Monsoons and the Intertropical Convergence Zone in the Presence of Zonal Asymmetries

Two-thirds of the global population lives in regions of the tropics and subtropics where the majority ofrainfall is provided by monsoons. These large-scale circulations dominate summertime climatology in partsof Africa, Asia, Australia and the Americas, playing an essential role in societal, economic and ecologicalwellbeing. However, our understanding of monsoons remains limited: general circulation models (GCMs)exhibit pronounced biases in both present-day (Kitoh et al., 2013) and paleoclimate (Boos & Korty, 2016)monsoon rainfall patterns, and there is stark disagreement between models in projections of future change(Wang et al., 2020). Importantly, improvement in monsoon representation between CMIP phases is lim-ited: common model biases and large inter-model spreads in projections persist in CMIP6. This suggeststhat increasing model complexity and resolution will not by itself resolve the large uncertainty. Improvedunderstanding of the processes and mechanisms governing monsoon rainfall, and their response to externalforcings, is vital.Our theoretical understanding of monsoons has been advanced through their recent re-framing as aglobal phenomenon. Traditionally, they were considered to be continental-scale land-sea breezes, howeverthey are now understood to be tightly coupled to the tropical overturning circulation, forming the over-land component of the seasonal excursions of the Intertropical Convergence Zone (ITCZ) (Biasutti et al.,2018). A substantial body of work has linked the latitude of the zonal-mean ITCZ to the cross-equatorialatmospheric energy transport, and consequently, global energy constraints (e.g. Bischoff & Schneider, 2014).Similar work is ongoing to characterise the zonal-mean ITCZ width and strength (Byrne et al., 2018).Furthermore, monsoon-like behaviour has been produced on a shallow aquaplanet and characterised in termsof angular momentum (e.g. Bordoni & Schneider, 2008). However, such theories disregard the significantzonal variability in the ITCZ, meaning they poorly represent regional climate, particularly during solstitialmonths (Adam et al., 2016). Some work on understanding the impact of zonal asymmetries on the ITCZhas been done, both using simple models (e.g. Priv 'e & Plumb, 2007) and full-complexity GCMs (Maroonet al., 2016), but many unresolved questions remain.
The aim of this project is to study the impact of zonal asymmetries caused by land and SST patterns onthe ITCZ, with a particular focus on results over land, i.e. monsoon regions. To achieve this, an idealisedrepresentation of the Earth's surface coupled to the atmospheric component of a GCM will be used. Present-day and climate change scenarios (4xCO2) will be studied. In both cases, the role of cloud radiative effectswill be considered.CMIP6 and reanalysis data will be used to inform the development of research questions and providea point of comparison. In the later stages of the project, results from the idealised studies will be used tounderstand the behaviour of monsoons in these more complex data sets, and consequently contribute to thenarrowing of uncertainties in monsoon projections.