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

Summer Winter Seasonal winds – West Coast Rottnest Island

Summer Winter Seasonal winds – South Coast Esperance

Global distribution of tidal conditions

Fremantle water level de-composition U(t) U p (t) U s (t) Sea Level (t) = Mean Sea Level(t) +Tide(t) + Surge(t) U(t) = Z o (t) + U p (t) + U s (t)

Spring and Neap tides Last quarter Full Moon New Moon First quarter Sun Spring tides = King tides Neap tides = Dodge tides

Tidal characteristics

Fremantle water level de-composition SolsticeEquinox

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 Capes Current Ningaloo Current Cresswell Current

The Leeuwin Current System

WASTAC, 1998 The Leeuwin Current warmer, lower salinity lower 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 Current Ridgway and Condie, 2004

The Leeuwin Current

Leeuwin Current Winter Summer Seasonal

Leeuwin Current La Nina El Nino Inter-annual

LC Leeuwin Current System – Water Masses

South 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) n Dominant current in southern region n Wind stress curl drive FC n Centered at 600m depth, max at 400m n Interconnect with LC at shelf break n Part of FC flows beneath LC, imitate LU Middleton and Platov,2003.

Flinders Current Leeuwin 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

May 1981 March 1981 Higher chlorophyll water on the continental shelf 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.