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Abstract Spatiotemporal variability on multi-decadal time scales can be observed in weather conditions associated with wildfires. The climatic factors that drive this variability in fire weather conditions have been studied using bias-corrected 20th Century Reanalysis data of the mean state and extreme values of the McArthur Forest Fire Danger Index (FFDI) from 1876 to 2014. In particular, the association of this time series with the climate drivers of El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Southern Annular Mode (SAM) are analysed using several techniques. The potential for long-term changes in the relationships between the climate drivers and seasonal FFDI values is also explored by breaking the total time period into two epochs. Composite analysis was performed to separate the phases of ENSO, IOD and SAM (given their interrelationships) and determine which phase combinations influence severe fire weather conditions the most. The most severe fire weather conditions were observed during a specific combination of ENSO, IOD and SAM phases, with results mapped to show spatial and seasonal variations in this over Australia. Long-term changes in the relationships between fire weather conditions and the three climate drivers (ENSO, IOD and SAM) are documented. These findings are based on a longer time period than similar previous studies, intended to help contribute to the range of guidance available for long-range fire weather outlooks and enhanced decision-making for fire agencies.

A substantial number of studies have been published since the Ninth International Workshop on Tropical Cyclones (IWTC-9) in 2018, improving our understanding of the effect of climate change on tropical cyclones (TCs) and associated hazards and risks. These studies have reinforced the robustness of increases in TC intensity and associated TC hazards and risks due to anthropogenic climate change. New modeling and observational studies suggested the potential influence of anthropogenic climate forcings, including greenhouse gases and aerosols, on global and regional TC activity at the decadal and century time scales. However, there are still substantial uncertainties owing to model uncertainty in simulating historical TC decadal variability in the Atlantic, and the limitations of observed TC records. The projected future change in the global number of TCs has become more uncertain since IWTC-9 due to projected increases in TC frequency by a few climate models. A new paradigm, TC seeds, has been proposed, and there is currently a debate on whether seeds can help explain the physical mechanism behind the projected changes in global TC frequency. New studies also highlighted the importance of large-scale environmental fields on TC activity, such as snow cover and air-sea interactions. Future projections on TC translation speed and medicanes are new additional focus topics in our report. Recommendations and future research are proposed relevant to the remaining scientific questions and assisting policymakers.

Assessing the role of anthropogenic warming from temporally inhomogeneous historical data in the presence of large natural variability is difficult and has caused conflicting conclusions on detection and attribution of tropical cyclone (TC) trends. Here, using a reconstructed long-term proxy of annual TC numbers together with high-resolution climate model experiments, we show robust declining trends in the annual number of TCs at global and regional scales during the twentieth century. The Twentieth Century Reanalysis (20CR) dataset is used for reconstruction because, compared with other reanalyses, it assimilates only sea-level pressure fields rather than utilize all available observations in the troposphere, making it less sensitive to temporal inhomogeneities in the observations. It can also capture TC signatures from the pre-satellite era reasonably well. The declining trends found are consistent with the twentieth century weakening of the Hadley and Walker circulations, which make conditions for TC formation less favourable.

Spatiotemporal variability on multi-decadal time scales can be observed in weather conditions associated with wildfires. The climatic factors that drive this variability in fire weather conditions have been studied using bias-corrected 20th Century Reanalysis data of the mean state and extreme values of the McArthur Forest Fire Danger Index (FFDI) from 1876 to 2014. In particular, the association of this time series with the climate drivers of El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and Southern Annular Mode (SAM) are analysed using several techniques. The potential for long-term changes in the relationships between the climate drivers and seasonal FFDI values is also explored by breaking the total time period into two epochs. Composite analysis was performed to separate the phases of ENSO, IOD and SAM (given their interrelationships) and determine which phase combinations influence severe fire weather conditions the most. The most severe fire weather conditions were observed during a specific combination of ENSO, IOD and SAM phases, with results mapped to show spatial and seasonal variations in this over Australia. Long-term changes in the relationships between fire weather conditions and the three climate drivers (ENSO, IOD and SAM) are documented. These findings are based on a longer time period than similar previous studies, intended to help contribute to the range of guidance available for long-range fire weather outlooks and enhanced decision-making for fire agencies. [ABSTRACT FROM AUTHOR]