Interannual and decadal variability of Australian monsoon rainfall and variability in the Pacific Ocean

Interannual and decadal variability of Australian monsoon rainfall and variability in the Pacific Ocean

The aim of this thesis is to improve our understanding of key global climate drivers and their connection with Australian monsoon rainfall variability. Specifically, I explore the El Niño-Southern Oscillation (ENSO) and its decadal interaction with the Interdecadal Pacific Oscillation (IPO). I investigate how these two climate modes affect Australian monsoon rainfall (AUMR). Through the review of literature and supporting analysis, ENSO is identified to be the primary remote driver for interannual AUMR variability by altering the zonal winds, as well as location, magnitude and frequency of large-scale tropical convection and rainfall. The strength of the ENSO-AUMR relationship fluctuates on a decadal timescale, aligned with the positive (warm) and negative (cool) phases of the IPO. AUMR also exhibits a significant positive long-term trend. An outcome from this review is the need for further research with regards to drivers of decadal AUMR variability and the need to attribute the cause of the long-term increase in AUMR. To further elucidate the relationship between AUMR variability and Indo-Pacific climate variability, I next investigate the decadal modulation of the ENSO-AUMR teleconnection associated with the IPO, using observational and reanalysis data. For central Pacific ENSO events, there are substantial differences in AUMR between ENSO events that occurred in positive, compared to negative IPO phases. This is due to differences in the large-scale atmospheric circulation and moisture advection during both central Pacific El Niño and central Pacific La Niña events. The research indicates a stronger remote connection between AUMR and the central Pacific during negative IPO phases compared to positive IPO phases. Finally, I show that ENSO events exhibit decadal differences in how they are spatially structured, how strong they are, how long they last and how often they occur. I utilise observations of sea surface temperature anomalies (SSTa) in the Pacific Ocean to demonstrate that the warm equatorial SSTa during eastern Pacific El Niño events are significantly stronger and extend much further into the subtropics in both hemispheres in positive IPO phases compared to negative IPO phases. La Niña events occur more than three times as often in negative than in positive IPO phases. Typically, these kind of events are characterised by meridionally broad cool SSTa around the equatorial Pacific that extend further into both hemispheres during negative IPO phases, in contrast to more equatorially confined SSTa in positive IPO phases. Significant differences in north Pacific SSTa between positive and negative IPO phases during ENSO neutral years indicate that not all of the IPO structure can simply be explained by decadal variations in ENSO activity.

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