physical oceanography of south western marine region charitha pattiaratchi school of environmental...

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