Abstract:
Subtropical Anticyclones located over the subtropical oceans of both hemispheres are a crucial element of large-scale atmospheric circulation. Subtropical Anticyclones dominate midlatitude weather and climate by influencing moisture transport, cyclone tracks, sea surface temperature, and precipitation. However, the underlying mechanisms driving the formation and seasonal cycle of Subtropical Anticyclones, as well as how they will respond to global warming still need further investigation, especially in the Southern Hemisphere. The South Pacific subtropical anticyclone is the only subtropical anticyclone that maximizes in area and strength during austral summer, whereas the other four subtropical anticyclones peak in intensity during boreal summer with strong Northern Hemisphere monsoonal heating. Given that the Southern Hemisphere summer monsoon is relatively weak due to less Southern Hemisphere landmass, this raises the question: what drives the austral summer intensification of the South Pacific subtropical anticyclone? Using reanalysis data and heating experiments conducted with an atmospheric general circulation model, the first component of this thesis finds that increased heating over the South Pacific Convergence Zone triggers a Rossby wave response that increases the strength of the South Pacific subtropical anticyclone in austral summer. The second component of this thesis investigates the mechanisms that drive changes in the Southern Hemisphere subtropical anticyclones under global warming within the CMIP5 & CMIP6 multi model ensembles. We find that a combination of local diabatic heating and baroclinicity changes (through static stability) drive the projected changes in Southern Hemisphere subtropical anticyclones under future global warming scenarios. Finally, sensitivity experiments conducted with an atmospheric general circulation model show that slow subtropical sea surface temperature warming primarily forces the projected changes in subtropical anticyclones through baroclinicity change. Fast CO2 atmospheric radiative forcing on the other hand plays a secondary role, with the exception of the South Atlantic subtropical anticyclone in austral winter, where it opposes the forcing by sea surface temperature changes resulting in a muted net response. Lastly, we find that tropical diabatic heating changes only significantly influence Southern Hemisphere subtropical anticyclone changes through tropospheric wind shear changes during austral winter.
