The onset and termination of the rainy season shape the planning and operation of water resources systems. Thus, we identify the onset and termination in Central Chile (32–40°S) from 1960 to 2021 based on the occurrence of frequent wet spells (consecutive days with rainfall ≥1 mm). The method yields a latitudinal gradient on both the onset and termination dates consistent with the latitudinal variation of annual precipitation. We report robust shortening trends for the rainy season north of 35°S over the last 30 years (up to 7 weeks per decade), primarily due to a delay in the onset date. Between 1990 and 2021, the number of wet spells exhibited a significant negative trend north of 35°S and a significant negative trend in the average precipitation per wet spell in 32–40°S. The changes in precipitation timing and the significant decline in precipitation observed in Central Chile are then affecting the current water infrastructure.
On 5–6 January 2022, a deepening cyclone approaching the coast of east Antarctica brought a vigorous downslope windstorm and prolonged blizzard conditions to Casey (66.28°S, 110.52°E), an Australian research station in Antarctica. Sustained storm-force winds persisted for ~24 h, and the maximum gust of 213 km h–1 (59.2 m s–1 or ~115 knots) set a new January wind gust record for the station. As the cyclone approached the Antarctic coast, the system caused a broad-scale warm-air advection over the Casey–Law Dome area and strengthened an easterly jet in the low-level atmosphere, resulting in the formation of bullet-shaped wind profiles. This distinctive upwind aerological structure, together with the interaction between the low-level jet and the topography of Law Dome, facilitated the development of wave breaking, flow transition and a hydraulic jump over moderate topography. These mesoscale processes are identified as the main mechanisms responsible for the downslope acceleration of the flow that produced the record windstorm at Casey.
Compound weather extremes in southeastern Australia are increasing in frequency and complexity, yet the mechanisms driving their interactions remain poorly understood. This study examines how compound drought and heatwave (CDH) events in Victoria transition into heavy rainfall and flooding (CDH_R) during extended austral summers (November–March) from 1980–81 to 2018–19. Using threshold-based criteria and joint indices, we identify 28 CDH_R events and compare them with 34 CDH events not followed by extreme rainfall (CDH_noR) at the regional scale. Composite analysis reveals that eastern Victoria is particularly susceptible to rapid transitions from CDH to wet extremes. CDH_R events are typically preceded by shorter, more humid heatwaves during drought, in contrast to the longer, drier CDH_noR events that may exacerbate water stress. Two key synoptic-scale mechanisms are found to drive CDH_R transitions: (i) equatorward intrusion of a transient midlatitude trough via Rossby wave breaking, which enhances tropical disturbances prior to CDH onset; and (ii) poleward transport of tropical moisture into Victoria during peak CDH conditions under a persistent high-pressure anomaly, which helps sustain the dry and hot regime. This moisture influx is facilitated by an active Madden–Julian Oscillation phase and enhanced rainfall in the tropical north, followed by eastward propagation of a midlatitude low-pressure system that promotes southward moisture advection. At CDH termination, the interaction between a midlatitude cold front and warm, moist air over Victoria triggers high atmospheric instability and dynamic uplift, supported by a persistent upper-level anticyclonic circulation over the Tasman Sea. These findings provide new insights into the interconnected drivers of compound extremes, valuable for improving the predictability of hydroclimate risks and enhancing early warning and preparedness.
Spatio-temporal characteristics of extreme rainfall over western Java across interdecadal-to-multidecadal periods allow assessment of the impact of various large-scale climate events on the region. In this paper, we describe these characteristics using extreme rainfall indices, based on a quality-assured daily rainfall dataset from 185 rain gauges with varying temporal coverage, ranging from 1950 to 2020. The characterisation was done by calculating Sen's slopes of the extreme indices across three interdecadal (10-15 and 16-25 years) and multidecadal (>25 years) periods. Our study shows that the interdecadal and multidecadal periods were dominated by Sen's slopes with no statistical significance, and likewise for their regression with elevation. Thus, there is no compelling evidence of any spatial dependency (e.g. topographical effect) of Sen's slopes in any of the extreme indices and periods in the region despite its complex terrains and coastlines. Much steeper Sen's slopes in interdecadal compared with multidecadal periods signal regional responses to large-scale climate internal variabilities, such as the long-term modulation of the El Ni & ntilde;o-Southern Oscillation (ENSO). The significance of extreme rainfall in interdecadal periods is well exemplified by the decade 2001-10, which exhibits a larger number of locations with positive and significant Sen's slopes than 1991-2000 and later decades. The positive and significant Sen's slopes in 2001-10 may be associated with a shift towards negative phases of the Pacific Ocean climate drivers, such as La Ni & ntilde;a events. Our study suggests that interdecadal changes in extreme rainfall over western Java indicate a key regional climate impact concern that warrants mitigation efforts.
In Australia, drought has dramatic consequences for social, economic and environmental systems, making it critical to understand how this hazard may change in the future. The Australian Climate Service (ACS) is a national interagency collaboration to service Australia's climate needs. In this paper, we unpack insights from the ACS's drought and changes in aridity team derived using dynamically downscaled and bias-adjusted CMIP6 (sixth Coupled Model Intercomparison Project) projections for two emissions scenarios. By applying a global warming level framework, we report on both mean state shifts to meteorological drought, as well as outlier ensemble members useful for high-impact, low-probability event planning. Key findings indicate high-confidence increases to time spent in drought across southern and south-western parts of Australia, with some areas projected to experience meteorological droughts up to 30% longer and 75% more frequently compared to the current climate. In regions of lower confidence change, such as the tropics and parts of the Murray-Darling Basin, a range of diverging drought futures are unpacked. The findings of this analysis can enable decision makers to make informed choices in regions of higher confidence change, and to develop adaptive strategies where there may be more uncertainty. As climate change intensifies, such planning will be critical to sustaining the long-term resilience and vitality of drought-prone ecosystems and communities.
This study analysed the influence of intraseasonal variability on the persistence of generalised frost (GF) events in the Pampa H & uacute;meda, Argentina. The events were classified into three categories: no persistence (0 DP), 1-day persistence (1 DP) and 2 or more days of persistence (2 DP+). The 0-DP events were associated with short-lived synoptic systems, such as transient anticyclones and high-pressure systems, which promote temporary stability but lack dynamic support for consecutive frost days. In 1-DP events, a Rossby wave train propagating from the central Pacific Ocean to South America favoured cold air advection, with additional modulation from tropical forcing patterns linked to convective episodes in the region. The 2-DP+ events were associated with a persistent tropical forcing signal that led to the excitation of well-structured Rossby wave trains, sustaining cold air advection and stable conditions over multiple nights. Overall, the results show that short-lived frosts are driven by transient synoptic systems, whereas persistent events are more strongly linked to Rossby wave activity modulated by tropical-extratropical interactions. These findings enhance the understanding of the frost formation mechanisms in south-eastern South America and provide valuable input for improving forecasting and agricultural risk management strategies.
Fire-generated tornadic vortices (FGTVs) were observed at two high intensity fires in New South Wales during the 2019-20 'Black Summer' Australian bushfires. At the Green Valley Fire, a fully laden fire truck was lifted and overturned by a confirmed FGTV, with estimated wind speeds of similar to 300 km h(-1) (Enhanced Fujita, EF, scale rating of 3 to 4). At Wandella, impact to vehicles at the Badja Forest Fire indicates winds may have exceeded 350 km h(-1) (EF5 tornado), making it one of the strongest pyrogenic winds documented globally. We use a combination of observations and insights from simulations using the coupled fire-atmosphere model ACCESS-Fire to investigate the FGTV environments. Radar and satellite observations showed both FGTVs occurred coincident with rapid acceleration of the fire's updraft and rapid growth of pyroconvective clouds; towering pyrocumulus cloud (pyroCu) at the Green Valley FGTV and pyrocumulonimbus cloud (pyroCb) tops higher than 12 km at the Wandella FGTV. Coupled fire-atmosphere simulations resolved transient, small scale features consistent with observations, including split flow around the fire, reverse lee side inflow, fire updrafts extending to the mid-troposphere, and split fire fronts with convective towers at the head of the flanks. A likely source of vorticity for each was a zone of near ground vertical wind shear between the fire's reverse inflow in a valley, and north-westerly winds above. The case studies contribute to growing knowledge of these destructive FGTVs, highlight the benefits of fire-atmosphere modelling, and demonstrate the operational utility of radar and satellite observations to inform warnings to communities and fire-fighting operations.
The National Partnership for Climate Projections (NPCP) was established as a collaborative effort of the Australian climate projections community to develop a consistent approach to deliver future climate information. As bias correction of climate model outputs is important for many applications, a NPCP bias correction intercomparison project was initiated. The first phase of the intercomparison aimed to support the production of national-scale climate projections by the Australian Climate Service. It focused on five methods – Equidistant Cumulative Density Function matching (ECDFm), Quantile Matching for Extremes (QME), Quantile Delta Change (QDC), N-Dimensional Multi-Variate Bias Correction (MBCn) and Multivariate Recursive Nesting Bias Correction (MRNBC) – and applied them to daily timescale Coordinated Regional Climate Downscaling Experiment (CORDEX) data produced by NPCP partner organisations. Each method was assessed over a calibration period and also via cross-validation on several metrics relating to the temperature and precipitation climatology, variability, distribution, extremes and trends. The best-performing bias correction methods were QME and MRNBC. The ECDFm method also performed well on most metrics, but under certain circumstances it could dramatically increase the model bias. The QDC method is a delta change method (i.e. it perturbs the observations rather than correcting model data) and compared very favourably to the four bias correction methods. The QME, MRNBC and QDC methods were subsequently used by the Australian Climate Service to produce climate projections datasets for Australia.
A numerical weather prediction (NWP) system that represents continental Australia at a convection-permitting resolution presents both advantages and challenges. It must represent diverse weather regimes over areas of variable observation coverage, which complicates the land and atmospheric modelling and data assimilation. A single-domain prototype system, ACCESS-A (Australia Community Climate and Earth System Simulator – Australia), has been developed and tested. Compared with the current operational NWP system comprising seven small domains (ACCESS-C), ACCESS-A incorporates improvements to satellite, conventional and radar data assimilation and uses an upgraded model science configuration. ACCESS-A was extensively evaluated over two 3-month periods. Qualitative and quantitative precipitation verification indicates that ACCESS-A reproduces the seasons’ weather patterns and observed behaviour of convective precipitation. Objective verification of defined subdomains shows that forecast skill of near-surface weather is variable across the continent. It is found more skilful in better-observed regions that coincide with areas dominated by more predictable, synoptically driven weather systems. Regions with lower skill, particularly corresponding to areas not covered by ACCESS-C, suggest a focus for future research. A comparison with ACCESS-C confirmed the anticipated improved skill related to the forecast model’s upgraded land and atmospheric physics, and provides confidence in the combined impact of all upgrades implemented in ACCESS-A. ACCESS-A is demonstrated to be ready to prepare for operational NWP and ongoing research at the Australian Bureau of Meteorology.
Tasmania, Australia's largest producer of hydroelectric power, receives most of its rainfall from extratropical cyclones (ETCs) and cold fronts. Western Tasmania experiences up to 3 m of annual rainfall, primarily driven by midlatitude weather systems and their interaction with local topography, which supports hydroelectric power generation in the state. However, the weather systems influencing rainfall variability in the east - where most Tasmanians live and where rainfall is vital for agriculture - remain less studied. Using combined datasets of ETCs, cold fronts and thunderstorms spanning 1979-2015 over 5-50 degrees S and 110-160 degrees E, we examined the key weather systems driving Tasmania's spatial and temporal rainfall variability. These weather systems collectively contribute over 80% of the state's annual rainfall. A large proportion of total rainfall in eastern Tasmania is due to ETCs, whereas cold fronts play a greater role in the west. ETCs south of 40 degrees S, particularly those passing through the Tasmanian region, are associated with heavy rainfall across the state. A statistically significant decline in rainfall (1979-2023) has been observed over western Tasmania, particularly during the warm season (November-April), raising concerns for hydroelectric resources. Our findings highlight the central role of midlatitude weather systems in sustaining Tasmania's hydroclimate and underscore the need to better understand their future changes in a warming world.
In coastal areas where Southern Ocean swells are observed, the Southern Annular Mode (SAM) climate index has been shown to be a useful predictor for wave conditions. However, the relationship between SAM and beach morphology change in these areas is not well known. In this study, empirical orthogonal function statistical analysis was applied to satellite-derived shoreline data from 1987 to 2021 at Grassy Beach, King Island, Australia. The dominant modes of shoreline variability were calculated and the interconnection between SAM and shoreline position investigated. Cross-shore beach movement and seasonal beach rotation were the dominant sources of shoreline variability, accounting for similar to 64 and similar to 18% of total shoreline variability respectively. Shoreline retreat (advance) was observed at the eastern (south-western) end of the embayment during winter and, conversely, shoreline retreat (advance) at the south-western (eastern) end during summer. A clear connection between the SAM and shoreline variability was found, the SAM modulating beach rotation depending on the SAM phase. When SAM was positive, beach rotation increased with a greater difference in beach orientation between summer and winter owing to an increased shoreline retreat at the eastern end of the embayment by 4.1 m on average. This increased retreat was attributed to more powerful south-westerly waves in winter for positive SAM. This research demonstrated a strong relationship between SAM and shoreline change at a sandy embayment in southern Australia and may have implications for the current and future morphodynamics at other beaches influenced by the Southern Ocean wave climate.
Dorothea Mackellar famously wrote that Australia is 'a land of droughts and flooding rains'. But has this always been true, and will these extremes intensify in a warmer world? Australia's vast continent spans diverse hydroclimate regions - from the wet tropics in the north-east to the arid central rangelands - a land of climatic contrasts. Our study examines changes in these regions from past to future by: 1, assessing hydroclimate anomalies from 1963-2022; 2, exploring compound events and their link to disasters (1993-2022); and 3, projecting compound events under wet, dry and mid-range storylines (1976-2005 v. 2036-2065). Currently, southern and some central regions show drying trends in precipitation, runoff and soil moisture, alongside rising temperatures. Natural disasters are increasingly tied to compound events, where hazards cycle through communities already under strain. Worryingly, these events appear to be on the rise. Future projections show more frequent 'hot and dry' and 'wet and windy' extremes across all regions under at least one storyline. This suggests larger-scale droughts, longer fire seasons and more extreme fire danger days, across most of Australia as well as heavier rainfall, storms and stronger winds in the northern and central regions under the wet storyline, signalling an increased risk of flooding through extreme runoff. Our findings indicate that Australia's hydroclimate extremes are changing and compounding, with significant implications for communities and disaster preparedness.
Amid the growing challenges of climate change impact and the fairly limited availability of observations of some Earth system parameters, this article highlights the potential of ground-based Global Navigation Satellite Systems (GNSS) atmospheric monitoring as a supplementary satellite observing technique to improve the monitoring and forecasting of weather and climate extremes. It spotlights current barriers and future opportunities, aiming to heighten public and institutional awareness of the research and application status and the prospective role of GNSS atmospheric monitoring for weather and climate resilience. The innovative uptake of diverse ground-based GNSS atmospheric parameters could support improved systems and policies for risk management and climate adaptation, thus empowering communities to better withstand hazardous weather and climate extremes.
This paper documents AUS2200, a community-driven, high-resolution limited-area modelling project for Australia based on the Met Office Unified Model (UM) coupled to the Joint UK Land Environment Simulator to represent the land surface. Developed through a national partnership involving the Australian Research Council Centre of Excellence for Climate Extremes (CLEX), Bureau of Meteorology (BoM), National Computational Infrastructure (NCI) and Australian Earth System Simulator National Research Infrastructure (ACCESS-NRI), AUS2200 marks a significant advancement in limited-area modelling efforts in the Australian university community. AUS2200 features a convection-permitting configuration with 2.2-km grid spacing, covering the entirety of the Australian continent and portions of surrounding oceans. Its large domain at convection-permitting scales allows simultaneous resolution of both large- and small-scale atmospheric processes. This capability supports scientific investigations into key atmospheric phenomena, including multiscale interactions, across a broad range of spatial and temporal scales, from continent-wide systems to localised events, and across diverse climatic regions spanning the tropics to the mid-latitudes. This paper provides an overview of the AUS2200 project, detailing its overarching aims, modelling framework including model configuration and optimisation efforts, contributions to scientific research and community development, and future directions. Early results from collaborative, cross-organisational efforts to study diverse atmospheric processes and high-impact weather events, including the 2019–20 Black Summer bushfires and record-breaking extreme rainfall events, are also presented. These investigations are contextualised within the broader scope of seasonal and climate variability, highlighting the project’s importance for advancing scientific research and addressing broader societal challenges.
Verification of atmospheric reanalysis products is crucial for their application in extreme weather and climate-related research. This study evaluates tropical cyclone (TC) characteristics and related variables from 1990 to 2018 over Australia (95-160 degrees E and 0-30 degrees S) in three reanalysis products - the recently developed Australian Bureau of Meteorology's Atmospheric high-resolution Regional Reanalysis for Australia Version 2 (BARRA-R2), its predecessor BARRA-R (both at 12-km spatial resolution) and the widely used European Centre for Medium-Range Weather Forecasts (ECMWF)'s Global Reanalysis Version 5 (ERA5) at 31-km spatial resolution. TCs detected in these reanalyses, using the Okubo-Weiss-Zeta Parameter detection and tracking scheme, are compared with observations from the Bureau's TC database. All three products simulated more than 50% of the observed TC frequency, with ERA5 achieving a higher hit rate of 77% compared with BARRA-R2 (68%) and BARRA-R (53%). Most missed cases involved non-severe TCs. ERA5 showed a clear decline in annual TC frequency consistent with observations, whereas BARRA-R displayed a weak upward trend and BARRA-R2 a statistically insignificant decline. Large differences emerged in surface wind speed and gusts: BARRA-R represented TC surface winds better than ERA5, and BARRA-R2 produced slightly improved gusts compared with ERA-5. Case studies show that temporal evolution is generally well represented in all products, though ERA5 tends to maintain peak intensity for longer, whereas BARRA products sometimes show shifted timing of peak intensity. General discrepancies are attributed to resolution limitations, inherent differences between best-track and gridded data, underestimation of TC peak intensity and wider model forecast constraints.
This study examines over 13 years (March 2010 to May 2023) of data from 12 Southern Ocean Flux Station (SOFS) mooring deployments to explore the characteristics and temporal climatology of air–sea heat flux in the Southern Ocean. SOFS, the only currently operational moored buoy in the Southern Ocean (anchored at ~47°S, 142°E), provides high-resolution (1-min) climate-quality meteorological and marine data, facilitating detailed air–sea heat flux analysis. Before analysis, the 1-min SOFS flux data were rigorously evaluated, and their high quality confirmed by comparing net heat flux against nearby overlapping moorings and research vessels. Over the study period, the average annual net heat flux at the SOFS site is −14.6 ± 5.4 W m−2 (a net ocean heat gain). This is the first estimate of the net heat exchange at a Southern Ocean site that is based on a multi-year record of high-quality measurements, offering direct evidence of the ocean region’s absorption of heat. Seasonal heat flux variabilities and extreme heat flux events are investigated. Additionally, a case study highlights a strong horizontal sea surface temperature gradient (3.4°C over 35.5 km) that resulted in a significant net heat flux difference of up to 242.5 W m−2, which showed that the environmental conditions in this region may shift dramatically over short temporal or spatial scales.
The method of archetypal analysis is used to generate a set of monthly timescale rainfall archetypes for the Australian region. The patterns associated with the archetypes reflect continental and regional-scale wet and dry. The dominant pattern in terms of occurrence and persistence is one in which most of the continent is dry. This pattern is typically expressed over winter and spring. The next most frequent pattern is one where most of the continent is wet, mostly expressed during summer. It is rare to find periods where the whole continent is wet outside summer, though this does occur and is associated with very wet years for the continent. The archetype patterns have preferred seasonal expressions, and preferred transitions from one pattern to another. The continent-wide dry pattern is mostly followed by patterns in which both south-west and south-east Australia are wet during the autumn and winter. However, if the dry continental archetype persists through to spring, then it is usually followed by a pattern that is wet in the south-east but not the south-west. The analysis reveals pivotal months, such as April and November. These months mark the end of periods when only a few archetypes are expressed, allow expression of almost all the archetypes, and are then succeeded by periods when a smaller number of archetypes are expressed again. The archetype patterns successfully capture the large-scale spatial patterns of monthly rainfall in Australia, and provide a diagnostic tool to evaluate the onset, duration and transitions between wet and dry periods.
By using finer resolution modelling and locally representative model physics, regional climate models (RCMs) have the potential to improve the information provided by global climate models (GCMs). However, RCMs have their own biases and limitations due to remaining unresolved processes. It is therefore necessary to carefully assess RCM outputs through added value analyses. An ensemble of CMIP6-based 12–17-km regional climate projections has been produced for the Australian Climate Service (ACS) based on the Bureau of Meteorology’s regional climate modelling system (BARPA) and CSIRO’s Conformal Cubic Atmospheric Model (CCAM). The historical and potential future added value of this ensemble is assessed, focusing on extremes (cold, hot, wet and dry). Despite variations in added value across different GCM–RCM experiments, quantities, seasons and regions, BARPA and CCAM generally improve on their driving models for the historical period. Added value over ERA5 is generally small, and often negative for wet and dry extremes, especially for CCAM. The most consistent improvements in all GCM–RCM pairs are found for quantities containing daily minimum temperature, whereas hot days above 40°C show the least improvements. CNRM-ESM2-1-CCAM appears to have significant issues in most analysed quantities, especially related to maximum temperature and might not be recommended for downscaling or use by the community. Additionally, RCMs often predict different climate change signals than their driving models, for example the Murray Basin, which combined with the historical added value indicate plausible improvements in future climate projections.
Low pressure systems are associated with a number of climate hazards in Australia, including heavy rainfall, strong winds and coastal erosion. Here, we use a new ensemble of 40 CMIP6 (Sixth Coupled Model Intercomparison Project)-based regional model projections to assess future changes in low pressure systems across Australia, with a focus on vertically developed (deep) cyclones that extend between the surface and 500 hPa. Results show robust future declines in extratropical lows in southern Australia throughout the year, with large uncertainty for lows in northern Australia. Projections for strong, rapidly intensifying and slow-moving low pressure systems are also assessed, and are all projected to decline in frequency. The strongest declines in lows are identified for models that also have larger increases in the intensity of 500-hPa zonal winds to the south of Australia (40-50 degrees S), with observed trends in both indices at the high end of the model range. This suggests the potential for constraining future projections of Australian low pressure systems based on monthly mean zonal winds.
