The dynamics of the South Pacific split jet in austral winter
The wintertime atmospheric circulation in the southern hemisphere is characterised by a zonally asymmetric, spiral-like pattern. This includes a strong jet over the Indian Ocean region, which bifurcates downstream into subtropical and polar front branches, known collectively as the South
Pacific split jet. The location of the split jet, over Australia and New Zealand, means that this system exerts a considerable influence on the climate of these countries, while the jets also affect the climate of East Antarctica and southern South America. In this thesis I investigate three major themes concerning the split jet.
Firstly, I examine the surface boundary conditions which give rise to the observed split jet structure via a set of idealised, atmosphere-only model
experiments. A particularly novel finding is that the presence of Antarctic
orography plays a key role in shaping the split jet structure. Flattening
Antarctica results in a weakened Indian Ocean jet and the destruction of
the polar front branch of the split jet.
Secondly, I study the low frequency variability of the split jet, with a focus
on the eddy-driven, polar front jet. I find that much of this variability can
be captured by the Southern Annular Mode, while the presence of high
latitude atmospheric blocking is closely correlated with equatorward jet
shifts.
Finally, I evaluate the split jet and atmospheric blocking in CMIP5 mod
els, before examining changes to the jet structure under the RCP8.5 cli
mate change scenario. I find that circulation in the split jet region un
dergoes substantial changes in comparison to other regions, with zonal
wind strengthening between the jets, causing the split jet to become less
distinct. Following this, I demonstrate that these changes can largely be
explained by considering the stationary wave response to changes in the
subtropical Pacific.