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 processes
Forcing, seasonal changes, upwelling
 Conclusions
Study Region
Seasonal winds – West Coast
Summer
Winter
Rottnest Island
Seasonal winds – South Coast
Summer 2000
Winter 2000
N
30 %
NW
20 %
N
30 %
NE
NW
10 %
10 %
W
E
SW
NE
20 %
SE
W
E
SW
SE
Wind speed scale in m/s
S
Wind speed scale in m/s
S
3 6 9 12 15 18
Summer
3 6 9 12 15 18
Winter
Esperance
Global distribution of tidal conditions
Fremantle water level de-composition
U(t)
Sea Level (t) = Mean Sea Level(t) +Tide(t) + Surge(t)
Up(t)
U(t) = Zo (t) + Up(t) + Us(t)
Us(t)
Spring and Neap tides
First
quarter
Sun
New
Moon
Full
Moon
Last
quarter
Spring tides = King tides
Neap tides = Dodge tides
Tidal characteristics
Fremantle water level de-composition
Solstice
Equinox
Continental Shelf Wave generation
Summer:
Tropical cyclones
Winter
+ summer:
pressure
systems
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
Ningaloo
Current
Capes
Current
Leeuwin
Current
Leeuwin
Undercurrent
Cresswell
Current
The Leeuwin Current System
The Leeuwin Current
warmer, lower salinity
lower nutrient water
Ridgway and Condie, 2004
• flows all year around
• stronger in winter
• weaker in summer
• strength linked to SOI
• weaker during El Nino
• stronger during La Nina
WASTAC, 1998
• Relative strength measured
by mean sea level
The Leeuwin Current
Ridgway and Condie, 2004
The Leeuwin Current
Leeuwin
Current
Seasonal
Winter
Summer
Leeuwin
Current
Inter-annual
La Nina
El Nino
LC
Leeuwin
Current System – Water Masses
Leeuwin Current System – Water Masses
South Indian
Central
Water
SubAntarctic
Mode water
Antarctic
Intermediate
water
Circulation: South Coast
Middleton and Cirano, 2003
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.
Leeuwin Current/
Flinders Current
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
Higher chlorophyll water on the
continental shelf
March 1981
May 1981
April 2002
April 2002
Nov 2000
April 2002
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