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AUSTRALIAN GEOGRAPHER https://doi.org/10.1080/00049182.2023.2199465 Kimberley J. Reid ARC Centre of Excellence for Climate Extremes and School of Earth, Atmosphere and Environment, Monash University, Clayton, Australia ABSTRACT ARTICLE HISTORY Received 26 October 2022 Few meteorological terms have caused such debate as Accepted 2 April 2023 Atmospheric Rivers (ARs). ARs were proposed in 1992, but a formal deﬁnition for ARs was only developed in 2015 following KEYWORDS the observational campaigns of the 2000s that led to a deeper Atmospheric river; Australia; physical understanding. ARs can cause extreme rainfall, ﬂooding, ﬂooding; terminology; landslides, damaging winds, glacier melt, and even polar observations; extreme heatwaves. Despite their well-documented global impacts, there rainfall has been relatively little research on this phenomenon in Australia. Compound events involving ARs led to the devastating 2021 and 2022 ﬂooding in eastern Australia. The media were drawn to the metaphor of a ‘river in sky’ but the term has not been embraced by our national weather service, and some scientists still question their existence. This essay explores the controversy of the name Atmospheric River and highlights what could be achieved if we focused less on the name and more on the science. The El Niño part of the El Niño-Southern Oscillation was coined by Peruvian ﬁshermen in the 1600s who noticed that the periodic warm waters oﬀ the coast of South America typi- cally peaked in December. Given the proximity to Christmas, they referred to this current of water as El Niño de Navidad in reference to Jesus Christ (Philander 1990). In modern times, this has been shortened to El Niño. The Indian Ocean Dipole, on the other hand, is so named because it is a mode of variability with two poles that occurs in the Indian Ocean – far less poetic. Some names are arguably designed to be attention grabbing, while others are more technical. Few meteorological terms could be described as having been met with ‘everything from dismissive disdain for its oversimpliﬁcation, to wonder at its intuitive nature’ (Ralph et al. 2020). This essay will explore the frustrations and joy of being on the receiving end of both the disdain and wonder as a PhD student undertaking the ﬁrst thesis on Atmospheric Rivers (ARs) in Australia, and what could be achieved if we focus on research eﬀorts on this topic rather than the name. During 2021 and 2022, eastern Australia sustained multiple intense Atmospheric Rivers that contributed to widespread ﬂooding, extensive damage, and loss of life and CONTACT Kimberley J Reid firstname.lastname@example.org School of Earth, Atmosphere & Environment, Monash Uni- versity, Clayton, Victoria, Australia 3800 © 2023 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/ licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The terms on which this article has been published allow the posting of the Accepted Manuscript in a repository by the author(s) or with their consent. 2 K. J. REID livelihoods (Reid et al. 2021; Figure 2). The ﬁrst three months of 2022 were the wettest start to the year for Sydney since records began and images of the 14.4 m ﬂood peak in the town of Lismore dominated the news cycle for weeks. The concept of a ‘river in the sky’ delivering the torrential rain captivated the media. However, the ‘river in the sky’ analogy appeared to pave the way for non-scientiﬁc metaphors such as this ﬁrst sen- tence that appeared in WA Today (Brookes 2022): Experts warn Perth could be walloped with rain bombs described as ‘tsunamis from the sky’ as climate change intensiﬁes weather systems known as atmospheric rivers. Neither rain bombs nor tsunamis from the sky are real meteorological terms for the phenomena described in the article, but to be fair to the journalist, scientists can be inconsistent with the terms we use. Atmospheric Rivers, originally named Tropospheric Rivers, ﬁrst appeared in the scien- tiﬁc literature in 1992 (Newell et al. 1992). These long, narrow regions of enhanced meridional water vapour transport were suggested as a mechanism to explain the carbon monoxide problem. That is, why high concentrations of carbon monoxide were observed in regions far from a potential source and not explained by the prevailing winds. The phenomenon was a called a river because the magnitude of moisture trans- 6 −1 port was comparable to the Amazon River (∼165 × 10 kg s ). Although the name did not appear until 1992, the concept of low altitude, large-scale streamers of moisture transport had been discussed before by James and Anderson (1984). The term Atmospheric River was initially contentious and there was confusion between the concepts of ARs and Warm Conveyor Belts (WCBs). The WCB is a narrow, continuous band of ascending low-level moisture from the warm sector in an extratropical cyclone. WCBs are often recognisable on satellite images as the comma- shaped cloud associated with an extratropical cyclone (Figure 1). WCBs are associated with thunderstorms and rainfall (Browning 1971). Some scientists did not like the term Atmospheric River (e.g. Bao et al. 2006) because it implied that all the moisture originated from a source (i.e. the (sub)tropics) and ﬂowed to the poles when in reality a signiﬁcant amount of the moisture in an AR comes from Figure 1. Himawari-8 satellite image over Australia (01:47UTC 22nd June, 2021) with authors own edits labelling the WCB (orange arrows) and AR locations (blue arrow) relative to the extratropical cyclone centre (L), and cold front (black). Note the ‘comma’ shape of the WCB. AUSTRALIAN GEOGRAPHER 3 local convergence (Dacre et al. 2015). Knippertz and Martin (2007) even used the phrase Moisture Conveyor Belt because it ﬁt with the existing conveyor belt model of extratro- pical cyclones. To settle the confusion among scientists, experts in ARs and WCBs gathered to develop a clear deﬁnition for ARs in June 2015 at the Scripps Institution of Oceanogra- phy in California, USA. They concluded that an Atmospheric River is: ‘A long, narrow, and transient corridor of strong horizontal water vapour transport that is typically associ- ated with a low-level jet stream ahead of the cold front of an extratropical cyclone’. Despite the bumpy beginning, Atmospheric Rivers are now generally accepted in the global meteorological community. For example, the joint project, AR-Recon, involves multiple US universities, government research agencies, and the US military, in collab- oration with the European Centre for Medium-Range Weather Forecasting (ECMWF). They send aircraft over ARs oﬀ the Californian coast to gather data with the aim of improving observations and the predictability of these weather systems (Ralph et al. 2014). This reconnaissance program is planned to be conducted over Europe too. The Californian Department of Water Resources have developed a Forecast-Informed Reser- voir Operations Program to incorporate AR forecasts into their water release strategies to improve dam management, and approximately 140 people attended the fourth Inter- national Atmospheric River Conference in 2022. However, as our understanding of ARs grew globally, it became clear that this deﬁnition did not encapsulate all regional cases of the phenomena, especially on the western side of the Paciﬁc Ocean. While winter is the peak AR season in North America, multiple independent studies have found that ARs are most common in the warmer seasons in East Asia (Kamae et al. 2017; Pan and Lu 2020), New Zealand (Prince et al. 2021) and Australia (Reid et al. 2022), indicating that we cannot simply assume ARs will behave the same over Asia and Oceania as they do elsewhere and there- fore a regional understanding is important. This is the context in which we turn our attention to Australia. The Bureau of Meteorology’s Special Climate Statement on the February to March 2022 extreme rainfall andﬂooding did not mention the phrase ‘Atmospheric River’ in theentirereport(TheBureauofMeteorology 2022). The meteorology of the event was instead described as: Ablockinghighpressure system near New Zealand, combined with a series of low-pressure systems, fed a large volume of tropical air over eastern Australia. There is nothing scientiﬁcally wrong with this description, butitisstrikingthattheoﬃcial report on an event that involved a strong AR made no mention of ARs. This is not an isolated occurrence. In March 2021, the AR-induced rainfall event (Figure 2) was described by the Bureau of Meteorology as ‘like atmospheric taps’ and ‘moisture feeds’ (Figure 3). This is despite the term ‘Atmospheric River’ existing in the literature for 30-years, and despite the recent collaboration between the Bureau of Meteorology and Chinese Meteorological Agency to investigate ARs in the Australia-Asia region (Ye et al. 2020). It is unclear why there has been resistance to the term Atmospheric River. It is not only the Bureau of Meteorology who appears to have an aversion to the term. As the ﬁrst person in Australia to complete a PhD thesis on Atmospheric Rivers, I experi- enced the resistance to the concept ﬁrst-hand. I felt colleagues took me more seriously if I described my PhD work in the context of large-scale regions of enhanced water vapour transport instead of Atmospheric Rivers despite referring to the same thing. I spent the ﬁrst year of the PhD frustrated after every presentation I gave because, without 4 K. J. REID Figure 2. Figure from Reid et al. (2021) showing the integrated water vapour transport during the March 2021 extreme rainfall and ﬂoods. Figure 3. Tweet from the Bureau of Meteorology describing ‘atmospheric taps’. fail, I would always be questioned on the existence of Atmospheric Rivers by senior scien- tists. The interrogation of new ideas is fundamental to scientiﬁc progress, and I assure you, reader, I have no intention of stiﬂing debate nor declaring AR science settled. What was tedious was that these conversations had already been had elsewhere years earlier (Ralph et al. 2020). Decades of scientiﬁc literature in high quality journals had already been written on the topic. I was not inventing a new concept, but merely expand- ing our regional understanding of ARs to include Australia. The name used to describe strong moisture transport could certainly be argued as being somewhat trivial, however it is inconceivable that anyone could be simultaneously up to date with the literature in this area, engaged with international forecasting and research agencies on the topic, and not AUSTRALIAN GEOGRAPHER 5 use the widely accepted name. It is not the name itself that is important, but the avoid- ance of the term is a symptom of deeper concerns. We are lagging the rest of the world. This is problematic because ARs can cause extreme weather events in Australia. They have been found to cause signiﬁcantly enhanced melting and associated discharge over Australia’s snowpack (Mcgowan et al. 2020)and areassoci- ated with around 30% of the top 100 heaviest rainfall days in southeast Australia (Reid et al. 2022). The recent ﬂoods of March 2021 and February–March 2022, were associated with ARs making landfall over eastern Australia. In addition, the AR associated ﬂooding in February–March 2022 in southern Queensland and Northern New South Wales was not well forecast. At just a 24-hour lead-time, most of the extreme rainfall appeared oﬀshore in the ECMWF high-resolution model, which is usually regarded as the best weather fore- casting model. At a lead-time of two weeks, global models indicated that rainfall over these regions were likely to be below average … rather than record breaking (ECMWF 2022). Additionally, reconnaissance eﬀorts over the eastern Paciﬁc have shown that enhanced dropsonde observations during strong AR events can improve precipitation forecasts at 24-hour lead-times (Lord et al. 2023). There has been one observational study of an AR in Australia (Rauber et al. 2020) and we have the potential to grow and become a regional leader in this area. Improved regional understanding of ARs is important for improving our preparedness and resilience to climate extremes in Australia, and targeted observa- tional campaigns could potentially improve short-term forecasts of extreme events (Lavers et al. 2020). Between the initial submission of this essay and a later edit, the Bureau of Meteorology and Commonwealth Scientiﬁc and Industrial Research Organisation released their State of the Climate report for 2022 (CSIRO & Bureau of Meteorology 2022), which did include a mention of ARs. The widespread ﬂooding of 2022 may be the impetus needed to spur more research in Australia on the mechanisms that move vast amounts of water vapour through the atmosphere. Perhaps the tide is turning, and atmospheric rivers will be seen as more than just a buzzword from the 90s. Atmospheric Rivers are large-scale regions of enhanced water vapour transport that are associated with extreme weather events globally. While the concept of a river in the sky captivated the media, it polarised scientists. Following years of debate, a formal deﬁnition was constructed for the American Meteorological Society’s weather glossary. Research eﬀorts globally exploded in recent years, but Australia is behind in our regional understanding of ARs. This is a concern due to the role of ARs in extreme rainfall over eastern Australia. Despite decades of established literature, there is resistance to accept the term Atmospheric River within Australia, which undoubtedly slows research eﬀorts. I cannot help but wonder: if El Niño was discovered today, would the name be accepted? Probably not. It is neither a scientiﬁc term nor descriptive of the actual phenomenon, but that has not hindered research eﬀorts on the topic. The name itself does not matter, but the avoidance of it does. 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Australian Geographer – Taylor & Francis
Published: Apr 3, 2023
Keywords: Atmospheric river; Australia; flooding; terminology; observations; extreme rainfall
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