Dust transporting wind systems in the lower Lake Eyre Basin, Australia: A preliminary study

Aeolian Research 2 (2011) 205–214

Contents lists available at ScienceDirect

Aeolian Research journal homepage: www.elsevier.com/locate/aeolia

Dust transporting wind systems in the lower Lake Eyre Basin, Australia: A preliminary study C.L. Strong a,⇑, K. Parsons a, G.H. McTainsh a, A. Sheehan b a b

Desert Knowledge CRC and Atmospheric Environment Research Centre, Griffith School of the Environment, Griffith University, Australia Moolawatana Station, SA, Australia

a r t i c l e

i n f o

Article history: Received 24 March 2009 Revised 4 November 2010 Accepted 12 November 2010 Available online 22 December 2010 Keywords: Wind erosion Dust storms Cold fronts Troughs Wind systems

a b s t r a c t Australia does not have named dust transporting winds, like most other global dust source regions. Previous studies indicate that Australian dust is transported offshore via two dust paths to the southeast and northwest and that these dust paths are fed by three wind systems associated with the west–east passage of frontal weather systems across the southern half of the continent. This preliminary study uses 2 years of meteorological observations and mean sea level synoptic analyses to quantify the main weather systems and resultant wind systems responsible for dust entrainment and transport in the lower Lake Eyre. Of the 160 dust event days recorded in 2005 and 2006, 51% were associated with fronts and pre-frontal troughs; which generate pre-frontal northerlies, frontal westerlies and post-frontal southerlies that feed dust into the two dust paths. Heat troughs accounted for 24% of dust event days; with pre-trough northerlies and post-trough southerlies and westerlies from thunderstorms feeding the two dust paths. High pressure systems accounted for 22% of dust event days; with southeasterlies entraining most of the dust and low pressure systems account for only 3% of dust event days associated with variable winds. There is a distinct seasonality of dust entrainment; during late (austral) spring to mid summer in the north and extending to summer further south, driven by heating of the southern hemisphere in summer resulting in a poleward shift of fronts, troughs and pressure systems as spring and summer progress. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction This paper examines the main wind systems, and underlying weather systems, responsible for entraining and transporting dust in the lower Lake Eyre Basin (LLEB) over a 2-year period. There are a number of major wind systems worldwide which are known for their dust transporting capacity. The ‘Harmattan’ is the dry, cool Saharan wind that transports dust from the Bodele Depression in the Chad Basin across West Africa (McTainsh and Walker, 1982), then out across the Atlantic Ocean to the West Indies (Prospero et al., 1981) and South America (Prospero and Nees, 1977). The Mediterranean has a number of dusty wind systems emanating from the northern Sahara; including the ‘Khamsin’, ‘Sharav’ and ‘Sirocco’ which are all hot dry southerly winds that blow from the interior of North Africa over Egypt in late boreal spring and early summer. Southern California experiences the ‘Santa Ana’; a

Abbreviations: LLEB, lower Lake Eyre Basin; DSI, Dust Storm Index; BoM, Bureau of Meteorology. ⇑ Corresponding author. Address: Griffith School of Environment, Griffith University, Nathan 4111, Australia. Tel.: +61 7 3735 3509; fax: +61 7 3735 7459. E-mail address: Craig.Strong@griffith.edu.au (C.L. Strong). 1875-9637/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aeolia.2010.11.001

foehn-type wind that carries dust from the dry south-west of the USA (Raphael, 2003). The Japanese recognize the ‘Kosa’ (meaning ‘‘yellow sand’’), a westerly wind that carries dust from China (Nishikawa and Kanamori, 1991). Despite its arid nature and moderately high frequency of dust storms (McTainsh et al., 2005) Australia does not have internationally known and named dust transporting winds, and the names that do exist are seldom used these days. In south east Australia, the ‘Southerly Buster’ describes the sudden shift in wind direction from north to south, accompanied by an increase in wind speed and drop in temperature. While this wind entrains considerable dust in inland regions, the name is most commonly used in coastal areas of south east Australia. The ‘Brickfielder’ is a historical term; once used in Sydney to describe the hot northerly winds that carry dust during summer months. The name, which came from wind picking up dust from a local brick works was later extended to any hot dry wind from the north. In western Queensland the haboob-style dust storms which emanate from the Simpson Desert used to be referred to as ‘‘Bedouries’’; after the small town on the eastern edge of the Simpson Desert. That these names are seldom used now may evidence decreased frequency and intensity of wind erosion from the 1930 and 40s

206

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

dust bowl decades (McTainsh and Leys, 1993), and/or it may reflect the diverse nature of dust entraining and transporting winds in Australia. The importance of climate as a driver of dust transport systems has been reinforced through the work on the role of drought upon wind erosion. The spatial correlation between dust storm occurrence and arid and semi-arid regions in Australia was noted first by Loewe (1943), and later Middleton (1984) with McTainsh and Pitblado (1987) providing the first continent-wide maps of dust activity. The all important role of drought in reducing soil moisture and vegetation cover; leading to increased soil erodibility to wind was described by McTainsh and colleagues using a suite of empirical climate index models (Burgess et al., 1989; McTainsh et al., 1990; McTainsh, 1998). While these climate index models evidence the role of the climate driver of wind erosion, they provide few process insights into which wind systems are responsible for dust entrainment.

2. The Sprigg Model of dust entraining wind systems Bowler (1976) was first to identify that there are two major dust paths in Australia; the South East Dust Path and the North West Dust Path (Fig. 1A). He noted the semi-circular, continental scale pattern of linear dunes, mapped by Jennings (1968) and made the insightful inference that this dune pattern must also reflect the predominant dust transporting winds during the Quaternary. Through measurements of wind run, wind direction and wind speed in the dunefields of the Strzelecki and southern Simpson Deserts Sprigg (1982) proposed a conceptual model that described the main wind systems feeding dust into Bowler’s two dust paths (Fig. 1B). Sprigg concluded that three wind systems were responsible for dune formation, and by association dust storm entrainment, in Australia. These are: (i) Pre-frontal anti cyclonic northerlies, (ii) frontal westerlies which form the classic Haboob-style dust storms, and (iii) post-frontal southerlies (Fig. 1B). All three wind systems are associated with the eastward passage of cold fronts; which separate two air masses of contrasting wind, temperature and density typically forming between two eastward moving anticyclones (Sturman and Tapper, 1996). Strong pressure gradients can occur across the front line causing high wind speeds. Under the influence of the preceding high pressure cell, wind directions will be north to north-westerly. As a front traverses southern Australia in an easterly direction, wind directions back to the west, then the south. These fronts are typically shallow (rarely exceeding 1000–1500 m in height – McGowan and Clark, 2008) and become more vigorous at night (Reeder et al., 2000). Sprigg (1982) proposed that the wind systems generated by these cold fronts are responsible for dust entrainment in Australia. Wind erosion mapping studies by McTainsh and colleagues provided general support for the Sprigg Model, by showing that spatial patterns of dust movement correlate with these wind systems (e.g. McTainsh and Pitblado, 1987; McTainsh et al., 1990). Also, studies of individual dust storm events associated with frontal systems provided unequivocal evidence that cold fronts are important in forming very large dust storms (e.g. Raupach et al., 1994; McTainsh et al., 2005). Daily trajectory modelling over a 20-year period by McGowan and Clark (2008) also showed that dust transport in and beyond Australia followed the general directions outlined by Bowler’s two dust pathway model, and that the three wind system model proposed by Sprigg (1982) is indeed active. The spatial statistical study of synoptic weather conditions associated with dust storms by Ekstrom et al. (2004) provided further support for the Sprigg Model. Using mean sea level pressure as an indicator of synoptic weather conditions in different dust storm seasons Ekstrom et al. (2004) found that dust events occurring in

the austral spring–summer dust storm season were strongly related to the passage of frontal systems, while in the summer– autumn season the relationship was less clear. While the accumulated evidence that there is a relationship between frontal systems and dust storms is strong, there is little quantitative information on the overall role of fronts versus other weather systems, or the relative importance of the three dust transporting winds to produce dust events. The present paper aims to provide a preliminary study of the quantitative evidence of the weather systems and wind systems that are responsible for dust entrainment and transport in the lower Lake Eyre Basin, Australia during 2005 and 2006. 3. Study area The Lake Eyre Basin is the best region in Australia to study meteorological controls on dust processes as it is the most active dust storm region (Middleton, 1984; McTainsh and Pitblado, 1987) and the 8th most active dust source region in the world (Washington et al., 2003). Also, as one of the world’s largest internal drainage basins; covering almost one-sixth of the Australian continent and spanning 13° of latitude (20°S to 33°S), the Lake Eyre Basin experiences a range of meteorological conditions that are representative of most of arid Australia. The southern half of the basin (the lower Lake Eyre Basin or LLEB) was chosen for this study because it has a relatively high density of Bureau of Meteorology (BoM) stations. Seven BoM stations were used: three within the LLEB (Moomba, Marree and Arkaroola), and four close to the boundary of the basin (Tibooburra, Broken Hill, Hawker and Leigh Creek) (Fig. 2). To further increase the number of observation sites, a DustWatch station at Moolawatana (a cattle property at 29 54°S, 139 43°E) was added. The Moolawatana station provided dust observations as per the BoM observation protocols (Leys et al., 2008), except that as observations were not restricted to prescribed hours a more comprehensive observational record was gained. 4. Data and methods Dust entrainment within the LLEB, during the calendar years 2005 and 2006 was identified from surface based meteorological observations at any of the seven BoM stations or the DustWatch station. In this study a dust event day (DED) is defined as a day when P1 station within the LLEB (or within 250 km of the catchment boundary) recorded dust entrainment, using any of the 10 World Meteorology Organisation (WMO) dust codes (McTainsh and Pitblado, 1987). Note that the dust haze code (06) was not used, because this measures dust in transport, which could have originated from outside the LLEB. Dust event days based upon only one station record were common; reflecting: the size of the LLEB, the large distances between stations, and the variable frequency with which observations were made (3 hourly at best) at different stations. The dust event day definition has a significant effect upon the number of recorded days. Leslie and Speer (2006) recorded only 55 dust days for a decade (1995–2005) across southern Australia, however, their definition of a dust day was not clearly articulated. Bullard et al. (2008) restricted their dust event days to only regional dust storms (reducing visibility below 1 km) and consequently reported 26 dust event days over a 2-year period (mid 2004 to mid 2006). By broadening the DED definition here, we are able to describe a wider range of dust entrainment events. A total of 160 DEDs were recorded in the present study during 2005–2006, however, it is acknowledged that even this is an incomplete record because of the low spatial density of stations

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

207

Fig. 1. (A) Bowler’s map of dust paths (Modified after Bowler, 1974). (B) Sprigg’s model of dust transporting winds (Modified after Sprigg, 1982).

and variable observation frequencies. Increasingly, remote sensing is used to identify occurrences of dust events (Bullard et al., 2008), but it too provides an incomplete record, arising from cloud cover and the frequency with which images are taken, (e.g. MODIS offers the best coverage of the LLEB with two passes a day). The meteorological conditions associated with each DED, including; wind direction, wind speed, pressure, temperature and rainfall, were obtained from the BoM surface meteorological record at each station, or the DustWatch station, at the time of the minimum observed visibility for the day. The weather systems responsible for dust events were subdivided into five types, using

published definitions and conventional methods. (1) A front is defined as a region in which cooler air moves equatorwards causing warm air to be forced aloft over its surface slope (Raupach et al., 1994; BoM, 2003). These systems extend northwards from parent low pressure cells. (2) A pre-frontal trough is defined as a zone of cool air with a gusty wind shift at the leading edge, with the rear edge coinciding with a front (Garratt, 1987; Hanstrum et al., 1989). These systems may also initiate thunderstorm activity and intensify at the expense of the front (Hanstrum et al., 1989; Schultz, 2005). (3) A heat trough is defined as either a poleward extension of a low-latitude heat-low producing a meridionally elongated

208

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

Fig. 2. Location of Bureau of Meteorology (BoM) stations and the DustWatch site used in this study in and around the lower Lake Eyre Basin, Australia.

region of low-pressure often reaching the sub-tropical high-pressure belt, or as an independent trough system without a parent heat-low connection (Fandry and Leslie, 1984; Skinner and Leslie, 1999). These systems generally move in a west to east direction but are often quasi-stationary for several days (Yimin et al., 2001). (4) A high pressure system contains high pressures of varying strengths which move from west to east and produce southerly winds in southern Australia not associated with fronts. When these systems block, or alter the passage of other synoptic scale systems (including fronts) they are referred to as blocking highs (Sinclair, 1996). (5) Low pressure systems are defined as systems which contain low pressures of varying strengths which are not associated with trough or fronts. These above definitions are similarly described in the standard Australian and New Zealand university text, Sturman and Tapper (1996). These five weather systems were classified on pressure field charts at both mean sea level (MSLP) and 700 hPa for 2 days before and 2 days after the dust event day. Independent classifications were made by two authors examining the synoptic charts and two events were removed when consensus was not reached. Visual assessment of MSLP charts has been used to classify weather events, including; cold fronts in central Australia (Smith et al., 1995), fires (Mills, 2005) and dust storms (Brazel and Nickling, 1986; Tao et al., 2006). A semiobjective synoptic classification technique was used by Ganor et al. (2010) in which manual classification of synoptic charts over a short time periods served as a ‘training’ data set to a more automated modified descriminant analysis. A range of statistical techniques have been used for characterising patterns of weather systems (McInnes et al., 1994; Kidson, 2000) and pressure patterns influencing dust storm patterns (Ekstrom et al., 2004), all of which were at broad spatial scales and/or long temporal scales, but these methods are not useable on the small data sets involved in the classification of weather systems on an event basis. The dust entraining wind systems associated with these weather systems were then classified using BoM wind direction, temperature and pressure data.

On days when more than one station recorded dust (n = 39), data from the station with the minimum visibility was used to determine meteorological conditions. On only four of these occasions did wind directions vary between the stations by greater than 90°, reflecting the effect of a small scale synoptic weather system, such as a heat trough (discussed later). 5. Results and discussion Table 1 summarises the weather systems (columns) and wind systems (rows) associated with DEDs during 2005 and 2006. Of the 160 DEDs observed: 51% were associated with cold fronts (and pre-frontal troughs), 24% with heat troughs, 22% with high pressure systems, and only 3% with low pressure systems (Table 1a). Examples of the mean surface level pressure charts for the four weather systems are presented in Fig. 3. Although 2006 (Table 1c) had almost twice as many DEDs as 2005 (Table 1b) the relative frequencies of the four weather systems were similar. Table 1 also shows that three wind systems are the most common, with; 31% of DEDs associated with the northerlies (wind directions 22.5–292.5°), 14% with westerlies (wind directions 292.5–202.5°), 46% with southerlies (wind directions 202.5– 112.5°), and only 9% with winds from the east (wind directions 112.5–22.5°) (Table 1a). The relative frequencies of these wind systems were also similar between 2005 (Table 1b) and 2006 (Table 1c). Although most of the weather systems produce some dust from all wind directions, some wind directions are more commonly associated with certain weather systems. The winds associated with fronts and pre-frontal troughs are predominantly northerly (pre-frontal) and southerly (post-frontal) winds, with a smaller number of frontal westerlies. This result is entirely consistent with the three wind systems described in the Sprigg Model (Fig. 1), which feed dust into both the South East Dust Path and North West Dust Path. Heat troughs produce mainly northerly (pre-trough) and southerly (post-trough) winds which feed both dust paths.

209

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

Table 1 The number of dust event days (percentage frequency in brackets) associated with weather systems in columns (F, frontal system; PFT, pre-frontal trough; HT, heat trough; HP, high pressure; LP, low pressure) and wind systems in rows, for (a) both years combined; (b) year 2005 and (c) year 2006. F

PFT

HT

HP

LP

Total (%)

(a) Both years combined Northerlies 22.5–292.5° Westerlies 292.5–202.5° Southerlies 202.5–112.5° Easterlies 112.5–22.5° Total (%)

Bearing

22 12 19 3 56 (35)

9 3 13 0 25 (16)

12 5 16 5 38 (24)

3 2 26 5 36 (22)

3 0 0 2 5 (3)

49 (31) 22 (14) 74 (46) 15 (9) 160

(b) 2005 Northerlies Westerlies Southerlies Easterlies Total (%)

22.5–292.5° 292.5–202.5° 202.5–112.5° 112.5–22.5°

11 10 7 1 29 (50)

1 2 5 0 8 (14)

1 2 4 1 8 (14)

1 0 9 0 10 (17)

1 0 0 2 3 (5)

15 (26) 14 (24) 25 (43) 4 (7) 58

(c) 2006 Northerlies Westerlies Southerlies Easterlies Total (%)

22.5–292.5° 292.5–202.5° 202.5–112.5° 112.5–22.5°

11 2 12 2 27 (26)

8 1 8 0 17 (17)

11 3 12 4 30 (30)

2 2 17 5 26 (25)

2 0 0 0 2 (2)

34 (33) 8 (8) 49 (48) 11 (11) 102

Fig. 3. Mean sea level pressure charts showing examples of the four main dust entraining weather systems, with location of dust event observation (star). (A) Cold front with pre-frontal trough (Moolawatana – 14 August 2005 with 70 km/h NW winds). (B) Heat trough (Moolawatana – 25 January 2006 with 70 km/h NNE winds). (C) High pressure system (Tibooburra – 4 April 2005 with 20 km/h SE winds). (D) Low pressure system (Moolawatana – 28 October 2005 with 40 km/h NNE winds).

High pressure systems produce mainly southerlies, feeding the North West Dust Path, and low pressure systems mainly northerlies (though based on a very small data set) (Table 1a). Although there were some inter-annual variations in the wind systems associated with the four weather systems (for example there were more frontal westerlies in 2005 than 2006, and more northerlies associated with heat troughs in 2006), overall the relationships appear robust. Over the 2-year study period the seasonality of dust entrainment in the LLEB was clearly bimodal; with a small mode in austral spring (October) followed by a larger mode in austral summer

(January) (Fig. 4). There were also differences in the seasonality of weather systems. Fronts associated with dust (example in Fig. 3A) were observed at any time of the year, though with a higher frequency between August and March (Fig. 4). Pre-frontal troughs, showed well defined seasonality occurring in late spring through to late summer (Fig. 4). Heat troughs (example in Fig. 3B) were similar to pre-frontal troughs, though they had an even stronger seasonality. The seasonality of high pressure systems (example in Fig. 3C) was less well defined, and similar to fronts. The frequency of low pressure systems (example in Fig. 3D) was too low to determine any seasonal pattern.

210

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

Fig. 4. Seasonal occurrence of dust events (bars). Seasonal trends of monthly maximum temperature (Marree) (line plot). Occurrence of heat troughs (HT), high pressure systems (HP), fronts (F), pre-frontal troughs (PFT) and low pressure systems (LP).

6. Discussion 6.1. How representative are 2005 and 2006 of the longer term? Ideally studies of climate patterns and dust activity should cover long time periods to account for inter-annual variability. Data limitations, however usually mean that inter-annual and interdecadel studies are rare (McGowan and Clark, 2008). The use of reanalysis data can overcome the temporal problem, albeit with a reduction in data quality compared with observational data. For example, McGowan and Clark (2008) calculated 20 years of air parcel trajectories in their study of Australian dust pathways, and Schwanghart and Schutt (2008) investigated the meteorological causes of the Harmattan over a 15-year period. Most dust studies in Australia are either; event-based (e.g. Raupach et al., 1994; McTainsh et al., 2005), use case studies (e.g. Leslie and Speer, 2006), use annual average data over long time periods (e.g. McTainsh et al., 1998), or cover short time periods (e.g. Cattle et al., 2009 – 12 months, Bullard et al., 2008 – 3 years). As the present preliminary study covers only 2 years (2005 and 2006) information is needed on where these years fit into longer term climate patterns and dust activity trends. The Southern Oscillation Index (SOI) and cycles of El Niño and La Niña have a pronounced effect on the occurrence of drought in eastern Australia (Carberry et al., 2007). Negative SOI values frequently indicate El Niño conditions which commonly produce lower than average rainfall and drought conditions (Carberry et al., 2007). Conversely, positive SOI values are associated with La Niña conditions, which usually result in moister conditions. The SOI in 2005 was 3.6; indicating a relatively weak El Niño (Fig. 5), which weakened further to 1.9 in 2006. This trend continued into 2008 when the SOI became strongly positive (10.2) with La Niña conditions prevailing (Fig. 5).

These slightly negative SOI conditions were accompanied by levels of dust activity which ranged from slightly above to below the long term average (Fig. 6). The Dust Storm Index (DSI) (McTainsh, 1998) provides a measure of the frequency and intensity of dust activity. In 2005 DSI for the LLEB was slightly above the 47-year long term average and in 2006 the DSI was half of the long term average. The apparent discrepancy between the DSI values for 2005 and 2006 and the total DEDs for these years is, in itself, interesting and will be discussed below. In summary, 2005–2006 were not unusually dry or dusty years, therefore, on the face of it, the weather systems and wind systems that entrain and transport dust may also not have been unusual. 6.2. Weather systems Most of the studies which have built upon Sprigg’s (1982) classic work on the three dust transporting frontal winds (summarised in McTainsh and Leys, 1993) have found support for Sprigg’s hypothesis relating to the importance of frontal systems in dust entrainment. Also, more recently, McGowan and Clark (2008) suggested that ‘‘widespread dust entrainment within the (Lake Eyre) Basin is most often associated with the passage of sub-tropical cold fronts’’ (McGowan and Clark, 2008). The results of the present study indicate that only 35% of the dust activity observed in 2005–2006 was associated with cold fronts, and even when the associated pre-frontal troughs are included, the proportion only increases to about half (51%). While the short time period of the study precludes a definitive statement (for example, in 2005 it was 64% and in 2006 43%) it is now clear that the dust transporting weather systems and wind systems in Australia are more complex and variable than previously thought. Cold fronts and pre-frontal troughs occur in the col of two high pressure systems. Pre-frontal troughs can either precede a front

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

211

Fig. 5. Southern Oscillation Index (SOI) for 2004–2008 (Source: BoM 2009; Department of the Environment, Water, Heritage and the Arts 2006). Horizontal line separates wet and dry cycles of SOI, shaded area highlights the study period.

Fig. 6. Annual Dust Storm Index (DSI) for 4 BoM stations within the lower Lake Eyre Basin (1960–2008). Mean long term DSI value indicated by dashed horizontal line, shaded area highlights the study period.

(here called a pre-frontal trough), and/or extend equatorward from the end of a cold front (here called an extended frontal trough) (Fig. 3A). Cold fronts and pre-frontal troughs traverse the continent from west to east and their latitudinal position moves poleward with the seasonal migration of the sun (details in Section 6.3). Of the non-frontal weather systems, responsible for the other half of the DEDs in the LLEB, heat troughs appear to be the next most important in dust entrainment during the pilot study period of 2005–2006. Heat troughs extend poleward from monsoonal low pressure systems in northern Australia (Fig. 3B) in the austral summer, and migrate south as summer progresses. These troughs often have thunderstorms associated with them which can produce intense local haboob-style dust storms (like the Birdsville dust storm of 25th January, 2006 recorded by the iconic image which appeared on the front page of ‘‘The Australian’’ newspaper, 27 January 2006 – http://www.newstext.com.au/frontpage/find.asp?sed=27&sem=01&sey=2006&P=AUS&sbs=Search&s=1). Unlike pre-frontal troughs which regularly traverse the continent in an easterly direction, heat troughs are generally quasi-stationary, semi permanent features during the summer months (Leslie and Skinner, 1994; Skinner and Leslie, 1999; Yimin et al., 2001). The heat troughs can either be anchored to their more stationary sub-tropical low pressure hosts, and appear as near stationary trough lines over the

lower latitudes, or they can link up with cold fronts or pre-frontal troughs, causing the heat troughs to swing in a pendulum fashion. High pressure systems are associated with a similar number of DEDs as heat troughs. They move eastward, separated by fronts (Fig. 3A). In winter, they track across the centre of the continent at around latitude 25°S (Fig. 3A) and can become stationary over the continent producing stable conditions with little dust entrainment. In summer their track is around latitude 35°S, and south easterly winds associated with the pressure gradient between them and sub-tropical low pressure systems (Fig. 3C) entrain dust. Easterly moving low pressure systems can produce intensely unstable conditions over small areas (Fig. 3D), but these produce the smallest number of DEDs. Globally, frontal systems are recognized as the most important weather system for transporting dust over large spatial scales (Tsoar and Pye, 1987; Livingstone and Warren, 1996). They play an important role in dust entrainment and transport in major global dust source regions, including: Northern Africa and the Middle East (Tsoar and Pye, 1987), the United States (Brazel and Nickling, 1986), and the Kalahari in southern Africa (Bullard and Nash, 1998). Other global weather systems responsible for dust entrainment include; large synoptic-scale depressions, thunderstorms and

212

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

mountain range winds. Large synoptic-scale depressions occur off the Mediterranean coast of North Africa in the boreal spring. Strong temperature and pressure gradients between Saharan and Mediterranean air masses travelling eastward can entrain dust in the northern Sahara (Bullard and Livingstone, 2009). Also, northeast winds in the boreal winter associated with pressure differentials produce the Harmattan wind (Schwanghart and Schutt, 2008); one of the major dust transporting winds on earth (McTainsh and Walker, 1982). The weather system controls upon dust entrainment by these large synoptic-scale depressions over the Lake Chad Basin of the southern Sahara has parallels with dust entrainment by high pressure systems over the Lake Eyre Basin, Australia, however quantities of dust entrained by the Harmattan are several orders of magnitude greater than those in Australia (McTainsh, 1985). Thunderstorms create very fast downward moving cool air called downdrafts, or in Australia microbursts (D. Millhouse Bureau of Meteorology – personal communication, 2006) which often produces intense localised dust storms associated with trough systems (Brazel and Nickling, 1986; Tao et al., 2006). These dust storms are known in Africa as ‘haboobs’, and in northern Africa are associated with the northward movement of the intertropical front which brings moisture from the Atlantic over summer (Bullard and Livingstone, 2009). Thunderstorm-induced haboobs also occur in Israel (Offer and Goossens, 2001), the Arabian Peninsula (Miller et al., 2008) and in the USA (Brazel and Nickling, 1986). The heat troughs in these regions produce thunderstorms with intense thunder activity and frequent lightning strikes (Griffin et al., 1983), but often little or no rainfall, which would otherwise reduce dust entrainment. Winds associated with mountain ranges commonly produce strong dry winds capable of lifting dust. The warm, dry, turbulent airflows that descend from the mountains, known as foehn winds, are called ‘‘Norwesters’’ in the South Island of New Zealand (McGowan et al., 2002), in southern California they are called the ‘‘Santa Ana winds’’, and in southern Africa ‘‘Berg winds’’ (Soderberg and Compton, 2007). Gaps in mountain ranges can also funnel winds, increasing their wind speeds and turbulence. The funnelling of the Harmattan wind between the Tibesti and Innedi Massifs in the Chad Basin is a critical ingredient in dust entrainment from the Bodele Depression (Washington and Todd, 2005), the most active dust source region on earth (Prospero et al., 2002). 6.3. Wind systems The two dust pathways that traverse and exit the Australian continent, to the northwest and southeast (Fig. 1A) have been confirmed and expanded by subsequent studies. Studies of large dust storms by Raupach et al. (1994) and McTainsh et al. (2005), and of dust deposition patterns by Cattle et al. (2002) have confirmed these pathways. Air parcel trajectory studies of a trans-Tasman Sea dust event by McGowan et al. (2000) indicated that within the South East Dust Path there can be a range of trajectories. Mackie et al. (2008) identified three common trajectories; to the northeast over the Coral Sea, the east over the Tasman Sea and to the south into the Southern Ocean. The North West Dust Path may be subdivided into; the more common northwesterly trajectories passing over Broome, Western Australia, and the northerly trajectories which pass over the Gulf of Carpentaria, but these have received limited research attention. Maps of the air parcel trajectories originating from the Lake Eyre Basin by McGowan and Clark (2008) have shown that air parcels exiting to the northwest also frequently pass over Indonesia, leading these authors to the hypothesis that dust rainout with the onset of monsoonal rains could take place. In summary, although these studies have provided much new information on these two dust paths, they have

not added much to our knowledge of how different weather systems produce particular wind systems which feed dust into these dust paths. The present study quantitatively confirms and elaborates Sprigg’s (1982) three wind system model of dust transporting winds associated with fronts, over the short study time. Pre-frontal northerlies were active dust entraining winds (19%) associated with fronts and pre-frontal troughs (Table 1a). These winds feed dust into the southern sector of the South East Dust Path which passes over the Southern Ocean (McGowan et al., 2000; Boyd et al., 2004; Mackie et al., 2008). Frontal westerlies carrying dust occur much less frequently (9%) (Table 1a), but usually produce dramatic haboob-style dust storms (e.g. McTainsh et al., 2005) which feed dust into the eastern sector of the South East Dust Path, sending dust out over the Tasman Sea to New Zealand, as well as to the northeast over the Coral Sea. These haboobs differ from the thunderstorm haboobs commonly found overseas (discussed earlier) in that they are associated with fronts and not thunderstorms. As a result they cover much larger areas, for example the 23 October, 2002 haboob was 2400 km long, <400 km wide and traversed approximately half of the continent (McTainsh et al., 2005). There was a major decrease in frontal westerlies from 21% in 2005 (Table 1b) compared with 3% in 2006 (Table 1c). Given that the frontal haboobs associated with westerlies are capable of producing especially intense dust storms (e.g. McTainsh et al., 2005), this probably explains the discrepancy between the two measures of dust activity used here; DED frequency and DSI. While 2005 had a significantly lower DED frequency (n = 58) (Table 1b) compared with 2006 (n = 102) (Table 1c), the more frequent occurrence of westerlies in 2005 explains why the DSI for 2005 is 4.2 compared with 2.3 for 2006 (Fig. 6). Post-frontal south to southeast winds associated with fronts and pre-frontal troughs accounted for 20% of the events (Table 1a), and these were almost twice as frequent in 2006 (Table 1c) as in 2005 (Table 1b). These are the main winds feeding dust into the northwest dust path. It is unclear, however, what proportion of these events would have passed north into the Gulf of Carpentaria, or been caught in the sub-tropical southeasterly trade winds, which during summer feed into a sub-tropical ridge at about 37°S and transport dust to the northwest (McGowan and Clark, 2008). The winds associated with heat troughs are similar to the prefrontal troughs. Northwesterly winds (pre-trough) and southeasterlies (post-trough) winds can feed into the South East and North West Dust Paths (respectively), in a similar fashion to the pre-frontal troughs. However, the winds produced by downdraft thunderstorms associated with heat troughs may be westerlies, as occurred at Birdsville in northern Lake Eyre Basin on the 25 January, 2006 (as featured on the front page of ‘The Australian’). High pressure systems tend to produce mainly southeasterly winds feeding into the North West Dust Path, whereas winds associated with localised and infrequent low pressures systems appear to come from a variety of directions which do not regularly feed into the dust paths.

6.4. Seasonality of dust activity Dust storms in eastern Australia have a well defined seasonality. McTainsh and Leys (1993) describe two dust storm regions with different seasons; a northern region (north of 33°S – Broken Hill in Fig. 2) which experiences most dust storms during the austral spring and early summer (September to December), and the southern region (south of Broken Hill), which experiences most dust storms in summer/autumn (December to April).

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

As the LLEB is located north of Broken Hill it might be expected (based on the work of McTainsh and Leys, 1993), to have peak dust event day occurrence in spring/early summer (as for the northern region). The seasonality of DEDs in the LLEB, is however more similar to that of the southern region; with a small peak in spring (October) followed by a larger peak in summer (January) (Fig. 4). There may be two reasons for this. Firstly, the onset of summer rainfall which usually shuts down dust storms in north eastern Australia happens less often in the more arid LLEB. Secondly, during summer, fronts over south east Australia tend to track further south (of the Broken Hill line) than over the LLEB because of blocking highs over the Tasman Sea. Eastward moving fronts in the mid-latitudes occur near continuously throughout the year (Fig. 4), however, their latitudinal position is controlled by the position of large, semi-permanent high-pressure cells (Colquhoun et al., 1985; Reeder and Smith, 1992 and Sturman and Tapper, 1996). During the austral winter high pressure systems are positioned over the middle of the continent creating stable conditions, with limited dust entrainment (Fig. 4), then during late spring and early summer the high pressure systems move southwards (Pook, 2002), with frontogenesis arising from the greater contrast in temperature and density between the airmasses. One high pressure system brings cold polar marine air in contact with warm continental air of another high pressure system (Fig. 3A) (Sturman and Tapper, 1996). The December decrease in dust storm activity in the northern region is linked with this southward migration of high pressure systems, plus the onset of summer rains in the northeastern parts of the continent. As summer progresses the southward migration of the high pressure systems brings the fronts and the dust storms to south east Australia in late summer/autumn (McTainsh et al., 1998). In contrast to fronts, pre-frontal troughs are highly seasonal; occurring only in spring and summer. Increased surface temperatures during spring and early summer, increase atmospheric instability producing trough lines extending north into the subtropics (Reeder and Smith, 1992; Pook, 2002) increasing dust storm activity in the northern region and further south as summer progresses. Heat troughs are associated with monsoonal low pressure systems and are therefore common in summer (Reeder et al., 2000; Fandry and Leslie, 1984). Summer heating of the southern hemisphere results in the southward migration of the Intertropical Convergence Zone to south of the equator, impacting Australia’s tropical north. Low pressure systems in the tropical regions produce heat troughs (HT in Fig. 3B) that extend southward over central Australia increasing dust activity in the northern part of LLEB in summer (Fig. 4). Although high pressure systems influence weather at all times of the year, they mainly influence dust activity during summer (Fig. 4) when they are centred around latitude 35°S producing southeasterly winds over the LLEB. High pressure systems can also have a blocking effect on frontal systems; pushing them south of the continent (Trenberth and Mo, 1985; Sinclair, 1996) and reducing the potential for dust entrainment from these fronts. The major eastward-tracking low pressure systems are mainly south of the continent (less than latitude 30°S) and don’t directly produce dust entraining winds, however, on a few occasions small intense low pressure systems can develop over the continent (Fig. 3D) producing high local wind speeds. A longer term study is needed to better understand the role of these less frequently occurring weather systems in dust entrainment.

7. Summary and conclusions The results of this preliminary study provide direct process links between the different weather systems driving the main dust

213

transporting wind systems, and how these feed dust into the South East and North West Dust Paths of Australia. In so doing, they quantitatively confirm and extend both the two dust path model of Bowler (1976) and the frontal three wind system model of Sprigg (1982). In Australia the major dust entraining weather systems were thought to be frontal systems. While the present paper confirms a clear relationship between frontal systems and dust storms in the lower Lake Eyre Basin of Australia, this only occurred on 51% of dust event days (with 35% associated with fronts and 16% with pre-frontal troughs) in 2005 and 2006. This percentage changed between the 2 years (from 64% in 2005 to 43% in 2006), and is likely to be different when averaged over a longer time period. Heat troughs and high pressure systems are more or less equally responsible (24% and 22%) for the remaining dust event days, with only 3% associated with low pressure systems. To further support these preliminary findings the analysis of a long term data set is needed. Acknowledgements The project reported here was supported by the Australian Government Cooperative Research Centres Programme through the Desert Knowledge CRC; however, the views expressed herein do not necessarily represent the views of Desert Knowledge CRC or its participants. We also wish to acknowledge the generosity of the keen DustWatchers throughout Australia who observe and report dust activity. Thanks also to two anonymous reviewers whose constructive criticisms contributed significantly to the quality of the final paper. References Bowler, J.M., 1976. Aridity in Australia – age, origins and expression in aeolian landforms and sediments. Earth-Science Reviews 12, 279–310. Boyd, P.W., McTainsh, G.H., Sherlock, V., Richardson, K., Nichol, S., Ellwood, M., Frew, R., 2004. Episodic enhancement of phytoplankton stocks in New Zealand subantarctic waters: contribution of atmospheric and oceanic iron supply. Global Biogeochemical Cycles 18, GB1029. doi:10.1029/2002GB002020. Brazel, A.J., Nickling, W.G., 1986. The relationship of weather types of dust storm generation in Arizona (1955–1980). Journal of Climatology 6, 255–275. Bullard, J.E., Baddock, M.C., McTainsh, G.H., Leys, J.F., 2008. Sub-basin scale dust source geomorphology detected using MODIS. Geophysical Research Letters 35, L15404. Bullard, J.E., Livingstone, I., 2009. Dust. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments, 2nd ed. Springer B.V., pp. 613–639. Bullard, J.E., Nash, D.J., 1998. Linear dune pattern variability in the vicinity of dry valleys in the southwest Kalahari. Geomorphology 23, 35–54. Bureau of Meteorology, 2003. Manual of Aviation Meteorology. Commonwealth Bureau of Meteorology, Melbourne. Burgess, R.C., McTainsh, G.H., Pitblado, J.R., 1989. An index of wind erosion in Australia. Australian Geographical Studies 27, 98–110. Carberry, P., George, D., Buckley, D., 2007. The Southern Oscillation Index and Southern Australia. Primefact 615, 1–2. Cattle, S.R., McTainsh, G.H., Elias, S., 2009. Æolian dust deposition rates, particlesizes and contributions to soils along a transect in semi-arid New South Wales Australia. Sedimentology 56, 765–783. Cattle, S.R., McTainsh, G.H., Wagner, S., 2002. Characterising æolian dust contributions to the soil of the Namoi Valley, northern NSW, Australia. Catena 47, 245–264. Colquhoun, J.R., Shepherd, D.J., Coulman, C.E., Smith, R.K., McInnes, K., 1985. The Southerly Buster of South eastern Australia: an orographically forced cold front. Monthly Weather Review 113, 2090–2107. Ekstrom, M., McTainsh, G.H., Chappel, A., 2004. Australian dust storms: temporal trends and relationships with synoptic pressure distributions (1960–99). International Journal of Climatology 24, 1581–1599. Fandry, C.B., Leslie, L.M., 1984. A two layer quasi-geostrophic model of summer trough formation in the Australian subtropical easterlies. Journal of the Atmospheric Sciences 41, 807–818. Ganor, E., Osetinsky, I., Stupp, A., Alpert, P., 2010. Increasing trend of African dust, over 49 years, in the eastern Mediterranean. Journal of Geophysical Research 115, D07201. Garratt, J.R., 1987. Summertime cold fronts in Southeast Australia – behaviour and low-level structure of main frontal types. Monthly Weather Review 116, 636– 649. Griffin, G.F., Price, N.F., Portlock, H.F., 1983. Wildfires in the Central Australian Rangelands, 1970–1980. Journal of Environmental Management 17, 311–323.

214

C.L. Strong et al. / Aeolian Research 2 (2011) 205–214

Hanstrum, B.N., Wilson, K.J., Barrell, S.L., 1989. Pre-frontal troughs over Southern Australia Part I: Climatology. Weather and Forecasting 5, 22–31. Jennings, J.N., 1968. A revised map of the desert dunes of Australia. The Australian Geographer 10, 408–409. Kidson, J.W., 2000. An analysis of New Zealand synoptic types and their use in defining weather regimes. International Journal of Climatology 20, 299–316. Leslie, L.M., Skinner, T.C.L., 1994. Real-time forecasting of the Western Australian summertime trough: evaluation of a new regional model. Weather and Forecasting 9, 371–383. Leslie, L.M., Speer, M.S., 2006. Modelling dust transport over central eastern Australia. Applied Meteorology 13, 141–167. Leys, J., McTainsh, G., Strong, C., Heidenreich, S., Biesaga, K., 2008. DustWatch: using community networks to improve wind erosion monitoring in Australia. Earth Surface Processes and Landforms 33, 1912–1926. Livingstone, I., Warren, A., 1996. Aeolian Geomorphology: An Introduction. Longman, London. Loewe, F., 1943. Duststorms in Australia. Australian Meteorological Bureau, Melbourne. Mackie, D.S., Boyd, P.W., McTainsh, G.H., Tindale, N.W., Westberry, T.K., Hunter, K.A., 2008. Biogeochemistry of Australian dust – from eolian uplift to marine uptake. Geochemistry, Geophysics, Geosystems 9, Q03Q08. doi:10.1029/ 2007GC001813. McGowan, H., Clark, A., 2008. Identification of dust transport pathways from Lake Eyre, Australia using Hysplit. Atmospheric Environment 42, 6915–6925. McGowan, H.A., Sturman, A.P., Kossmann, M., Zawar-Reza, P., 2002. Observations of foehn onset in the Southern Alps, New Zealand. Meteorology and Atmospheric Physics 79, 215–230. McGowan, H.A., McTainsh, G.H., Zawar-Reza, P., 2000. Identifying regional dust transport pathways: application of kinematic trajectory modelling to a transTasman case. Earth Surface Processes and Landforms 25, 633–647. McInnes, K.L., McBride, J.L., Leslie, L.M., 1994. Cold fronts over southeastern Australia: their representation in an operational and numerical weather prediction model. Weather and Forecasting 9, 384–409. McTainsh, G.H., 1985. Dust processes in Australia and West Africa: a comparison. Search 16, 104–106. McTainsh, G.H., 1998. Dust storm index. In: Indicators for Sustainable Agriculture, Sustainable agriculture: assessing Australia’s recent performance. SCARM Technical Report 70, 65–72. Available from: . McTainsh, G.H., Walker, P.H., 1982. Nature and distribution of Harmattan dust. Zeitschrift fur Geomorphologie 26, 417–435. McTainsh, G.H., Pitblado, J.R., 1987. Dust storms and related phenomena measured from meteorological records in Australia. Earth Surface Processes and Landforms 12, 415–424. McTainsh, G.H., Lynch, A.W., Burgess, R.C., 1990. Wind erosion in eastern Australia. Australian Journal of Soil Research 28, 323–339. McTainsh, G., Leys, J., 1993. Soil Erosion by wind. In: McTainsh, G., Boughton, W.C. (Eds.), Land Degradation Processes in Australia. Longman Cheshire, Melbourne, pp. 189–233. McTainsh, G.H., Lynch, A.W., Tews, E.K., 1998. Climatic controls upon dust storm occurrence in eastern Australia. Journal of Arid Environments 39, 457–466. McTainsh, G.H., Chan, Y., McGowan, H., Leys, J.F., Tews, E.K., 2005. The 23rd October 2002 dust storm in eastern Australia: characteristics and meteorological conditions. Atmospheric Environment 39, 1227–1236. Middleton, N.J., 1984. Dust storms in Australia: frequency, distribution and seasonality. Search 15, 46–47. Miller, S.D., Kuciauskas, A.P., Liu, M., Ji, Q., Reid, J.S., Breed, D.W., Walker, A.L., Al Mandoos, A., 2008. Haboob dust storms of the southern Arabian Peninsula. Journal of Geophysical Research 113 (D1), D01202. Mills, G.A., 2005. On the subsynoptic-scale meteorology of two extreme fire weather days during the Eastern Australian fires of January 2003. Australian Meteorological Magazine 54, 265–290.

Nishikawa, M., Kanamori, S., 1991. Chemical composition of KOSA aerosol (yellow sand dust) collected in Japan. Analytical Sciences 7, 1127–1130. Offer, Z.Y., Goossens, D., 2001. Ten years of aeolian dust dynamics in a desert region (Negev Desert, Israel): analysis of airborne dust concentration, dust accumulation and high-magnitude dust events. Journal of Arid Environments 47, 211–249. Pook, M., 2002. The roaring forties sometimes purr. Australian Antarctic Magazine 4, 31–32. Prospero, J.M., Glaccum, R.A., Nees, R.T., 1981. Atmospheric transport of soil dust from Africa to South America. Nature 289, 570–572. Prospero, J.M., Nees, R.T., 1977. Dust concentrations in the atmosphere of the equatorial North Atlantic: possible relationship to the Sahelian drought. Science 196, 1196–1198. Prospero, J.M., Ginoux, P., Torres, O., Nicholson, S., Gill, T., 2002. Environmental characterization of global sources of atmospheric soil dust identified with the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product. Reviews of Geophysics 40, 1002. Raphael, M.N., 2003. The Santa Ana Winds of California. Earth Interactions 7, 1–13. Raupach, M.R., McTainsh, G.H., Leys, J.F., 1994. Estimates of dust mass in recent major Australian dust storms. Australian Journal of Soil and Water Conservation 7, 20–24. Reeder, M.J., Smith, R.K., Deslandes, R., Tapper, N.J., Mills, G.A., 2000. Subtropical fronts observed during the 1996 Central Australian Fronts Experiment. Australian Meteorological Magazine 49, 181–200. Reeder, M.J., Smith, R.K., 1992. Australian spring and summer cold fronts. Australian Meteorological Magazine 41, 101–124. Schultz, D.M., 2005. A review of cold fronts with pre-frontal troughs and wind shifts. Monthly Weather Review 133, 2449–2472. Schwanghart, W., Schutt, B., 2008. Meteorological causes of Harmattan dust in West Africa. Geomorphology 95, 412–428. Sinclair, M.R., 1996. A climatology of Anticyclones and blocking for the Southern Hemisphere. Monthly Weather Review 124, 245–264. Skinner, T.C.L., Leslie, L.M., 1999. Numerical Prediction of the Summertime RidgeTrough System over Northeastern Australia. Weather and Forecasting 14, 306– 325. Smith, R.K., Reeder, M.J., Tapper, N.J., Christie, D.R., 1995. Central Australian Cold Fronts. Monthly Weather Review 123, 16–39. Soderberg, K., Compton, J.S., 2007. Dust as a nutrient source for fynbos ecosystems, South Africa. Ecosystems 10, 550–561. Sprigg, R.C., 1982. Some stratigraphic consequences of fluctuating Quaternary sea levels and related wind regimes in southern and central Australia. In: Wasson, R.J. (Ed.), Quaternary dust mantles. Australian National University, Canberra, China, New Zealand and Australia, pp. 211–240. Sturman, A.P., Tapper, N.J., 1996. The weather and climate of Australia and New Zealand. Oxford University Press, Melbourne. Tao, G., Yongfu, X., Xiao, Y., 2006. Synoptic characteristics of dust storms observed in Inner Mongolia and their influence on the downwind area (the Beijing– Tianjin Region). Meteorological Applications 13, 393–403. Trenberth, K.F., Mo, K.C., 1985. Blocking in the Southern Hemisphere. Monthly Weather Review 113, 3–21. Tsoar, H., Pye, K., 1987. Dust transport and the question of desert loess formation. Sedimentology 34, 139–153. Washington, R., Martin, T., Middleton, N., Goudie, A.S., 2003. Dust-storm source areas determined by the total ozone monitoring spectrometer and surface observations. Annals of the Association of American Geographers 93, 297–313. Washington, R., Todd, M.C., 2005. Atmospheric controls on mineral dust emission from the Bodélé Depression, Chad: the role of the low level jet. Geophysical Research Letters 32, L17701. doi:10.1029/2005GL023597. Yimin, M., Lyons, T.J., Blockley, J.A., 2001. Surface influences on the Australian West Coast Trough. Theory and Applied Climatology 68, 207–217.