Square Island on Aqua Planet: mechanisms of expansion of subtropical continental dry zones under CO2 increase

Subtropical land regions are projected to experience drying under increasing greenhouse gas concentrations due to widening of the tropical circulation. The magnitude and mechanisms of this response vary strongly across different regions. Using an idealised set-up for an atmospheric general circulation model coupled to a slab ocean, we investigate how the latitudinal position of a simplified square subtropical continental land mass influences the formation, extent and CO₂ sensitivity of continental dry zones (CDZ). For all land positions, a continental dry zone emerges on the equatorward side of the land mass in boreal summer, extending significantly further poleward than the zonally symmetric edge of the Hadley cell. The poleward extent of the emerging CDZ is consistently constrained to a narrow latitude band in which subtropical subsidence weakens and midlatitude eddy activity increases. The amount of CDZ widening under CO₂ increase strongly depends on the type of climatic dry zone established over land. Land configurations that produce persistent all-year round arid, continental-type dry climates exhibit weak sensitivity to circulation changes, while Mediterranean-type dry climates show enhanced dynamical drying associated with poleward CDZ expansion. These results provide a unifying framework for understanding why robust subtropical land drying in observations and projections is confined to very specific regions. The importance of differentiating continental dry zones by their climate regime is highlighted, underlining the heightened sensitivity of Mediterranean-type dry climates to circulation-driven drying under climate change.