4.2 Methods
4.2.2 Oceanographic Setting
Study region
The oceanographic setting is described in Bowie et al. (2011a). Figure 4.2, presents the SAZ- Sense station locations, south of Tasmania, Australia, with the relevant oceanographic features superimposed on an iso-surface of absolute salinity at 0 dbar (the surface). The major currents (and their flows) to consider are the: East Australian Current (EAC, 9.4 Sv (Ridgway and Godfrey,1994;
Ridgway, 2007)) ; Zeehan Current (ZC, 2.4 Sv (Cirano and Middleton, 2004; Ridgway, 2007)) ; and the Antarctic Circumpolar Current (ACC, 147 Sv (Rintoul and Sokolov, 2001), not shown). Approximate summer locations of the major oceanographic features within the ACC are shown: Subantarctic Front (SAF, 105 Sv (Rintoul and Sokolov, 2001)), which is split into north (SAF-N) and south (SAZ-S); and the Polar Front (PF, 5 Sv (Rintoul and Sokolov, 2001)). These frontal systems separate the distinct watermasses: Subtropical Zone (STZ); Subantarctic Zone north (SAZ- N); Subantarctic Zone south (SAZ-S); Polar Frontal Zone (PFZ).
Currents
The ACC dominates the SO, flowing from West to East (Rintoul and Sokolov, 2001). It is a composite of many smaller fronts and currents (which all vary in position through space and time), those relevant to this region have been mentioned above.
The EAC, is a well defined north-south current off the eastern coast of Australia, ∼200 km
wide (Nilsson and Cresswell, 1980;Mulhearn, 1983;Cresswell and Legeckis, 1986;Ridgway and Godfrey, 1994, 1997). In comparison to the ZC, the adjacent coastal region is densely populated, with farm land and forests that are subject to seasonal bush fires, which may be a source of highly soluble forms of micronutrients to the open ocean (Guieu et al.,2005;Sholkovitz et al., 2009;Ito,
2011;Sholkovitz et al., 2012). Soils are younger and rich in organic material and minerals easily soluble in water (Strand et al., 2002). The EAC passes through the middle of the SE atmospheric pathway (see Figure 4.3) (Mackie et al., 2008; McGowan and Clark, 2008; Strong et al., 2011) and is the beneficiary of aeolian dust transported off the Australian continent over the coastal zone (Mackie et al., 2008). It also interacts with shelf waters, which could entrain trace elements into the upper water column through vertical mixing. Physical entrainment of shelf waters has been observed by Baird and Ridgway (2012). As the EAC passes Bass Strait, it can incorporate Bass Strait Waters (BSW) as a discrete sub-surface lense inside anticyclonic eddies (Tomczak, 1981,
Chapter 4: Using dissolved aluminium concentrations to constrain trace element supply to the
subantarctic SO south of Australia 66
5.
Disc
ussio n
While not large in comparison to major Northern
Hemisphere dust sources, the Lake Eyre Basin, and in
particular dry Lake Eyre, is the most active dust source in
the Southern Hemisphere. Although
Bowler (1976)
first
described dust transport from this region more than 30
years ago, the spatial and temporal nature of his northwest
and southeast dust transport pathways has remained
unknown. Using HYSPLIT we have mapped the spatial
extent and frequency that air parcels originating over Lake
Eyre for a 20-year period have affected regions downwind
within eight days. Under favourable atmospheric condi-
tions where the coefficient of turbulent exchange is
<104
cm2
s!1
(Tsoar and Pye, 1987) in the absence of
precipitation as occurs during the passage of some frontal
systems and associated steep pressure gradients, then
these air parcels could transport PM10 dust particles and
other aerosols. Accordingly, we have shown that dust from
Lake Eyre may affect most of Indonesia, the Antarctic
mainland and much of the Southern and South Pacific
Oceans between 120"E and 90"W. On rare occasions
Fig.
2.
The spatial characteristics of summer air parcel trajectories computed forward from Lake Eyre: (a) all trajectories, (b) 0–500 m agl., (c) 500–1000 m agl.,
(d) 1000–1500 m agl., (e)1500–2000 m agl. and (f) 2000–5000 m agl.
density plots. This is due to divergent flow in this region
associated with upper level highs that are situated over the
continent during summer (Leslie, 1980). Subsidence within
these highs inhibits vertical transport, therefore resulting
in the much greater density of air parcel trajectories below
1500 m.
The air parcel trajectory density plots for autumn
display a similar pattern as those shown for summer,
particularly below 1000 m (Fig. 3b and c). This is due to the
sub-tropical ridge still being centered over southern
Australia resulting in southeasterly airflow over much of
the continent. Above 1000 m and particularly in the
2000–5000 m layer, there is greater southerly and easterly
transport due to Southern Hemisphere circulation patterns
repositioning in response to seasonal changes in radiation
receipt. At this time of year zonal westerly flow at mid-
levels is beginning to extend further north and this explains
transport densities of 1–25 per 10 m2
mapped to almost
120!W as shown in
Fig. 3f. Greater meridional transport
south toward Antarctica during this season we believe is
due to transport in northwesterly airflow ahead of very
active cold fronts embedded in the mid-latitude westerlies.
Fig. 5.
The spatial characteristics of spring air parcel trajectories computed forward from Lake Eyre: (a) all trajectories, (b) 0–500 m agl., (c) 500–1000 m agl.,
(d) 1000–1500 m agl., (e) 1500–2000 m agl. and (f) 2000–5000 m agl.
H. McGowan, A. Clark / Atmospheric Environment 42 (2008) 6915–6925
6922
Figure 4.3: A representation of the major atmospheric pathways over AUstralia by McGowan and Clark
(2008) using the HYSPLIT model. This represents the potential deposition field of dust from Australia into the Tasman Sea and the Southern Ocean. Most trajectories do not venture far offshore. The atmospheric pathways are referred to as the North West (towards Asia) and South East (towards New Zealand) components.
over the Australian continental shelf (depth<100 m) separating Tasmania from mainland Australia (Harris et al.,2005). Naturally they are in close proximity to terrestrial sources from both Tasmania and Victoria, with high likelihood of trace element enrichment from both aeolian and shelf sediment sources.
The ZC is a continuation of the Leuwin Current and flows from west to east along the southern edge of the Australian continent, before turning south down the west coast of Tasmania (Ridgway and Condie,2004). In comparison to the EAC, the adjacent coastal region is sparsely populated, mostly of lightly vegetated desert, with soils that are enriched with insoluble minerals (as the soils are old and have been heavily processed by erosive forces) (Mackie et al., 2008, and references therein). The instance of industrial pollution and bush fires (both of which generate particles with a greater soluble fraction (Guieu et al., 2005; Sholkovitz et al., 2009;Ito, 2011;Sholkovitz et al., 2012)) is lower than on the east coast of Australia. The ZC flows along the south eastern edge of the SE Atmospheric pathway (see Figure4.3) (Mackie et al.,2008;McGowan and Clark,2008), where the proportion of aeolian deposition is expected to be lower than in the middle of the pathway. Together all these factors reduce the potential trace element yield from aeolian dust deposited into the ZC.
Chapter 4: Using dissolved aluminium concentrations to constrain trace element supply to the
subantarctic SO south of Australia 67
Station 2
Station 2 was on the South-Western edge of a region with a strong seasonal signature in its dynamics, dominated by the ZC in winter and the EAC in summer (where the two currents are 180°out of phase) (Ridgway,2007). As such, the source waters of this region originate from totally different locations in summer compared to winter. The ZC, driven by the flow of the Leeuwin Current (on the Australian continental west coast), dominates from May to July (austral winter) driving waters from the Great Australian Bite down the west coast and around the southern tip of Tasmania, creating southerly and easterly flows (Ridgway, 2007). A weakening of the ZC in September, coupled with a strengthening of the EAC in October results in the EAC extension rounding the southern tip of Tasmania in November, where northerly and westerly flows then dominate the region until March (Ridgway,2007).
The meeting of the ZC, the EAC and the northern component of the ACC (at the STF) is a region of dynamic and dispersive oceanographic features (Herraiz-Borreguero and Rintoul,2011a), where lateral and vertical mixing could rapidly reduce the concentration of trace elements supplied to this region by the ZC and the EAC. MLDs are 150–200 m in winter and 50–100 m in summer (Herraiz- Borreguero and Rintoul,2011b).
Process station one (P1), station 4
P1 was in the middle of the SAZ, as were stations 5, 10 and 12 (see Figure4.2). It was expected to be exclusively under the influence of the ACC. The strong westerly winds that dominate the area make it an oceanic region with limited terrestrial influence. Although stations 5, 9, 10 and 12 are not near P1, they are within the SAZ, affected by similar oceanographic and atmospheric conditions between the PF and the STF. MLDs are 500–700 m in winter and 50–100 m in summer (Herraiz-Borreguero and Rintoul,2011b).
Process station two (P2), station 6
P2 was in the AZ. It was under the influence of the PFZ and the ACC and given its remoteness from the continents, has limited terrestrial influence. MLDs are 200–300 m in winter and 50–100 m in summer (Herraiz-Borreguero and Rintoul,2011b).
Process station three (P3), stations 17b and 17j
P3 was in a region with highly complex oceanographic features where the EAC extension interacts with the STF. Stations 19, 21, 23 and 24 were under the influence of similar oceanographic conditions as they were also within the EAC/STZ. The southern Tasman Sea, between Tasmania and New Zealand, is a region of relatively slow moving water, separated from the relatively fast ACC by the STF (Herraiz-Borreguero and Rintoul, 2011a). The STF has much slower velocities at P3 compared to its velocity near station 2 and P1. Meso-scale eddies, transported south in the EAC, enter the region and can persist for weeks to months (Mongin et al., 2011a,b). The entire region falls within the SE atmospheric pathway (Mackie et al., 2008). The slow moving nature of the watermasses increase the probability of aeolian deposition events to each watermass in the region. MLDs are 150–200 m in winter and 50–100 m in summer (Herraiz-Borreguero and Rintoul,
2011b).