The dynamics of the South Pacific split jet in austral winter

Patterson, Matthew. 2020 The dynamics of the South Pacific split jet in austral winter. University of Oxford, Oriel College, PhD Thesis, 236pp.

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Abstract/Summary

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.