Alison O'Donnell

Drought variability in the eastern Australia and New Zealand summer drought atlas (ANZDA, CE 1500–2012) modulated by the Interdecadal Pacific Oscillation

Agricultural production across eastern Australia and New Zealand is highly vulnerable to drought, but there is a dearth of observational drought information prior to CE 1850. Using a comprehensive network of 176 drought-sensitive tree-ring chronologies and one coral series, we report the first Southern Hemisphere gridded drought atlas extending back to CE 1500. The austral summer (December–February) Palmer drought sensitivity index reconstruction accurately reproduces historically documented drought events associated with the first European settlement of Australia in CE 1788, and the leading principal component explains over 50% of the underlying variance. This leading mode of variability is strongly related to the Interdecadal Pacific Oscillation tripole index (IPO), with a strong and robust antiphase correlation between (1) eastern Australia and the New Zealand North Island and (2) the South Island. Reported positive, negative, and neutral phases of the IPO are consistently reconstructed by the drought atlas although the relationship since CE 1976 appears to have weakened.

Tree Rings Show Recent High Summer-Autumn Precipitation in Northwest Australia Is Unprecedented within the Last Two Centuries

An understanding of past hydroclimatic variability is critical to resolving the significance of recent recorded trends in Australian precipitation and informing climate models. Our aim was to reconstruct past hydroclimatic variability in semi-arid northwest Australia to provide a longer context within which to examine a recent period of unusually high summer-autumn precipitation. We developed a 210-year ring-width chronology from Callitris columellaris, which was highly correlated with summer-autumn (Dec–May) precipitation (r = 0.81; 1910–2011; p < 0.0001) and autumn (Mar–May) self-calibrating Palmer drought severity index (scPDSI, r = 0.73; 1910–2011; p < 0.0001) across semi-arid northwest Australia. A linear regression model was used to reconstruct precipitation and explained 66% of the variance in observed summer-autumn precipitation. Our reconstruction reveals inter-annual to multi-decadal scale variation in hydroclimate of the region during the last 210 years, typically showing periods of below average precipitation extending from one to three decades and periods of above average precipitation, which were often less than a decade. Our results demonstrate that the last two decades (1995–2012) have been unusually wet (average summer-autumn precipitation of 310 mm) compared to the previous two centuries (average summer-autumn precipitation of 229 mm), coinciding with both an anomalously high frequency and intensity of tropical cyclones in northwest Australia and the dominance of the positive phase of the Southern Annular Mode.

Climatic anomalies drive wildfire occurrence and extent in semi-arid shrublands and woodlands of southwest Australia

Variation in the frequency, extent and intensity of wildfires can drive changes in the composition, structure, diversity and functioning of ecosystems in fire-prone regions worldwide. However, relationships between climatic variation and wildfire occurrence remain poorly understood in many fire- prone regions. We investigated fire occurrence and extent across 15,500 km 2 of semi-arid southwest Australia in relation to inter-annual and/or seasonal variation in regional climate and broad-scale circulation patterns. Superposed epoch analysis (SEA) was used to determine whether wildfire occurrence was related to anomalously high or low regional rainfall or temperature. In particular, we tested if years of minor fire extent (i.e., ,250 km 2 burnt) and major fire extent (i.e., .1,000 km 2 burnt) occurred under different climatic conditions. We also used SEA to determine if wildfires occurred during or following periods of extremes of drivers of regional climate, including the El Nio southern oscillation (ENSO), the Indian Ocean dipole, atmospheric blocking in the adjacent Southern Ocean, and the southern annular mode (SAM). Fire occurrence was linked to dry and hot conditions typically associated with the El Ni˜ no phase of ENSO, with few or no fires in years with cool and wet summers. However, major fire years tended to occur during drought conditions that followed wet and cool conditions in spring and summer of the preceding year. These wet and cool periods were typically associated with the presence of blocking highs in the Southern Ocean to the south of Western Australia. We hypothesise that high rainfall in spring and summer favours the growth of ephemeral plants while subsequent drought conditions promote fuel drying, resulting in more continuous and highly flammable fuel beds capable of sustaining larger fires. Regional climatic patterns are likely driven by interactions among the SAM, atmospheric blocking, and decaying tropical cyclones. As climatic extremes are expected to increase in intensity and frequency in the future, it is likely that the occurrence of extensive wildfires in semi-arid southwest Australia will also increase, potentially driving changes in the distribution and composition of fire-sensitive plant communities.

Evidence for climate‐driven synchrony of marine and terrestrial ecosystems in northwest Australia

The effects of climate change are difficult to predict for many marine species because little is known of their response to climate variations in the past. However, long-term chronologies of growth, a variable that integrates multiple physical and biological factors, are now available for several marine taxa. These allow us to search for climate-driven synchrony in growth across multiple taxa and ecosystems, identifying the key processes driving biological responses at very large spatial scales. We hypothesized that in northwest (NW) Australia, a region that is predicted to be strongly influenced by climate change, the El Niño Southern Oscillation (ENSO) phenomenon would be an important factor influencing the growth patterns of organisms in both marine and terrestrial environments. To test this idea, we analyzed existing growth chronologies of the marine fish Lutjanus argentimaculatus, the coral Porites spp. and the tree Callitris columellaris and developed a new chronology for another marine fish, Lethrinus nebulosus. Principal components analysis and linear model selection showed evidence of ENSO-driven synchrony in growth among all four taxa at interannual time scales, the first such result for the Southern Hemisphere. Rainfall, sea surface temperatures, and sea surface salinities, which are linked to the ENSO system, influenced the annual growth of fishes, trees, and corals. All four taxa had negative relationships with the Niño-4 index (a measure of ENSO status), with positive growth patterns occurring during strong La Niña years. This finding implies that future changes in the strength and frequency of ENSO events are likely to have major consequences for both marine and terrestrial taxa. Strong similarities in the growth patterns of fish and trees offer the possibility of using tree-ring chronologies, which span longer time periods than those of fish, to aid understanding of both historical and future responses of fish populations to climate variation.

Scale-dependent thresholds in the dominant controls of wildfire size in semi-arid southwest Australia

We aimed to characterize the size distribution of naturally occurring wildfires and to determine how fuel characteristics influence wildfire size in a vegetation mosaic of shrublands and woodlands in semi-arid southwest Australia. The shape of frequency-size distributions of fires can be used to elucidate shifts in the dominant drivers or constraints of fire size. We modeled the cumulative frequency-size distribution of wildfires in southwest Australia using a segmented linear model with two break points. Three fire size categories were defined by the break points of the segmented model: “small” ( 41,020 ha). Break points were interpreted as thresholds in fire size that may represent shifts in the dominant controls of fire spread. We tested whether the influence of fuel characteristics on fire spread varied among fire size categories by comparing the composition of vegetation types and fuel ages among burnt and adjacent unburnt areas using selection ratios (ωx). Small and medium fires burnt shrub-dominated vegetation types in proportion to their availability (ωx ≈ 1), but tended to burn smaller proportions of sparsely-vegetated woodlands (ωx 30 years) burnt in proportion to their availability (ωx ≈ 1). Fire size in semi-arid southwest Australia is generally limited by fuel mass in recently burnt areas and by fuel continuity in sparsely vegetated woodlands. However, following unusually wet years and subsequent growth of ephemeral plants, fuels in woodland vegetation likely become spatially continuous and capable of supporting the spread of large fires.

The paleoclimate context and future trajectory of extreme summer hydroclimate in eastern Australia

Eastern Australia recently experienced an intense drought (Millennium Drought, 2003-2009) and record-breaking rainfall and flooding (austral summer 2010-2011). There is some limited evidence for a climate change contribution to these events, but such analyses are hampered by the paucity of information on long-term natural variability. Analyzing a new reconstruction of summer (December-January-February) Palmer Drought Severity Index (the Australia-New Zealand Drought Atlas; ANZDA, 1500-2012 CE), we find moisture deficits during the Millennium Drought fall within the range of the last 500 years of natural hydroclimate variability. This variability includes periods of multi-decadal drought in the 1500s more persistent than any event in the historical record. However, the severity of the Millennium Drought, which was caused by autumn (March-April-May) precipitation declines, may be underestimated in the ANZDA because the reconstruction is biased towards summer and antecedent spring (September-October-November) precipitation. The pluvial in 2011, however, which was characterized by extreme summer rainfall faithfully captured by the ANZDA, is likely the wettest year in the reconstruction for Coastal Queensland. Climate projections (RCP 8.5 scenario) suggest that eastern Australia will experience long-term drying during the 21st century. While the contribution of anthropogenic forcing to recent extremes remains an open question, these projections indicate an amplified risk of multi-year drought anomalies matching or exceeding the intensity of the Millennium Drought.

Multidecadal variations in Southern Hemisphere atmospheric 14C: Evidence against a Southern Ocean sink at the end of the Little Ice Age CO2 anomaly

Northern Hemisphere-wide cooling during the Little Ice Age (LIA; 1650–1775 Common Era, C.E.) was associated with a ~5 ppmv decrease in atmospheric carbon dioxide. Changes in terrestrial and ocean carbon reservoirs have been postulated as possible drivers of this relatively large shift in atmospheric CO2, potentially providing insights into the mechanisms and sensitivity of the global carbon cycle. Here we report decadally resolved radiocarbon (14C) levels in a network of tree-ring series spanning 1700–1950 C.E. located along the northern boundary of, and within, the Southern Ocean. We observe regional dilutions in atmospheric radiocarbon (relative to the Northern Hemisphere) associated with upwelling of 14CO2-depleted abyssal waters. We find the interhemispheric 14C offset approaches zero during increasing global atmospheric CO2 at the end of the LIA, with reduced ventilation in the Southern Ocean and a Northern Hemisphere source of old carbon (most probably originating from deep Arctic peat layers). The coincidence of the atmospheric CO2 increase and reduction in the interhemispheric 14C offset imply a common climate control. Possible mechanisms of synchronous change in the high latitudes of both hemispheres are discussed.