School of Civil, Environmental and Mining Engineering

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Contact

Tanziha Mahjabin

Phone: (+61 8) 6488 8112


Start date

Aug 2014

Submission date

Aug 2017

Curriculum vitae

Tanziha Mahjabin CV
[doc, 791.57 kb]
Updated 16 Dec 2014

Tanziha Mahjabin

Tanziha Mahjabin profile photo

Thesis

Occurrence and controls on dense shelf water cascades around Australia

Summary

Dense shelf water is formed in coastal waters either by a decrease in temperature through cooling or an increase in salinity from evaporation or ice formation. In arid or semi-arid regions, high rates of evaporation with negligible rainfall and river input cause a net loss of fresh water from the inner continental shelf, which results more saline coastal water bodies than the surrounding ocean waters. This creates a horizontal density gradient with density increasing from the ocean toward the coast. The gradient drives a circulation of denser saline water along the sea bed to offshore, which is governed by vertical mixing resulting from turbulence generated by the wind and the tides (e.g. Pattiaratchi et al., 2011; Hetzel et al., 2013). This buoyancy-driven gravity current forms Dense Shelf Water Cascades: DSWC (Shapiro et al., 2002; Canals et al., 2009; Shearman & Brink, 2010; Pattiaratchi et al., 2011). Dense water cascades have been found in over 60 locations around the world with the majority located in Polar Regions resulting mainly from ice formation. Australia has the highest evaporation rates globally, around 2.5 per year (Yu, 2007), which results in higher salinity and density water in the shallow coastal waters around Australia.

The overarching objective of this research is to predict offshore export of water through DSWC in seven contrasting regions around Australia and to understand the dynamics, particularly the transport and mixing characteristics of DSWC due to various forcing mechanisms (e.g., tidal mixing, wind mixing and gravitational circulation). The objective will be addressed with the observations from Ocean gliders and numerical simulations of dense shelf water formation and transport.To achieve the overarching objective, this proposal comprises three specific aims:

(i) Identify and document the occurrence of Dense Shelf Water Cascades around Australia;

(ii) Describe the role of turbulent vertical mixing which controls the formation and transport of DSWC along the Rottnest continental shelf;

(iii) Numerically simulate the formation and transport of DSWC along the Rottnest continental shelf to define the role of vertical mixing and to predict offshore flux of water.

In the present study, 7 contrasting sites that exhibit DSWC around Australian coast have been chosen for comparison (Figure 1), based on their tidal and wind regimes : (1) Kimberley, northwest Australia: macro-tidal and moderate wind (Shearman and Brink, 2010) ; (2) Pilbara, northwest Australia: macro-tidal (Holloway, 1983b); (3) Two Rocks, Western Australia: mainly wind driven (Pattiaratchi et al., 1997; Zaker et al., 2007) with low tidal range with diurnal tides (Pattiaratchi and Eliot, 2008) ; (4) Investigator Strait, South Australia: mainly spring-neap tidal cycle driven (Nunes and Lennon, 1986, 1987); (5) Port Stephens, New South Wales: moderate tide dominated (McPherson et al., 2013) and strong wind (Geary, 1987); (6) Yamba, New South Wales: mostly wind driven and micro-tide [Pritchard et al., 2007]; and, (7) Capricorn Channel, Queensland: wind and tide driven (Andutta et al., 2011) .

Why my research is important

The continental shelves of Australia have been proposed to be the world’s largest contributor of dense water production from evaporation to the global water cycle as it is the largest oceanic region dominated by evaporation (Yu, 2007). Dense shelf water production is directly affected by evaporation and freezing. Considering certain tidal and wind conditions, dense shelf water cascades transport terrestrial carbon, particulate matter, nutrients, all dissolved matter and in some instances, anthropogenic contaminants from the coastal regions to the deeper ocean.

The carbon flux from the DSWC is basically dependent on the vertical mixing and residence within the region of dense shelf water formation. This region facilitates primary productivity, biogeochemical cycling and pollutant dispersal. Hence, the complex physical dynamics of the dense shelf water and its containing suspended particles has direct implications and consequences for Australian biodiversity on and off the shelf.

Effects of tides on stratified coastal and estuarine waters, have been studied before in many estuaries and regions of freshwater influence (ROFI) globally. But there have been few studies in regions of evaporation influence (ROEI) (Winant and Gutierrez de Velasco, 2003) and DSWC formation.

This research will advance the understanding of the influence of tidal and wind mixing on stratification and DSWC along the continental shelf in a region where evaporation dominates.

Funding

  • SIRF, UIS and UIS net top-up

 

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Last updated:
Thursday, 19 September, 2013 11:39 AM

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