The topic of “Flash Drought” is rapidly gaining attention within both the research and drought management communities. This literature review aims to synthesize the research to-date and provide a basis for future research on the topic. Specifically, our review is focused on documenting the range of definitions of “flash drought” being proposed in the research community. We found that the term first appeared in the peer-reviewed literature in 2002, and by 2020 has become an area of active research. Within that 18-year span, “flash drought” has been given 29 general descriptions, and 20 papers have provided measurable, defining criteria used to distinguish a flash drought from other drought. Of these papers, 11 distinguish flash drought as a rapid-onset drought event while eight distinguish flash drought as a short-term or short-lived, yet severe, drought event and one paper considers flash drought as both a short-lived and rapid onset event. Of the papers that define a flash drought by its rate of onset, the rate proposed ranges from 5 days to 8 weeks. Currently, there is not a universally accepted definition or criteria for “flash drought,” despite recent research that has called for the research community to adopt the principle of rapid-intensification of drought conditions.

<p>The topic of “Flash Drought” has rapidly gained attention within the research and drought management communities within the last decade. In preparation for a recent workshop on Flash Drought, the National Integrated Drought Information System (NIDIS) prepared a literature review to synthesize the research to-date (as of August 2020) and to provide a basis for future research on the topic. Specifically, this review is focused on documenting the range of definitions of "flash drought" that have been proposed by the research community. The term first appeared in the peer-reviewed literature in 2002, and by 2020, has become an area of active research. Within that 18-year span, 19 papers have provided measurable, defining criteria used to distinguish a flash drought from other drought. Of these papers, 11 distinguish flash drought as a rapid-onset or rapid-intensification drought event while seven distinguish flash drought as a short-term or short-lived, yet severe, drought event, and one paper considers flash drought as both a short-lived and rapid-onset event. Currently, there is no universally accepted definition or criteria for “flash drought,” despite recent research that has called for the research community to adopt the principle of rapid-intensification of drought conditions. This topic was further explored at the NIDIS-sponsored Flash Drought Workshop on 1-3 December 2020, where additional perspectives were shared about the key characteristics of flash drought that should inform its definition.  We will provide a review of the literature-derived definitions as well as a brief overview of this additional discussion.</p>

The annual Australian monsoon pattern includes an onset, or the much anticipated first active monsoon period of the season, but defining the monsoon onset has proven to be problematic. Since the first Australian monsoon onset definition by Troup in 1961 there have been many others presented. There appears to be no universally accepted method to define the Australian monsoon onset, and therefore, we present here an analysis of the methods that have been proposed. The aim of this paper is to systematically review the different methods used to define the Australian monsoon onset, adding to the work that has been done by other reviews for monsoon systems around the world. For the first time, we identify the 25 different methods that have been published for the Australian monsoon/wet season onset and compare them to identify how well they align. When considering the 57 seasons where more than one onset definition is provided, the range of dates within the season can range over several months, with an average range of 44 days and the largest range within a season of 78 days. Thus, we show that different onset definitions are capturing different events altogether and pin the ‘onset’ to different dates throughout the progression of the north Australian wet season. Some capture a ‘wet season onset’ while others capture the dynamical overturning of the atmosphere (i.e. the monsoon). In conclusion, our analysis finds that there is still a lack in real-time monitoring or prognostic capabilities of monsoon onset dates as well as limited operational applicability despite a plethora of definitions.

Abstract. The timing of the first monsoon burst of the season, or the monsoon onset, can be a critical piece of information for agriculture, fire management, water management, and emergency response in monsoon regions. Why do some monsoon seasons start earlier or later than others? Previous research has investigated the impact of climate influences such as the El Niño–Southern Oscillation (ENSO) on monsoon variability, but most studies have considered only the impact on rainfall and not the timing of the onset. While this question could be applied to any monsoon system, this research presented in this paper has focused on the Australian monsoon. Even with the wealth of research available on the variability of the Australian monsoon season, the timing of the monsoon onset is one aspect of seasonal variability that still lacks skilful seasonal prediction. To help us better understand the influence of large-scale climate drivers on monsoon onset timing, we recreated 11 previously published Australian monsoon onset datasets and extended these to all cover the same period from the 1950/1951 through the 2020/2021 Australian wet seasons. The extended datasets were then tested for correlations with several standard climate indices to identify which climate drivers could be used as predictors for monsoon onset timing. The results show that many of the relationships between monsoon onset dates and ENSO that were previously published are not as strong when considering the extended datasets. Only a strong La Niña pattern usually has an impact on monsoon onset timing, while ENSO-neutral and El Niño patterns lacked a similar relationship. Detrended Indian Ocean Dipole (IOD) data showed a weak relationship with monsoon onset dates, but when the trend in the IOD data is retained, the relationship with onset dates diminishes. Other patterns of climate variability showed little relationship with Australian monsoon onset dates. Since ENSO is a tropical climate process with global impacts, it is prudent to further re-examine its influences in other monsoon regions too, with the aim to evaluate and improve previously established prediction methodologies.