The Impact of Interacting Climate Modes on East Australian Precipitation Moisture Sources

Modes of climate variability can drive significant changes to regional climate affecting extremes such as droughts, floods, and bushfires. The need to forecast these extremes and expected future increases in their intensity and frequency motivates a need to better understand the physical processes that connect climate modes to regional precipitation. Focusing on east Australia, where precipitation is driven by multiple interacting climate modes, this study provides a new perspective into the links between large-scale modes of climate variability and precipitation. Using a Lagrangian back-trajectory approach, we examine how El Nino-Southern Oscillation (ENSO) modifies the supply of evaporative moisture for precipitation, and how this is modulated by the Indian Ocean dipole (IOD) and southern annular mode (SAM). We demonstrate that La Nina modifies large-scale moisture transport together with local thermodynamic changes to facilitate local precipitation generation, whereas below-average precipitation during El Nino stems predominantly from increased regional subsidence. These dynamic-thermodynamic processes were often more pronounced during co-occurring La Nina/negative IOD and El Nino/positive IOD periods. As the SAM is less strongly correlated with ENSO, the impact of co-occurring ENSO and SAM largely depended on the state of ENSO. La Nina-related processes were exacerbated when combined with +SAM and dampened when combined with -SAM, and vice versa during El Nino. This new perspective on how interacting climate modes physically influence regional precipitation can help elucidate how model biases affect the simulation of Australian climate, facilitating model improvement and understanding of regional impacts from long-term changes in these modes. Significance StatementHow climate modes modulate the oceanic and terrestrial sources of moisture for rainfall in east Australia is investigated. East Australia is wetter during La Nina because more moisture is transported into the region and is more easily turned into rainfall when it arrives, whereas drier conditions during El Nino are because local conditions inhibit the conversion of moisture into rainfall. Distant atmospheric changes over the Indian and Southern Oceans can intensify these changes. Our results can be used to better understand and predict the regional impact of long-term changes in these modes of climate variability, which are potentially altered under climate change.

Automated summary

This study investigates how climate modes modulate the sources of moisture for rainfall in east Australia. It finds that La Nina leads to wetter conditions in the region due to increased moisture transport and easier conversion into rainfall, while El Nino leads to drier conditions due to inhibiting local conversion of moisture into rainfall. Distant atmospheric changes over the Indian and Southern Oceans can amplify these changes. The results contribute to a better understanding and prediction of the regional impact of long-term changes in these climate variability modes, which can potentially be altered under climate change.