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Transport of inshore waters and suspended material off the continental shelf by Dense Shelf Water Cascades (DSWC) has important ecological and biogeochemical implications in Australian waters. Because of high rates of evaporation, denser saline water along the sea bed occurs in the shallow coastal regions around Australia, setting up horizontal density gradients that can form DSWC. Ocean glider data available in the Integrated Marine Observing System (IMOS), which is operated by the Australian National Facility for Ocean Gliders (ANFOG) located at the University of Western Australia, are being used to measure cross-shelf density profiles under varying wind and tide conditions for seven contrasting regions around the entire continent. Overall 97 sets of spatial and temporal resolution data from year 2008 to 2015 collected by the ocean gliders in the selected locations will be analyzed for this study. Analysis of 19 transects covering data from year 2012 to 2015 for Pilbara, which is in North-western continental shelf of Australia, indicate that cascades occur during the autumn and winter due to cooling of the coastal water which already have higher salinity due to evaporation during the summer months and intregrated by cooling later in the winter. The cross-shelf density gradient in this continental shelf is maximum in July which is about 14.23x10-6 kgm-4.
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Journal of Coastal Research
SI
75
XX-XX
Coconut Creek, Florida
2016
Factors influencing the occurrence of Dense Shelf Water Cascades
in Australia
Tanziha Mahjabin*, Charitha Pattiaratchi, and Yasha Hetzel
ABSTRACT
Mahjabin, T.; Pattiaratchi, C., and Hetzel, Y., 2016. Factors influencing the occurrence of Dense Shelf Water Cascades
in Australia. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th
International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. XX-
XX. Coconut Creek (Florida), ISSN 0749-0208.
Transport of inshore waters and suspended material off the continental shelf by Dense Shelf Water Cascades (DSWC)
has important ecological and biogeochemical implications in Australian waters. Because of high rates of evaporation,
denser saline water occurs in the shallow coastal regions around Australia, setting up horizontal density gradients that
can drive DSWC. Ocean glider data available from the Integrated Marine Observing System (IMOS), which is operated
by the Australian National Facility for Ocean Gliders (ANFOG) located at the University of Western Australia, were
used to measure cross-shelf density profiles under varying wind and tide conditions for seven contrasting regions
around the entire continent. Overall 97 sets of spatial and temporal resolution data from year 2008 to 2015 collected
by the ocean gliders and analysed with a subset presented here. Data from 19 transects covering the years 2012 to 2015
for the Pilbara region of Western Australia, indicated that cascades occur during the autumn and winter due to cooling
of the coastal waters which already have higher salinity due to evaporation during the summer months. The cross-shelf
density gradient in this continental shelf was found to be maximum in July with a value of 14.23x10-6 kg m-4.
ADDITIONAL INDEX WORDS: Cascades, gliders, horizontal density gradient.
INTRODUCTION
Dense shelf water is formed in coastal waters either by a
decrease in temperature through cooling or increase in salinity
from evaporation or ice formation (Figure 1). In Australian
waters, high rates of evaporation, up to 2.5m per year, (Figure
2) with negligible rainfall and run-off cause a net loss of fresh
water from the inner continental shelf. This results in coastal
waters having higher salinity than offshore. During the winter
months the shallower coastal waters lose heat due to convective
processes resulting in colder water near the coast. The
combination of higher salinity colder water closer to the coast
results in a horizontal density gradient (d𝜌/dx) with increasing
density from the ocean towards the coast. This gradient drives
a gravitational circulation with the offshore transport of denser
water along the sea bed. This is controlled by vertical mixing
resulting from turbulence generated by the wind and the tide
(Hetzel et al., 2013; Pattiaratchi et al., 2011). Under low wind
and tidal mixing conditions either a bottom gravity current or a
surface plume will be present depending on the sign of the
horizontal density gradient (Figure 1a, 1b) and under strong
wind and tidal mixing conditions the water column is well
mixed (Figure 1c, 1d).
The buoyancy-driven gravity current forms Dense Shelf Water
Cascades: DSWC (Canals et al., 2006; Pattiaratchi et al., 2011;
Shapiro et al., 2003; Shearman and Brink, 2010).
Dense water cascades have been found in over 60 locations
around the world and most of them happen in Polar Regions
due to ice formation (Ivanov et al., 2004). DSWC can play a
major role in transporting terrestrial carbon, nutrients, larvae,
low-oxygen water, sediments and also pollutants from coastal
regions to deeper ocean.
DSWC have been documented previously for in some
locations around Australia by research cruises and using
moorings, usually for individual events during single seasons:
E.g. North-west Australian shelf (Brink and Shearman, 2006;
Shearman and Brink, 2010), Shark Bay (Pattiaratchi and Woo,
2009), Great Australian Bight (Petrusevics et al., 2009),
Spencer Gulf (Bowers and Lennon, 1987).
Seasonal variation of DSWC has been identified as a major
feature through the deployment of ocean gliders in coastal
waters along the Rottnest continental shelf using 13 months of
data (Pattiaratchi et al., 2011). Since these previous studies
wide-ranging ocean glider data have become available, making
it possible to investigate DSWC in other areas around Australia
with different wind and tide conditions (Figure 2). In this paper,
we used high spatial and temporal resolution temperature and
salinity data collected using Teledyne Webb Research Slocum
Gliders (Schofield et al., 2007) to identify DSWC formation in
the Pilbara region of Western Australia and to investigate
DSWC dynamics in general.
____________________
DOI: 10.2112/SI75-XXX.1 received Day Month Year; accepted in
revision Day Month Year.
*Corresponding author: tanziha.mahjabin@research.uwa.edu.au
©Coastal Education and Research Foundation, Inc. 2016
School of Civil, Environmental and Mining Engineering & UWA
Oceans Institute, The University of Western Australia, Australia.
www.JCRonline.org
XX Mahjabin, Pattiaratchi, and Hetzel
_________________________________________________________________________________________________
Journal of Coastal Research, Special Issue No. 75, 2016
Figure 1. Effects of vertical mixing by wind and tide in the presence of a cross-shelf density gradient. Under low wind and tidal mixing conditions either
a bottom gravity current or a surface plume will be present depending on the sign of the horizontal density gradient (a,b). Under strong vertical mixing
conditions the water column is well mixed (c,d).
Figure 2. Annual evaporation rate in Australia (Yu, 2007) and selected
study sites with tidal range and mean wind speed in summer and winter:
(i) Kimberley; (ii) Pilbara; (iii) Two Rocks; (iv) Investigator Strait; (v)
Port Stephens; (vi) Yamba; (vii) Capricorn Channel .
METHODS
Glider data from seven sites (Figure 2) are available to study
DSWC under contrasting environmental: (i) Kimberley, north-
west Australia: macro-tidal and moderate wind (Shearman and
Brink, 2010); (ii) Pilbara, north-west Australia: macro-tidal
(Holloway, 1983); (iii) Two Rocks, Western Australia: mainly
wind driven (Pattiaratchi et al., 1997) with low tidal range with
diurnal tides (Pattiaratchi and Eliot, 2008); (iv) Investigator Strait,
South Australia: mainly spring-neap tidal cycle driven (Nunes
and Lennon, 1986; 1987); (v) Port Stephens New South Wales:
moderate tide dominated (McPherson et al., 2013) and strong
wind (Geary, 1987); (vi) Yamba, New South Wales: mostly wind
driven and micro-tide (Pritchard et al., 2007) ; (vii) Capricorn
Channel, Queensland: wind and tide driven (Andutta et al., 2011).
This paper focusses on data from the Pilbara site that has
moderate wind and tidal forcing. Analysis of the other sites is the
subject of future work.
We used Teledyne Webb Research Slocum Electric Glider
(Schofield et al., 2007) data which are operated by the Australian
National Facility for Ocean Gliders (ANFOG) located at the
University of Western Australia. The data are publicly available
through the Integrated Marine Observing System (IMOS).
Slocum gliders cover maximum depth range of 200 m. Gliders
can measure data from the surface to up to 5 m above the seabed
with mean speed of 25 km per day (Schofield et al., 2007).
Gliders traverse a saw-tooth pattern using buoyancy control
whilst moving forward to the target destination and navigating to
a series of pre-programmed waypoints using GPS, internal dead
reckoning and altimeter measurements. A Seabird-CTD,
Chlorophyll-a fluorescence measuring sensor, coloured dissolved
organic matter (CDOM) sensor, 660 nm Backscatter WETLabs
BBFL2SLO optical sensor and an Aanderaa Oxygen Optode were
attached to the ocean glider for this study. Slocum Gliders are
small in size (1.8 m), efficient and economical to sample for much
longer periods and higher spatial resolution compared to ships.
For this study, 19 transects from 13 sets of glider missions for
the Pilbara were analysed over the period 2012 to 2015
considering specific tide and wind conditions. This included a
total of 40248 vertical profiles and over 13.5 million data points.
Analysis aimed to identify the presence of DSWC and the effects
of wind and tide on the cascade formation. Quality control of the
data were done after the recovery of each glider and then actual
XX Factors influencing the occurrence of Dense Shelf Water Cascades around Australia XX
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Journal of Coastal Research, Special Issue No. 75, 2016
vehicle trajectory was transposed onto the Pilbara transects as a
straight line. Each variable was interpolated onto a grid with
vertical and horizontal resolutions of specific time and depth
respectively. Density gradients were calculated by defining
latitudes and longitudes of the starting shallow part and shelf
break of the transects, while the direction of shallow part to shelf
break was considered as positive. Tidal data were predicted using
the TPXO7.2 global database and wind data were obtained from
the European Centre for Medium-Range Weather Forecast
Interim Reanalysis (ECMWF ERA-I) (Dee et al., 2011).
Vertical temperature stratification has been compared with the
local wind speed cubed (W3) and bottom current speed cubed
(|Ub3|). These are proportional to available mixing energy from
the wind and tidal currents respectively (e.g. Nunes and Lennon,
1987; Nunes Vaz et al., 1990).
RESULTS
Analysis of the 19 transects of ocean glider data in the Pilbara,
collected between 2012~2015, indicated that DSWC were a
common occurrence during the winter months when cross-shelf
density gradients formed with denser water near the coast. Figure
3 represents the location of the glider path near Pilbara on July
2012 which is chosen for an example.
Figure 3. Glider path location for Pilbara July 2012 is shown on the Sea
Surface Temperature map.
Cross-shelf transects (Figure 4) indicated that both temperature
and salinity contributed to the dense water formation. The dense
water then flowed along the sea bed and spilled off of the
continental shelf reaching depths of up to 150m (Figure 4). This
DSWC was observed during July 2012 when high salinity water
had accumulated near the coast due to summer evaporation and
subsequently this water was cooled and the gradient became
strong enough to force the inshore water off the shelf. Spatial
patterns of fluorescence, sediment and oxygen closely followed
density, indicating that these properties were influenced by the
DSWC (Figure 5), and were likely transported along the sea bed
from the continental shelf to the open ocean.
Figure 4. Cross-shelf profile of DSWC as measured by a Slocum glider
on the Pilbara coast during July 2012.
Figure 5. Ancillary water properties (Fluorescence, sediment and oxygen)
as measured by a Slocum glider on the Pilbara coast during July 2012.
Driving force for the formation of cascades
Analysis of all 19 transects suggested that winter cooling was
an important contributor to DSWC formation in the Pilbara, with
a majority of the cascades occurring during winter months. In
winter the winds were weaker and the cross-shelf density
gradients were enhanced by cooling (Figure 6).sediment supply
and the concentration of wave energy removes the sand, causing
erosion (Figure 7).
XX Mahjabin, Pattiaratchi, and Hetzel
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Journal of Coastal Research, Special Issue No. 75, 2016
Figure 6. The monthly density gradient for Pilbara showing the mean
density gradient for all available data for each month. The error bar for
July indicated the range of values calculated using different transects.
Data from multiple seasons indicated that favorable density
gradients formed repeatedly in autumn and winter months of each
year and DSWC occurred whenever the vertical mixing from
wind and tide was weaker.
Using multiple transects acquired over several years we
calculated the average horizontal density gradient for each month
of the year (Figure 6). The calculated positive density gradient
(onshore offshore) in Pilbara was maximum in July as 14.23x
10-6 kgm-4. The gradient became negative during the summer. The
seasonality of cascade formation in Pilbara closely followed the
density gradient with all observed cascades requiring a positive
density gradient as occurred between April and September.
Control of cascade formation by turbulent vertical mixing
The data suggested that whenever the horizontal density
gradient is strong, we can expect cascade formation. However,
wind mixing and tidal mixing were also capable of inhibiting the
formation of DSWC and the relative importance of these factors
varies around Australia.
Figure 7. Pilbara transect in July 2012 with wind and tidal mixing energy
proxies (windspeed3 and tidal current speed3) with DSWC present.
Under low wind and tidal mixing conditions, the water column
stratifies and water flows offshore along the sea bed; whereas
high mixing inhibits stratification and offshore transport of water.
Tidal amplitudes vary along the Australian coast and influence
cross-shelf water movement. Two contrasting transects from
Pilbara plotted with proxies for wind and tidal mixing
(windspeed3 and tidal current speed3) illustrate the effects of
vertical mixing on DSWC formation (Figure 7 and Figure 8). The
first transect (Figure 7) showed a clear DSWC and weak tidal
currents and wind. Cascades were absent in the second transect
(Figure 8) with higher current speeds and slightly stronger winds.
Figure 8. Pilbara transect in September 2014 showing absence of cascade
and stronger tidal currents and windspeeds.
DISCUSSION
Dense water flows are the result of either intense cooling or
excess evaporation. Analysis of 19 ocean glider transects
collected between 2012 and 2015 in the Pilbara region of Western
Australia indicated that DSWC were a common occurrence,
particularly in Autumn and Winter seasons, even under relatively
strong tide and wind conditions. It has been shown previously that
cascades are controlled by turbulence generated by wind and tide
(Hetzel et al., 2013; Pattiaratchi et al., 2011). However this study
data suggests that the density gradients dominate over mixing
energy most of the time in the Pilbara during Autumn and Winter
seasons. Cascade occurs in this period of time with velocities
between 2~3 cm/s in Pilbara region (Bahmanpour et al., 2016),
which is comparable with other location where it was measured
before. DSWC have been documented previously for a single
season in North-west Australian shelf (Brink and Shearman,
2006; Shearman and Brink, 2010), but here several years data
allowed us to observe the seasonality of cascade formation for
Pilbara region.
The next step will be to analyse the data for other 6 study
regions. If the results also indicate that DSWC occur even where
tides or winds are strong, it will confirm that dense shelf water
cascades are an important process for cross shelf exchange around
the entire Australian continent. The broad range of ocean glider
deployments presents a unique opportunity to examine DSWC
over such a large and varied coastline.
XX Factors influencing the occurrence of Dense Shelf Water Cascades around Australia XX
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Journal of Coastal Research, Special Issue No. 75, 2016
CONCLUSIONS
An analysis of the dynamics of DSWC formation in the
Pilbara region of Western Australia was completed based on
19 individual transects covering three years (2012-2015). The
formation of DSWC in the Pilbara region of Western Australia
was found to depend on the balance between the cross shelf
density gradient and vertical mixing by wind and tide. When
the density gradient is strong, we can expect DSWC; but when
the mixing is strong enough to make the shallow water
vertically mixed DSWC will be absent. During autumn and
winter the cross shelf density gradient remains positive and
dominates over mixing. As a result DSWC occur in the North-
western Australian coast of Pilbara despite the relatively large
tidal range, with strongest DSWC events occurring during neap
tides and weak winds. Further analysis of 124 remaining
transects will be undertaken for other selected locations around
Australia to determine whether similar relationships exist
between cross-shelf density gradients, vertical mixing, and
DSWC.
ACKNOWLEDGMENTS
All ocean glider data used in this paper are from the Integrated
Marine Observing System (IMOS) which are operated by the
Australian National Facility for Ocean Gliders (ANFOG) located
at the University of Western Australia. IMOS is funded by the
National Collaborative Research Infrastructure Strategy and the
Super Science Initiative. The postgraduate research has been
funded by Scholarship International Research Fees (SIRF) and
University International Stipend.
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... A similar feature to that observed in 'inverse' estuarine systems located in Mediterranean climates with negligible or intermittent freshwater input and loss of freshwater through evaporation 9,10 . In coastal regions, the buoyancy-driven gravity current is defined as Dense Shelf Water Cascade (DSWC) 5,8,[11][12][13][14] and has mainly been considered as a high-latitude process 11,13 . DSWCs documented around the Mediterranean Sea are episodic and their formation ...
... Evidence for DSWC has been provided in locations around Australia based on single field experiments lasting over a period of about 1 month [6][7][8][9]13,14,[16][17][18] . Majority of these studies examined the export of higher salinity water from large inverse estuary systems 6,7,16,18 with only two studies examining shelf regions 13,17 . ...
... Schematic of the influence of wind-and tide-induced vertical mixing on the continental shelf in the presence of a cross-shelf density gradient: (a) Under low wind-and tidal-induced vertical mixing, a dense shelf water cascade is present; (b) Under strong vertical mixing, the water column is well mixed although a density gradient is present (modified from Mahjabin et al.8 ). ...
Article
Full-text available
Transport of water between the coast and the deeper ocean, across the continental shelf, is an important process for the distribution of biota, nutrients, suspended and dissolved material on the shelf. Presence of denser water on the inner continental shelf results in a cross-shelf density gradient that drives a gravitational circulation with offshore transport of denser water along the sea bed that is defined as Dense Shelf Water Cascade (DSWC). Analysis of field data, collected from multiple ocean glider data missions around Australia, confirmed that under a range of wind and tidal conditions, DSWC was a regular occurrence during autumn and winter months over a coastline spanning > 10,000 km. It is shown that even in the presence of relatively high wind- and tidal-induced vertical mixing, DSWCs were present due to the strength of the cross-shelf density gradient. The occurrence of DSWC around Australia is unique with continental scale forcing through air-sea fluxes that overcome local wind and tidal forcing. It is shown that DSWC acts as a conduit to transport suspended material across the continental shelf and is a critical process that influences water quality on the inner continental shelf.
... Chapter -2 : Background -------------------------------------------------------------------------------------------------------- Wobus et al., 20114 De Madron et al., 2005Gaudin et al., 2006;Palanques et al., , 2007Palanques et al., , 2009Puig et al., 2008Puig et al., , 2013Ulses et al., 2008;Sanchez-Vidal et al., 2008;Pasqual et al., 2010;Ribo et al., 2011;Salvado et al., 2012;Pusceddu et al., 20135 Ivanov et al., 2004Tesi et al., 2008;Chiggiato et al., 2016;Foglini et al., 20166 De Madron et al., 2005Estournel et al., 2005;Theocharis et al., 1999;Theocharis & Georgopoulos, 1993;Georgopoulos et al., 1992;Gertman et al., 19907 Roveri et al., 20148 Shapiro & Hill, 19979 Ivanov & Shapiro, 200510 Golovin, 2007Ivanov & Golovin, 200711 Banse, 199712-16 Ivanov et al., 200417 Lavin et al., 1998Valle-Levinson et al., 200318 Winant & Gutierrez de Velasco, 200319-36 Ivanov et al., 200437 Shearman & Brink, 2006, 2010Mahjabin et al., 201638 Logan & Cebulski, 1970Burling et al., 1999;39 Pattiaratchi et al., 201140 Petrusevics et al., 2009Lennon et al., 198741 Bowers et al., 1987Nunes & Lennon, 1987;Nunes Vaz et al., 1990;De Silva Samarasinghe & Lennon, 1987;Ivanov et al., 200442 Godfrey, 1980Tomczak, 1981Tomczak, , 1986Gibb, 1992;Sandery & Kampf, 2004;Ivanov et al., 2004Ribbe, 1996 Chapter -2 : Background ...
... Australia has a high rate of evaporation, around 2.5 m per year [15] with less rainfall and river run-off that generally results in coastal waters having higher salinity than offshore. Along the majority of Australian shallow coastal regions, summer evaporation leaves the shallow coastal waters more saline and subsequently in autumn and winter the nearshore waters become cooler due to heat loss by convection [7]. In combination, strong horizontal density gradients develop with density increasing from the ocean towards the coast. ...
... DSWC are controlled by vertical mixing resulting from either wind mixing and/or tidal mixing [5,10]. When wind and tidal mixing are weak either a bottom gravity current or surface plume will form and when vertical mixing is strong the water column will be well mixed [7]. The balance between the major destratifying and stratifying influences neglecting air-sea exchanges can be expressed as [8]: ...
Thesis
Full-text available
Along Australian continental shelves, high evaporation during summer and cooling during winter result in a cross-shelf density gradient that drives gravity currents transporting denser water along the sea bed offshore. This process is defined as Dense Shelf Water Cascade (DSWC). Multi-year transects (192) of ocean glider data from eight contrasting regions around Australia confirmed the existence of DSWC as a regular occurrence during autumn and winter periods. The main parameters controlling DSWC were identified as buoyancy input (cross-shelf density gradient) and vertical mixing through wind and tidal action. To examine the spatial variability of DSWC along the Perth Metropolitan continental shelf region, a three-dimensional hydrodynamic model was used. The model, validated using field measurements, confirmed the presence of DSWC throughout the model domain. Although there was gradual cooling of coastal waters during autumn and winter, there were periods of rapid heat loss during the passage of storm systems. During these periods the cross-shelf density gradients were enhanced and generated strong DSWC. Onshore winds associated with cold fronts enhanced the DSWC. The field and numerical model results confirmed the cross-shelf density gradient as the dominant forcing mechanism for DSWC formation. The influence of tidal mixing was small even in regions of high tidal range compared to the cross-shelf density gradient. In contrast, wind effects had a strong influence through: (1) inhibiting DSWC through vertical mixing; and, (2) enhancing during onshore winds. DSWC play an important role in ecological and biogeochemical processes in Australian waters as a conduit to the transport of dissolved and suspended materials offshore.
... Australia has a high rate of evaporation, around 2.5 m per year [1] with low rainfall and river run-off that generally results in coastal waters having higher salinity than offshore. Along the majority of Australian shallow coastal regions, summer evaporation leaves the shallow coastal waters more saline and subsequently in autumn and winter the nearshore waters become cooler due to heat loss via convection [2,3]. Combination of salinity and cooling effects causes strong horizontal density gradients to develop with density increasing from the ocean towards the coast. ...
... This horizontal density gradient is the driving force for the formation of buoyancy-driven flows along the sea bed, defined as dense shelf water cascades (DSWCs) [4][5][6][7]. DSWC have important ecological and biological implications as they provide an effective mechanism to transport nearshore water and dissolved and suspended material (e.g., terrestrial carbon, nutrients, larvae, low-oxygen water, sediments, and pollutants) off the continental shelves into the deep ocean [3]. Despite their ecological importance, DSWCs are rarely measured in detail because the process often consists of intermittent events occurring in the bottom layers that cannot be observed using satellite measurements [6]. ...
... The underlying driving force for DSWCs is the horizontal density gradient with inhibiting effects, mostly via vertical mixing resulting from either wind and/or tidal mixing [2,5,9,10]. In the majority of sites, data indicated that when wind and tidal mixing were weak, either a bottom gravity current or surface plume would form, or when vertical mixing was strong, the water column was well mixed [2,3]. On the continental shelf, numerous processes play important roles including, for example, the effects of: boundary currents, advection, eddies, topography, wind-driven currents, downwelling and upwelling, and ambient density fields [6]. ...
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Along the majority of Australian shallow coastal regions, summer evaporation increases the salinity of shallow waters, and subsequently in autumn/winter, the nearshore waters become cooler due to heat loss. This results in the formation of horizontal density gradients with density increasing toward the coast that generates gravity currents known as dense shelf water cascades (DSWCs) flowing offshore along the sea bed. DSWCs play important role in ecological and biogeochemical processes in Australian waters through the transport of dissolved and suspended materials offshore. In this study a numerical ocean circulation model of Rottnest continental shelf, validated using simultaneous ocean glider and mooring data, indicated that the passage of cold fronts associated with winter storms resulted in rapid heat loss through evaporative cooling. These conditions resulted in enhancement of the DSWCs due to modifications of the cross-shelf density gradient and wind effects. Specifically, onshore (offshore) directed winds resulted in an enhancement (inhibition) of DSWCs due to downwelling (vertical mixing). Consequently, the largest DSWC events occurred during the cold fronts when atmospheric temperatures reinforced density gradients and onshore winds promoted downwelling that enhanced DSWCs. Advection of DSWCs was also strongly influenced by the wind conditions, with significantly more transport occurring along-shelf compared to cross-shelf.
... Australia has a high rate of evaporation, around 2.5 m per year [15] with less rainfall and river run-off that generally results in coastal waters having higher salinity than offshore. Along the majority of Australian shallow coastal regions, summer evaporation leaves the shallow coastal waters more saline and subsequently in autumn and winter the nearshore waters become cooler due to heat loss by convection [7]. In combination, strong horizontal density gradients develop with density increasing from the ocean towards the coast. ...
... DSWC are controlled by vertical mixing resulting from either wind mixing and/or tidal mixing [5,10]. When wind and tidal mixing are weak either a bottom gravity current or surface plume will form and when vertical mixing is strong the water column will be well mixed [7]. The balance between the major destratifying and stratifying influences neglecting air-sea exchanges can be expressed as [8]: ...
... DSWC have previously been identified in the Kimberley region through examination of the oceanic response to large outgoing heat and freshwater fluxes using data collected on research cruises and moored instrument deployments for individual events in a single season [3,14]. DSWCs have also been identified in other locations around Australia [7]: Shark Bay [9]; Great Australian Bight [11]; Spencer Gulf [2]; and, the Rottnest continental shelf [10]. However, none of these studies focused on the seasonality of DSWC formation, which we considered in this work. ...
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High evaporation during the summer and cooling during autumn/winter along the coastal regions of the Australian North-West Shelf (NWS) results in a cross-shelf density gradient. This drives a gravitational circulation with the offshore transport of higher density water along the sea bed, defined as Dense Shelf Water Cascades (DSWC). Ocean glider data available from the Integrated Marine Observing System (IMOS) were used to measure cross-shelf density profiles under varying wind and tide conditions along Kimberley and Pilbara in the NWS. Analysis of 41 transects from 26 missions of high spatial and temporal resolution data of Kimberley and Pilbara covering the years 2011 to 2015 confirmed that DSWC occur on a regular basis during autumn and winter seasons, mainly due to cooling of the coastal waters that already have higher salinity due to evaporation during summer months. Cross-shelf transects (Figure 1a) indicated that both temperature and salinity contributed to the dense water formation. The dense water flow along the sea bed may be identified to depths of up to 150m. This DSWC was observed in Pilbara during July 2012 when higher saline water was present near the coast due to summer evaporation and subsequently this water was cooled and the cross-shelf gradient became sufficiently strong enough to establish the gravity current. The temporal variability and controlling mechanisms of the DSWC were investigated using data from the moorings deployed in the shallow regions of Kimberley and Pilbara. Although these two regions are macro-tidal and are subject to wind mixing, the vertical temperature stratification and monthly mean velocity profiles of cross-shore velocities indicated the presence of cascades during autumn and winter seasons. It is shown that even in the presence of high tidal mixing DSWC persists due to the strength of the cross-shelf density gradient.
... The cross-shelf density gradient drives a bottom gravity current under low wind and 41 tidal mixing conditions ( Mahjabin et al., 2016b;Pattiaratchi et al., 2011) and is controlled by 42 wind-and tide-induced vertical mixing ( Hetzel et al., 2013;Pattiaratchi et al., 2011). These 43 buoyancy-driven gravity currents are defined as dense shelf water cascades (DSWCs), and 44 have important ecological and biological implications for coastal waters (Canals et al., 2006;45 Chen et al., 2019;Mahjabin et al., 2016aMahjabin et al., , 2016bMahjabin et al., , 2019Pattiaratchi et al., 2011Pattiaratchi et al., , 201746 Shapiro et al., 2003;Shearman and Brink, 2010). ...
... The cross-shelf density gradient drives a bottom gravity current under low wind and 41 tidal mixing conditions ( Mahjabin et al., 2016b;Pattiaratchi et al., 2011) and is controlled by 42 wind-and tide-induced vertical mixing ( Hetzel et al., 2013;Pattiaratchi et al., 2011). These 43 buoyancy-driven gravity currents are defined as dense shelf water cascades (DSWCs), and 44 have important ecological and biological implications for coastal waters (Canals et al., 2006;45 Chen et al., 2019;Mahjabin et al., 2016aMahjabin et al., , 2016bMahjabin et al., , 2019Pattiaratchi et al., 2011Pattiaratchi et al., , 201746 Shapiro et al., 2003;Shearman and Brink, 2010). The circulation in a DSWC is similar to that 47 observed in 'inverse' estuarine systems located in Mediterranean climates with negligible or 48 ...
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... The gravity currents may be interrupted due to vertical mixing provided by the local wind and tidal conditions. DSWC have been identified in other locations around Australia such as [15]: Shark Bay; Great Australian Bight; Spencer Gulf; and, the Rottnest continental shelf [20]. ...
... The study site is located in a region of internal tide generation [11,22]. The region is also associated with the generation of dense shelf water cascades [15]. ...
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