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Physical Oceanography of South Western Marine Region
Charitha Pattiaratchi
School of Environmental Systems Engineering
The University of Western Australia
Outline
Background Wind, tide and wave regime Leeuwin Current
Forcing, water masses, eddy generation
Continental shelf processesForcing, seasonal changes, upwelling
Conclusions
Study Region
Summer Winter
Seasonal winds – West Coast
Rottnest Island
Summer Winter
Seasonal winds – South Coast
Esperance
10 %
20 %
30 %
Summer 2000 N
NE
E
SE
S
SW
W
NW
Wind speed scale in m/s
3 6 9 12 15 18
10 %
20 %
30 %
Winter 2000 N
NE
E
SE
S
SW
W
NW
Wind speed scale in m/s
3 6 9 12 15 18
Global distribution of tidal conditions
Fremantle water level de-composition
U(t)
Up(t)
Us(t)
Sea Level (t) = Mean Sea Level(t) +Tide(t) + Surge(t)
U(t) = Zo (t) + Up(t) + Us(t)
Spring and Neap tides
Lastquarter
FullMoon
NewMoon
Firstquarter
Sun
Spring tides = King tidesNeap tides = Dodge tides
Tidal characteristics
Fremantle water level de-composition
Solstice Equinox
Winter + summer:pressure systems
Summer:Tropical cyclones
Continental Shelf Wave generation
Sub-tidal water levels
Water Level Changes
Cyclone Track: 1993 (Naomi)
Shelf currents: DWCM (100m)
Naomi
Shelf currents: SWANB (8m)
Naomi
Southern Surveyor Voyages 2003, 2006
The Leeuwin Current System
Leeuwin Undercurrent
Leeuwin Current
CapesCurrent
NingalooCurrent
Cresswell Current
The Leeuwin Current System
WASTAC, 1998
The Leeuwin Current
warmer, lower salinitylower nutrient water
• flows all year around• stronger in winter• weaker in summer
• strength linked to SOI• weaker during El Nino• stronger during La Nina
• Relative strength measured by mean sea level
Ridgway and Condie, 2004
The Leeuwin CurrentRidgway and Condie, 2004
The Leeuwin Current
Leeuwin Current
Winter Summer
Seasonal
Leeuwin Current
La Nina El Nino
Inter-annual
LCLeeuwin Current System – Water Masses
Leeuwin Current System – Water MassesSouth Indian
Central Water
Sub-Antarctic
Mode water
Antarctic Intermediate
water
Middleton and Cirano, 2003
Circulation: South Coast
The Leeuwin Current – South Coast
Flinders Current (FC)
Dominant current in southern region
Wind stress curl drive FC Centered at 600m depth, max
at 400m Interconnect with LC at shelf
break Part of FC flows beneath LC,
imitate LU
Middleton and Platov,2003.
FlindersCurrent
LeeuwinCurrent
Leeuwin Current/Flinders Current
Leeuwin Current/Flinders Current
Leeuwin Current: Higher in temperature
FC/Undercurrent:
Higher in salinity
Transect P
Flinders Current feeds Leeuwin Undercurrent
Leeuwin Current: Eddies
May 1981March 1981
Higher chlorophyll water on the continental shelf
April 2002
April 2002
April 2002
Nov 2000
The Leeuwin Current – Eddy generation
Shark bay
Abrolhos Islands
Perth Canyon
Albany
Esperance
The Capes Current: A northward counter current during the summer
Balance of forces: Capes Current
Summary – southern section
Cresswell Current?
Pygmy Blue Whale aggregations: Perth Canyon
Pygmy Blue Whales:
• > 25 m long• Found in the Perth
Canyon Feb - May• Consume 4-5 tonnes
of food per day
Upwelling in South Australia
Conclusions - I The wind regime is seasonal with summer/spring sea breezes and winter
storms.
The wave climate responds to changes in the winds with higher swell waves during winter.
The tides are diurnal and do not follow the moon’s phases for spring and neap tides – lowest water levels occur during the December solstice.
Sub-tidal forcing is important for the whole study region: Shark Bay to Kangaroo Island
Conclusions - II
Leeuwin Current is the dominant surface forcing in the offshore regions – it interacts with particular regions of the coast (e.g. Jurien Bay) due to eddy generation.
In the subsurface the Leeuwin Undercurrent and Flinders Current are important
Continental shelf currents are dominated by wind forcing. There are seasonal changes with upwelling occurring during the summer and downwelling in winter.
http://www.sese.uwa.edu.au/~pattiara/CoastalOceanography
