Subtropical dynamics and change and their influence on the monsoons, especially in the Southern Hemisphere.

Large-scale tropical and extratropical responses to anthropogenic warming are well studied. These include mean poleward shifts of the extratropical jet streams and expansion of Hadley cells. Debate about the interaction between the two in changing the poleward limit of the Hadley cell continues. However, this literature has limited focus on subtropical drying. Hadley cell expansion does appear a leading candidate for the observed and projected winter drying across the mediterranean climates of the Southern Hemisphere. But the most unambiguous drying signal in climate model projections is in the southern monsoons and their extensions during austral spring (September-November). In this contribution, we argue that understanding this drying requires new climate theory developed with a specific subtropical dynamics lens.

Subtropical westerly flow on the edge of tropical convective hotspots allows the propagation of synoptic-scale Rossby waves into low latitudes. The propagation of these extatropical upper-level westerly waves towards the tropics is known to modulate rainfall across subtropical deserts, monsoons, and monsoon extensions.

The unique geographic distribution of ocean and land in the Southern Hemisphere preferentially supports such wave propagation and absorption into three well-defined subtropical convergence zones in the South Pacific, Atlantic, and Indian Oceans. While the subtropical belt is a zone of mean subsidence, hence the large deserts, frequent synoptic-scale interaction between upper-level westerly waves and tropically-sourced warm humid air intermittently overcomes this mean subsidence in these subtropical convergence zones. The resulting tropical-extratropical cloud bands produce much of the rainfall supporting the water resources and agro-economies across southern Africa, the South Pacific Islands, and South America.

Here, we present contemporary trends of decline in these cloud bands which are projected to continue under planetary warming. These trends are most robust in austral spring (October-November), coinciding with delays to the onset of southern monsoons. Declines in cloud bands are partially associated with poleward shifts of the eddy-driven jet, however, analysis of the annual cycle shows that across the CMIP model ensembles the equinoctial switch of the Hadley cell from the southern into the northern hemisphere is delayed about one month. This delayed switch explains a relative enhancement of subtropical subsidence during austral spring which is reflected in monsoonal dynamics, especially over South America and Southern Africa.