Atmospheric connection between the Australian climate and sea-salt aerosol concentration in East Antarctic snowfall

Hydroclimate extremes like persistent droughts and intense rainfall threaten water and food security, infrastructure, and human health and lives. Development of appropriate polices and infrastructure to mitigate the impact of future hydroclimate extremes in Australia is challenging due to a combination of substantial natural climate variability and relatively short instrumental climate records (~100 years). These factors also make it difficult to determine the underlying climate dynamics associated with hydroclimate extremes, especially the onset and termination of droughts. This limits our ability to evaluate the representation of past, current, and future hydroclimate extremes in climate model simulations.
Multi-century palaeoclimate proxy records sensitive to rainfall variability (e.g., tree rings) allow us to extend hydroclimate records beyond the short instrumental period, providing a longer-term context to the frequency, duration, and magnitude of hydroclimate extremes. However, suitable hydroclimate proxies with seasonal to annual resolution that extend beyond 500 years are rare in Australia. Therefore, exploring the possibility of remote palaeoclimate records with climatological links to Australia is critically important. Previous research shows that the concentration of sea salt aerosol preserved in ice cores from Law Dome, East Antarctica, are a useful proxy for regional wind speed and associated weather systems across the Southern Ocean. At the Law Dome, Dome Summit South (DSS) ice core site, sea-salt aerosol concentrations preserved during austral summer correlate significantly with annual rainfall variability in subtropical eastern Australia. However, the physical mechanisms underpinning this connection have not been established.
This thesis investigates the climate mechanisms that link the Law Dome summer sea-salt (LD_sss) aerosol concentration with an Australian rainfall signal. It was hypothesised that the teleconnection between the two continents occurs at a synoptic weather scale, through a combination of favourable weather patterns that influence the strength and position of the mid-latitude westerly winds in the Law Dome region and the surface wind regime over eastern Australia. To investigate this, synoptic weather typing was used to define the dynamic relationship between East Antarctica and Australia. Self-organising maps (SOM) grouped daily 500-hPa geopotential height anomalies into nine regional synoptic types based on their dominant patterns over the southern Indian Ocean (30- 75°S, 40-180°E) from January 1979-October 2018. The local meteorology of each synoptic type was then explored over an extended study domain (10-75°S, 40-180°E) to include subtropical eastern Australia, with a focus on precipitation and surface wind variability during late austral spring?summer (November to February). Precipitation and surface wind variables were chosen as they influence the generation, transport, and deposition of sea-salt aerosols through snowfall at the Law Dome ice core site. Time series analysis was also used to explore relationships between seasonal synoptic type frequency, LD_sss variability, large-scale modes of climate variability (El Niño-Southern Oscillation, Southern Annular Mode (SAM) and Indian Ocean Dipole), Antarctic stratospheric polar vortex strength and seasonal breakdown, and selected eastern Australian climate variables (rainfall, temperature, soil moisture, humidity, and Forest Fire Danger).
The average precipitation and surface wind anomalies of two self-organised maps (SOM3 and SOM7) supported the hypothesis and were consistent with LD_sss lower and upper tercile composite conditions respectively. Decreased ice core sea-salt aerosol concentrations and drier eastern Australian conditions (reflected in SOM3) are associated with an equatorward shift in the mid?latitude westerly winds, consistent with negative SAM conditions. In contrast, increased ice core sea?salt aerosol concentrations and wetter conditions in eastern Australia (reflected in SOM7) are associated with a regional, asymmetric contraction of the mid-latitude westerly winds. This asymmetric contraction reflects part of the positive SAM signal and is a key component to the direct synoptic scale connection between the two continents. The asymmetry provides favourable conditions for increased westerly wind strength and sea-salt aerosol production upstream of Law Dome at the same time as moist surface easterly geostrophic flow into subtropical eastern Australia and an upper-level trough over southeast Australia enhancing vertical uplift. These conditions result in widespread rainfall across subtropical eastern Australia. The seasonal synoptic persistence enabling the teleconnection appears to be driven by the strength and breakdown timing of the Antarctic stratospheric polar vortex in austral spring. Both the LD_sss concentration record, and the key synoptic types (SOM3, SOM7) were significantly correlated with the strength and seasonal breakdown timing of the Antarctic stratospheric polar vortex. Decreased sea-salt aerosol concentrations and increased SOM3 frequency are more likely during weakened vortex years with early seasonal breakdowns. In contrast, increased ice core sea-salt aerosol concentration and SOM7 frequency are more likely during strengthened vortex years with delayed seasonal breakdowns.
The identified synoptic connection and link with the Antarctic stratospheric polar vortex suggests that the LD_sss record could provide a 2000-year perspective for additional climate applications in the southern Indian and western Pacific sector, including East Antarctica, southern and eastern Australia, and New Zealand. As a case study, relationships between LD_sss and fire promoting weather and climate conditions in Australia were explored and found to be significantly correlated with late austral spring-summer fire hazard and related climate variables across eastern Australia. The strength of the significant correlation between LD_sss and fire hazard across southern and eastern Australia was stronger than the rainfall teleconnection. Increased sea-salt aerosol concentrations are associated with reduced fire hazard in subtropical eastern Australia and increased fire hazard in southwest Tasmania, consistent with the dipole between the two regions associated with positive SAM and La Niña conditions. Conversely, decreased sea-salt aerosol concentrations are associated with increased fire hazard in subtropical eastern Australia and decreased fire hazard in southwest Tasmania.
The combination of a synoptic bridge linking East Antarctica and Australian climate, and the annually resolved multi-centennial ice core record (2000+ years) provides an opportunity for a dynamically based hydroclimate reconstruction that extends the ~100-year instrumental climate records in eastern Australia. This enables a long-term baseline of plausible hydroclimate extremes to be derived which assists in disentangling the influence of natural climate variability and anthropogenic climate change on recent droughts, fires, and floods. The synoptic-scale weather pathway demonstrated in this thesis may also assist in evaluating the representation of dynamic rainfall processes in historical climate model simulations to help reduce uncertainties associated with regional rainfall projections across eastern Australia. Furthermore, this research highlights possibilities that climate records (e.g., rainfall, temperature, fire hazard) for other Southern Hemisphere continents could be derived from annually resolved Antarctic coastal ice cores.