- Title
- Historical variability of east coast lows (ECLs) and their impact on Eastern Australia’s hydroclimate
- Creator
- Twomey, Callum
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The hydroclimate of eastern Australia is highly variable, with a multitude of large-scale climate processes bearing considerable influence on spatial and temporal rainfall characteristics. One phenomenon known for its contribution to rainfall and which operates on daily timescales, are East Coast Lows (ECLs). These intense low-pressure systems which take place over the subtropical east coasts of southern and northern hemisphere continents are typically associated with gale force winds, large seas, storm surges, heavy rainfall and flooding. While ECL impacts are usually seen as negative (e.g. flooding, storm damage etc.), the rainfall associated with ECLs is also very important for urban water security within the heavily populated eastern seaboard of Australia (ESA). This region of Australia contains a high number of city centres which are forecast to undergo disproportionate rates of growth compared to other areas in Australia. As a result, considerable pressure will be placed on water infrastructure and its resilience to climate variability. This thesis investigates the historical variability of ECLs, and their impact on eastern Australia’s hydroclimate, with particular emphasis placed on the ESA. Within the last decade, several comprehensive ECL databases have been developed. Despite this, inconsistencies remain as to what constitutes an ECL. This has hindered our ability to understand these systems and their impacts. In this thesis, we demonstrate that the definition of an ECL should include classification of the various ECL sub-types based on the synoptic-scale environments from which they form. ECL sub-types have different spatial distributions, seasonal cycles, and rainfall characteristics. Consequently, regions of eastern Australia and in particular the ESA, are influenced differently by different ECL sub-types. An investigation of rainfall across Australia and within the ESA suggests that the ESA is different to the rest of Australia and also not homogenous itself. For winter three separate divisions are identified: (i) the most northerly division from Moreton in Queensland (QLD) to the Manning region of New South Wales (NSW); (ii) the Hunter region south to the metropolitan Sydney area; and (iii) from Illawarra (NSW) to Eastern-central Victoria. For summer, autumn, and spring rainfall, two clear divisions are present: (i) the two most northerly divisions identified in winter combined and (ii) and the equivalent of the third and most southerly outlined for winter. The results suggest that the observed spatial inhomogeneity in rainfall across the ESA is at least in part due to ECLs and their sub-types. Though ECLs may only last a few days, they do have the capacity to provide considerable contributions to water storage reservoirs. Approximately one-third of ECL related rainfall occurs in the 48-hours prior to the system entering the Tasman Sea. Furthermore, given the trajectory of sub-types such as Inland Troughs (IT), Continental Lows (CL) and Southern Secondary Lows(SSL), much of this rainfall provides relief to western flowing headwaters, inland of the Great Dividing Range (GDR). An examination of seasonal rainfall contributions reveals that rainfall associated with Easterly Trough Low (ETL) is statistically significant along the central and northern latitudes of the ESA within winter. SSLs are also found to be significant for the southern extent of the ESA and extends its influence into spring, while CLs establish significance across Victoria. On daily time-scales ITs and ex-tropical cyclones are found to have significantly higher rainfall totals than non-ECL sources (and a number of other ECL sub-types) for their regions of preference. However, due to their infrequent nature, this did not translate into significant seasonal contributions, signifying an important difference in what sub-types present a risk to flooding and those, or rather their absence, that present a risk to water security. ECL sub-types and how their variability impact eastern Australia’s hydroclimate is also shown to be affected by large-scale climate processes. Changes in the spatial distribution of ECLs is found to reflect changes in the proportion of ECL sub-types. When in the La Niña phase, the El Niño Southern Oscillation (ENSO) (and its variant ENSO Modoki) tend to shift the spatial distribution of ECLs north. In winter, this also corresponds to an increase in overall ECL activity. This results in more than a 50% increase in ECL related winter rainfall, while similar magnitude of decrease was observed during the El Niño phase. Other mechanisms such as the Indian Ocean Dipole (IOD) were also found to have a considerable influence on the spatial distribution of ECLs and their associated rainfall. During negative IOD conditions, increases in rainfall west of the GDR corresponded to an increase in the proportion of westerly ECLs. Conversely, during positive IOD, ECL rainfall increases within the ESA owing to a change in the proportion of ECL sub-types. This thesis also provides insights into the importance of ECLs and their sub-types to a key streamflow monitoring station within the Hunter region of NSW. As an indicator for inflowpotential to the Grahamstown Dam, ECLs are responsible for 74% of all streamflow ≥ 99th percentile within the Williams River catchment. Likewise, the absence of ECLs is also shown to be associated with times where the Williams River is experiencing its lowest flow rates. The findings of this thesis are significant and demonstrate the influence ECL sub-types have on hydroclimatic variability in eastern Australia. It also reveals that existing climate related risks are different across the ESA and suggests that how those risks change into the future is also likely to be inconsistent across the ESA – and will likely depend heavily on what eventuates in terms of changes to ECL, and the various ECL sub-types and behaviour (e.g. frequency, timing, location, duration, magnitude and sequencing). This reinforces the need for locally relevant and practically useful climate science information and adaptation strategies - as opposed to State- or Countrywide information and adaptation approaches that are commonly used.
- Subject
- east coast lows; ECL; climate; variability; ENSO; Indian Ocean Dipole; atmospheric blocking; cut-off lows; Pahsa Bulker; hydroclimate; Eastern Seaboard of Australia; ESA; storm; cyclone; rainfall; precipitaion; water security
- Identifier
- http://hdl.handle.net/1959.13/1356112
- Identifier
- uon:31615
- Rights
- Copyright 2017 Callum Twomey
- Language
- eng
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