Master of Science

thesisThe Great Salt Lake (GSL) in the western United States has been identified as the most mercury laden body of water in the United States with a median water mercury concentration of 42 ng L-1. When Hg enters an aquatic ecosystem, it can be converted to the toxic organic mercury compound, methylmercury. Methylmercury bioaccumulates up the food chain and has been the cause of consumption advisories for game fish in many lakes and rivers in the historically pristine Intermountain West. In 2005, the Utah Department of Health and the Fish and Wildlife Service placed a similar consumption advisory on waterfowl on the GSL. The primary goal of this study is to identify the pathway of greatest influx of Hg pollution to the GSL to give insight toward the source and an eventual solution to the Hg pollution problem. Speciated atmospheric mercury measurements were collected at a field site on the eastern shore of the GSL for a 1-year period beginning on July 1, 2009. These atmospheric mercury concentrations, along with turbulence measurements, were used as input to a resistance-in-series dry deposition model (based on Wesley and Hicks 1977). The dry deposition flux of mercury was determined from the modeled dry deposition velocity and the measured concentrations. This dry deposition flux was compared to the wet deposition flux measured by the National Deposition Network and the riverine influx measured by the USGS

Developing a better understanding of the Australian monsoon and wet season onset climatology

By some estimates, about 40-60% of the world's population lives within a monsoonal climate. For all of these people, the timing of the monsoon onset is an annual event that is critical for sustainable agriculture, fire management, water management, travel and tourism, and so much more. A late monsoon onset can create serious issues in ways that are similar to drought conditions in higher latitudes but they may onset faster and last only a few weeks. The topic of this thesis focuses on the Australian monsoon, a singular monsoon region in a global weather pattern, which experiences high wet season rainfall variability, including in the timing of the precipitation, which can cause short-term, rapid-onset droughts. For example, by most definitions the Australia monsoon onset has a standard deviation of more than ±2 weeks and a range of onset dates of nearly two months from the earliest to the latest. This research has the following two objectives: 1. Determine which monsoon onset definitions provide the most predictability at seasonal time scales, and which seasonal-scale climate drivers provide the strongest influence on onset timing. 2. Investigate the frequency of 'false onsets'—when an onset criterion is met, but follow-up rainfall is not received—and if these lead to 'flash drought' conditions over northern Australia. These objectives were accomplished by, first performing a systematic literature review of Australian monsoon onset definitions. Second, recreating 11 dynamical monsoon onset datasets and extending them to the same time period to test their seasonal predictability through correlations with large-scale seasonal climate drivers. And, third, when considering a standard wet season rainfall onset criterion, the date after 1 September that 50 mm of precipitation is accumulated, quantify the frequency of occurrence of false onsets as a physical characteristic of the north Australian climate, rapid soil moisture declines and drought development. Results presented in this thesis from the first research objective demonstrate that while the wet season rainfall onset (first rainfall of the season, usually mesoscale features and not the global monsoon) is highly predictable on a seasonal time scale, the dynamical monsoon onset (i.e. the global-scale weather pattern) is not easily predictable at these timescales by traditional seasonal climate influences. Only a strong (<-1 standard deviation) La Niña pattern shows a statistically significant correlation with an early onset of the dynamical monsoon. A weak La Niña, ENSO neutral, and a weak or strong El Niño pattern has only a weak or non-statistically significant correlation and should not be used to make monsoon onset predictions. A negative and neutral Indian Ocean Dipole (IOD) do not have a statistically significant correlation with onset dates, but a strong positive IOD correlates with a delayed monsoon onset and could be used in monsoon onset predictions. The outcomes of the second objective show that false wet season onsets are relatively common across northern Australia; 30% to 50% of wet seasons experience a false onset. False onsets are more common during La Nina and negative IOD events. False onsets do not always coincide with a 'flash drought' (investigated here as a rapid drop in soil moisture). These rapid drops in soil moisture are relatively common across northern Australia in the wet season, occurring on average at least once within about 25% of seasons. These rapid drops in soil moisture are common enough that they probably should not be a considered a drought (i.e. a climatological extreme). The findings presented in this thesis significantly advance our knowledge of Australian monsoon temporal variability. This includes: A systematic and comprehensive assessment of the literature on Australian monsoon onset definitions and timing; An analysis of monsoon onset dates and the correlations of onset timing with climate drivers; A study of wet season onset variability, false onsets and flash drought. The significance of this work extends well beyond the Australian monsoon. Similar analysis could be applied to other monsoon regions. It is also very likely that, given the variability of global monsoon patterns, other monsoon regions may experience seasons with false onsets. Investigation of the frequency of occurrence of false onsets would give residents of other monsoon regions an understanding of their climatological propensity toward drought

Seasonal climate influences on the timing of the Australian monsoon onset

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

Defining the Australian monsoon onset: a systematic review

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 (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 the 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 a limited operational applicability despite a plethora of definitions

Wet season rainfall onset and flash drought: The case of the northern Australian wet season

In this paper, we report on the frequency of false onsets of wet season rainfall in the case of the Northern Australian wet season and investigate the role of large-scale tropical climate processes such as the El Nino–Southern Oscillation, Indian Ocean Dipole (IOD) and Madden–Julian Oscillation. A false onset occurs when a wet season rainfall onset criterion is met, but follow-up rainfall is not received for weeks or months later. Our analysis of wet season rainfall data from 1950 through 2020 shows a false onset occurs, on average, between 20 and 30% of wet seasons across all of northern Australia. This increases at a regional and local level such as at Darwin, the Northern Territory (NT), and parts of Queensland's north coast to over 50%. Seasonal climate influences, such as a La Niña pattern and a negative IOD that typically expedite the wet season rainfall onset, also increase the likelihood of a false onset over northern Australia. Our analysis also finds that periods of false onsets can sometimes, but not always, coincide with periods of rapid soil moisture depletion. The false rainfall onsets that develop into flash drought can be potentially disruptive and costly and are of potential significance for agriculture and fire management in northern Australia, and in other monsoonal climates that also typically experience a slow build-up to the seasonal monsoon. In conclusion, effective rainfall indicates that many seasons experience ‘false onsets’ with dry conditions after early rainfall. We propose that false onsets are a physical characteristic of the climate of northern Australia which occurs with relatively high frequency. In addition, these false onsets may sometimes co-occur with a flash drought

Getting ahead of Flash Drought: From Early Warning to Early Action

Flash droughts, characterized by their unusually rapid intensification, have garnered increasing attention within the weather, climate, agriculture, and ecological communities in recent years due to their large environmental and socioeconomic impacts. Because flash droughts intensify quickly, they require different early warning capabilities and management approaches than are typically used for slower-developing “conventional” droughts. In this essay, we describe an integrated research-and-applications agenda that emphasizes the need to reconceptualize our understanding of flash drought within existing drought early warning systems by focusing on opportunities to improve monitoring and prediction. We illustrate the need for engagement among physical scientists, social scientists, operational monitoring and forecast centers, practitioners, and policy-makers to inform how they view, monitor, predict, plan for, and respond to flash drought. We discuss five related topics that together constitute the pillars of a robust flash drought early warning system, including the development of 1) a physically based identification framework, 2) comprehensive drought monitoring capabilities, and 3) improved prediction over various time scales that together 4) aid impact assessments and 5) guide decision-making and policy. We provide specific recommendations to illustrate how this fivefold approach could be used to enhance decision-making capabilities of practitioners, develop new areas of research, and provide guidance to policy-makers attempting to account for flash drought in drought preparedness and response plans

The tropical circulation in the Australian/Asian region-November 2010 to April 2011

A summary of the broad-scale tropical circulation from 70°E to 180°, for the six months from November 2010 to April 2011 is presented. A strong La Niña state of the El Niño Southern Oscillation (opposite of El Niño) persisted during the season with signs of weakening from February 2011. The sea surface temperature remained cooler than normal in the central and eastern equatorial Pacific during the season. The coolest waters in the equatorial Pacific remained in the central Equatorial Pacific. The Southern Oscillation Index remained strongly positive throughout the season. Onset of the Australian monsoon over northern Australia occurred around 12 December 2010, earlier than the climatological onset date and remained active until mid-March. However, monsoonal rains affected northern Australia until April and Darwin rainfall for the season was the highest on record. The Madden-Julian Oscillation signal was not clear and was erratic during the season as the convection during the season was mostly driven by a strong La Niña pattern. A total of seventeen tropical cyclones, sixteen less than average, developed in the Regional Specialised Meteorological Centre area during the period

The tropical circulation in the Australian/Asian region-May to October 2010

A summary of the broadscale tropical circulation from 70º E to 180º, for the six months May to October 2010, is presented. During this period the El Niño/La Niña-Southern Oscillation (ENSO) state shifted gradually from neutral conditions to La Niña conditions by July 2010. The Southern Oscillation Index (SOI) remained mostly above +10 during the season and reached a peak value of +25 in September. The mean SOI for the season was +15.8. Above average convection persisted throughout the season south of the equator between 90°E and 140°E and the South Pacific Convergence Zone (SPCZ). Mostly weak mean sealevel pressure (MSLP) anomalies persisted in the Darwin Regional Specialised Meteorological Centre (RSMC) analysis area except over parts of Australia, where it remained higher than normal. Easterlies in the tropical western Pacific remained stronger than normal. Sea Surface Temperatures in the tropical western Pacific gradually shifted from a warm to cooler pattern during the season and in the tropical Indian Ocean were warmer than their long-term mean. Weak active convective phases of the Madden-Julian Oscillation (MJO) were observed in the the beginning of each month during the season with a periodicity of 30 to 35 days. However, it was difficult to analyse the eastward propagation of the MJO signal mainly due to the developing La Niña conditions. A total of sixteen tropical cyclones (including eight typhoons/severe cyclones) were analysed during the period, less than the mean of 30 for the Darwin RSMC analysis area

Making sense of flash drought: definitions, indicators, and where we go from here

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