Impact of drought on temperature extremes in Melbourne, Australia

Journal of Climate, 2014

The Australian decade-long ''Millennium Drought'' broke in the summer of 2010/11 and was considered the most severe drought since instrumental records began in the 1900s. A crucial question is whether climate change played a role in inducing the rainfall deficit. The climate modes in question include the Indian Ocean dipole (IOD), affecting southern Australia in winter and spring; the southern annular mode (SAM) with an opposing influence on southern Australia in winter to that in spring; and El Niño-Southern Oscillation, affecting northern and eastern Australia in most seasons and southeastern Australia in spring through its coherence with the IOD. Furthermore, the poleward edge of the Southern Hemisphere Hadley cell, which indicates the position of the subtropical dry zone, has possible implications for recent rainfall declines in autumn. Using observations and simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), it is shown that the drought over southwest Western Australia is partly attributable to a long-term upward SAM trend, which contributed to half of the winter rainfall reduction in this region. For southeast Australia, models simulate weak trends in the pertinent climate modes. In particular, they severely underestimate the observed poleward expansion of the subtropical dry zone and associated impacts. Thus, although climate models generally suggest that Australia's Millennium Drought was mostly due to multidecadal variability, some late-twentieth-century changes in climate modes that influence regional rainfall are partially attributable to anthropogenic greenhouse warming.

EarthArXiv (California Digital Library), 2023

We examine the characteristics and causes of southeast Australia's Tinderbox Drought (2017-2019) that preceded the Black Summer fire disaster. The Tinderbox Drought was characterised by cool season rainfall deficits of around-50% in three consecutive years, which was exceptionally unlikely in the context of natural variability alone. The precipitation deficits were initiated and sustained by an anomalous atmospheric circulation that diverted oceanic moisture away from the region, despite traditional indicators of increased drought risk in southeast Australia generally being in neutral states. Moisture deficits were later intensified by unusually high temperatures, high vapour pressure deficits and sustained reductions in terrestrial water availability. Anthropogenic forcing intensified the rainfall deficits of the Tinderbox Drought by around 18% with an interquartile range of 34.9% to-13.3% highlighting the considerable uncertainty in attributing droughts of this kind to human activity. Skillful predictability of this drought was possible by incorporating multiple remote and local predictors through machine learning, providing prospects for improving forecasting of multi-year droughts. Teaser Australia's Tinderbox Drought was an unusually extreme natural event that was worsened by human-caused climate change.

Journal of Climate, 2009

The variability of the north Australian wet season is examined by performing cluster analysis on the wind and thermodynamic information contained in the 2300 UTC radiosonde data at Darwin for 49 wet seasons (September–April) from 1957/58 to 2005/06. Five objectively derived regimes of the wet season are obtained and are found to differ significantly in their synoptic environment, cloud patterns, and rainfall distributions. One regime is primarily associated with the trade wind regime. Two regimes are associated with the lead up to and break periods of the monsoon at Darwin. A fourth regime is clearly identified with the active monsoon at Darwin and is offered as a definition of monsoon onset. This regime captures the active monsoon environment associated with significant widespread rainfall. The fifth regime is a mixed regime, with some days associated with the inactive monsoon, a period of westerly zonal winds at Darwin associated with relatively suppressed convection compared with...

International Journal of Climatology, 2004

Drought of 3 to 7 years' duration has devastated the flora, fauna, and regional economies in rangeland grazing districts over eastern and central Australia every 15 to 25 years throughout the 20th century, in some cases degrading the land beyond recovery. Recently, these drought and degradation episodes have been associated with a global interdecadal oscillation (IDO) of period 15 to 25 years. This IDO signal brought cooler sea-surface temperatures (SSTs) to the western extratropical South Pacific Ocean in association with reduced onshore transport of moisture over eastern/central Australia during the summer monsoon. Here, we utilize optimized canonical correlation analysis (CCA) to forecast principal components of summer precipitation (PCP) anomalies over Australia from the persistence of principal components that dominate spring SST anomalies across the Southern Hemisphere. These summer PCP forecasts are cross-validated with the CCA forecast model for each year independent of that year's variability. Resulting cross-validated forecasts are best over Queensland, correlating with those observed at >0.40 from 1890 through to 2001, significant at >99% confidence level. More importantly, 6 of 10 drought episodes (but only three of seven degradation episodes) observed in eastern/central Australia during the 20th century are forecast. luxurious plant and animal growth . Other times, however, the productivity of land for grazing purposes did not return, leaving scalds and woody weeds in its wake . investigated the source of drought and degradation episodes during the summer rainy season over eastern/central Australia. They found summer PCP variability during the 20th century characterized by quasi-biennial, interannual, quasi-decadal, and interdecadal signals. Subsequently, they found these signals associated with corresponding signals in global sea-surface temperature (SST) and sea-level pressure (SLP) anomalies. These global climate signals are the familiar 2 to 3 year period quasi-biennial oscillation (QBO), the 3 to 7 year period El Niño-southern oscillation (ENSO), the 7 to 13 year period quasi-decadal oscillation (QDO), and the 15 to 25 year period interdecadal oscillation (IDO; . found the Pacific extra-tropical cool phase of each of these global climate signals associated with higher SLP over northwest Australia and less onshore geostrophic transport of tropospheric moisture over eastern/central Australia during the summer monsoon. The latter resulted in summer dry conditions over land. also found the interdecadal signal of 15 to 25 year period in summer PCP anomalies over eastern/central Australia dominating the higher frequency signals and fluctuating in phase with the global IDO signal. This accounted for the drought episodes of 3 to 7 years' duration that plagued Australia every 15 to 25 years throughout the 20th century. These drought episodes occurred during the decade straddling 1900, 1915, 1929, 1944, 1963, and 1982, coinciding with the Pacific extra-tropical cool phase of the global IDO signal . The latter displayed the largest extra-tropical SST and SLP contrast across Australia from the south Indian Ocean to the South Pacific Ocean of the four QBO, ENSO, QDO, and IDO signals . This may explain why drought is endemic to Australia.

Australia has been exposed to a vast array of extreme weather regimes over the past few years, and the frequency and intensity of these events are expected to increase as a result of anthropogenic climate change. However, the predictability of extreme droughts, heat waves (HWs), bushfires and floods, is still hampered by our inability to fully understand how these weather systems interact with each other and with the climate system. This study brings new insight into the regional and large scale dynamics of some extreme events in Australia, by describing and comparing the climate signature of summer HWs and extreme rainfall events which have occurred in the states of Victoria and Queensland respectively, during 1979-2013. Our analyses highlight the importance of mid-latitude dynamics operating during HWs, in contrast with more tropical interactions at play during extreme rainfall events. A 'common' blocking high pressure system is observed over the Tasman Sea during the two types of extreme events, and may explain why some southeastern HWs (only about 25 %) occur in close succession with floods in Queensland. However, our results suggest that there is no dynamical link between these two types of events, since the HW-related anticyclone evolves as part of a baroclinic wave train, whereas in the case of rainfall events, this structure emerges as an equivalent barotropic response to tropical convection. Sub-regional surface temperatures and air-sea fluxes also suggest that distinct processes may be operating in the lead up to these two events. Indeed, HWs tend to occur when the wave train propagates from the south Indian to the Pacific Ocean, inducing a quasi-stationary blocking high system over the Tasman Sea. This anticyclonic anomaly can then advect hot dry air towards the southern Victorian coast, where it produces HW conditions. On the other hand, extreme rainfall events mostly occur when the background conditions correspond to a La Niña state. The convection induced in the western Pacific can trigger a tropical-extratropical teleconnection over Queensland. This may generate an anticyclonic anomaly over the Tasman Sea, able to divert air parcels over a warm and humid area where conditions are, this time, favorable for more extreme rainfall along the Queensland coast.

Droughts are a recurrent and natural part of the Australian hydroclimate, with evidence of drought dating back thousands of years. However, our ability to monitor, attribute, forecast and manage drought is exposed as insufficient whenever a drought occurs. This paper summarises what is known about drought hazard, as opposed to the impacts of drought, in Australia and finds that, unlike other hydroclimatic hazards, we currently have very limited ability to tell when a drought will begin or end. Understanding, defining, monitoring, forecasting and managing drought is also complex due to the variety of temporal and spatial scales at which drought occurs and the diverse direct and indirect causes and consequences of drought. We argue that to improve understanding and management of drought, three key Climatic Change research challenges should be targeted: (1) defining and monitoring drought characteristics (i.e. frequency, start, duration, magnitude, and spatial extent) to remove confusion between drought causes, impacts and risks and better distinguish between drought, aridity, and water scarcity due to over-extractions; (2) documenting historical (instrumental and preinstrumental) variation in drought to better understand baseline drought characteristics, enable more rigorous identification and attribution of drought events or trends, inform/evaluate hydrological and climate modelling activities and give insights into possible future drought scenarios; (3) improving the prediction and projection of drought characteristics with seasonal to multidecadal lead times and including more realistic modelling of the multiple factors that cause (or contribute to) drought so that the impacts of natural variability and anthropogenic climate change are accounted for and the reliability of long-term drought projections increases.

Climate

The aims of this study are to assess the impacts of accelerated climate change on summer maximum temperatures since the early 1990s in the Australian city of Sydney’s eastern coastal and western inland suburbs. Western Sydney currently experiences far more intense summer (December–March) heat waves than coastal Sydney, with maximum temperatures exceeding those of coastal Sydney by up to 10 °C. Aside from increased bushfire danger, extreme temperature days pose health and socio-economic threats to western Sydney. Permutation tests of consecutive summer periods, 1962–1991 and 1992–2021, are employed to determine the differential climate change impacts on maximum summer temperatures at two locations: Sydney and Richmond, representative of eastern and western Sydney, respectively. Attribution of observed maximum summer temperature trends in Sydney and Richmond was performed using machine learning techniques applied to known Australian region oceanic and atmospheric climate drivers. It w...

Journal of geographical research, 2021

An extensive search has been carried out to find all major flood and very heavy rainfall events in Victoria since 1876 when Southern Oscillation (SOI) data became available. The synoptic weather patterns were analysed and of the 319 events studied, 121 events were found to be East Coast Lows (ECLs) and 82 were other types of low-pressure systems. Tropical influences also played a large role with 105 events being associated with tropical air advecting down to Victoria into weather systems. Examples are presented of all the major synoptic patterns identified. The SOI was found to be an important climate driver with positive SOIs being associated with many events over the 144 years studied. The 1976 Climate Shift and its influence on significant Victorian rainfall events is studied and negative SOI monthly values were shown to dominate following the Shift. However, one of the most active periods in 144 years of Victorian heavy rain occurred after the shift with a sustained period of positive SOI events from 2007 to 2014. Therefore, it is critical for forecasting future Victorian heavy rainfall is to understand if sequences of these positive SOI events continue like those preceding the Shift. Possible relationships between the Shift and Global Temperature rises are also explored. Upper wind data available from some of the heaviest rainfall events showed the presence of anticyclonic turning of the winds between 850hPa and 500hPa levels which has been found to be linked with extreme rainfall around the Globe.

Geophysical Research Letters, 1992

Temperature records from rural station networks are being assembled and analyzed for many Northern Hemispheric locations. In this study, we examine the annual mean, maximum, and minimum temperatures from a network of rural stations in Australia over the period 1911-1990. Our findings show general cooling from 1911 to 1978 with a steady decrease in the diurnal temperature range. The cooling in the mean temperature results from a decrease in maximum temperature that is larger than the increase in minimum temperature. However, in the late 1970s, maximum, minimum, and mean temperatures all rose sharply, although the daily temperature range continued its steady decline. This discontinuity in the Australian temperature record is similar in magnitude and timing to discontinuities found in the Northern Hemispheric climate system. Whether it is related to the buildup of greenhouse gases or some other cause, is unknown.

Scientific Reports, 2023

The cool-season (May to October) rainfall decline in southwestern Australia deepened during 2001-2020 to become 20.5% less than the 1901-1960 reference period average, with a complete absence of very wet years (i.e., rainfall > 90th percentile). CMIP5 and CMIP6 climate model simulations suggest that approximately 43% of the observed multi-decadal decline was externally-forced. However, the observed 20-year rainfall anomaly in 2001-2020 is outside the range of both preindustrial control and historical simulations of almost all climate models used in this study. This, and the fact that the models generally appear to simulate realistic levels of decadal variability, suggests that 43% might be an underestimate. A large ensemble from one model exhibits drying similar to the observations in 10% of simulations and suggests that the external forcing contribution is indeed larger (66%). The majority of models project further drying over the twenty-first century, even under strong cuts to greenhouse gas emissions. Under the two warmest scenarios, over 70% of the late twenty-first century years are projected to be drier than the driest year simulated during the 1901-1960 period. Our results suggest that few, if any, very wet years will occur during 2023-2100, even if strong cuts to global emissions are made. The strong spatial gradient in rainfall in southwest Western Australia (SWWA) is evident on the ground: from the tall trees in the far southwest through the wheat belt and into the dry interior, over a distance of approximately 500 km 1. Thus, any small shift in the weather patterns north or south can strongly influence rainfall totals across the region, with consequences for dryland farming, ecosystems, and regional water supplies 2. SWWA has very dry summers and wet winters indicating a region highly influenced by the seasonal cycle of weather systems. Rainfall in the cool season (May to October) contributes about 70% of the annual total, so the May to October period is the main focus of this study. Cool season rainfall comes primarily from the fronts and lows that cross the region from the west 3,4 , while warm season rainfall is linked to the west coast trough, thunderstorms or the breakdown of tropical lows and cyclones typically moving down from the tropics 5. Cool season rainfall in southwest of Australia is strongly influenced by mid-latitude weather systems, while climate modes including the El Niño Southern Oscillation (ENSO) have minimal impact 6. Cool season rainfall (Fig. 1) prior to the 1960s exhibited interannual variability, with some years experiencing well above average rainfall, while some of the other years experienced well below average rainfall. After the 1960s, however, those very wet seasons disappeared, and the number of troughs and rainfall from those weather systems both declined 7-11. A further downward step in decadal rainfall in SWWA occurred in the late 1990s 12,13. This coincided with a downward step in other regions around the world 14-16. A decline in SWWA cool season rainfall is evident in almost all CMIP5 climate models under all historical forcings 17. By the early 1990s, Nicholls and Lavery 18 presented a discussion of a potential role for human-caused climate change. Since the 1990s, multiple generations of climate models and scientific assessments have supported a