaced into the subtropics. The SAM circulation includes the establishment of the Somali Jet (SJ), which has been argued to provide moisture transport essential to the monsoonal precipitation. Against conventional thinking, we have recently shown that removal of the African topography in a comprehensive general circulation model (GCM) results in a weakening of the cross-equatorial mass transport associated with the SJ without a similar weakening of the SAM precipitation (Wei and Bordoni 2016). This highlights how important aspects of the dynamics of the coastally-trapped SJ and its relation to the SAM precipitation strength and location remain poorly understood.

Emerging energetic constraints on the position of the Intertropical Convergence Zone (ITCZ) might help shed light on these open questions. In particular, a recent observational study has shown that a large fraction of the global cross-equatorial energy transport in boreal summer is effected by the SJ (Heaviside and Czaja 2012). The zonally-averaged cross-equatorial energy transport has in turn been related to the position of the zonally averaged ITCZ by several recent studies. While this energetic framework is well understood in the context of zonally symmetric dynamics, the relation between zonally asymmetric transports, such as that provided by the SJ, and zonally localized ITCZs, such as the SAM convergence zone, remains relatively unexplored.

In this study, we perform experiments with an idealized aquaplanet GCM to understand how zonally asymmetric circulations driven by an idealized African-like topography affect the cross-equatorial energy transport and the zonally asymmetric ITCZ location. The simplified model physics and lower boundary allow for conceptual progress in the understanding of the fundamental workings of this relationship.