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02.03.2015 05:53 Age: 3 yrs
Category: WA-IMOS

Modelling of ocean currents gives swimmers the edge at ‘Rotto’

Just before Western Australia’s iconic ocean event, the Rottnest channel Swim, hundreds of individuals, eager to get the edge on their competitors, took a peek at IMOS Ocean Current’s forecast of currents between the mainland and Rottnest Island.


Web statistics on OceanCurrent’s news page show that more than 200 additional individuals viewed the site on 21 February, the morning of the race, downloading imagery showing the direction of currents. Some swimmers were so keen to get the information they were calling Ocean Current to see if they could get the ‘good oil’ before it was posted on the site.

Ocean current forecasts were produced by the University of Western Australia’s Oceans Institute using wind forecasts from the Bureau of Meteorology. Local oceanographer and swimmer, Chari Pattiaratchi, provided an overview of the two predominant local weather patterns and the ocean currents off Perth which they produce. The complicating factors of offshore ocean influence, weather system strength and the transition time between the two weather systems is managed with the ocean modelling. Charts showing the forecast currents were uploaded to the IMOS Ocean Current website. 

The OceanCurrent forecast published on the afternoon before the swim was for northward currents throughout the race, weak near the coast and strongest after the 15 km mark. Northward currents were anticipated to increase from midday due to the sea breeze, making the approach to the island most difficult near the cut-off time.

IMOS High Frequency radar measurements of surface currents (made during the race) confirm the general pattern as predicted by the models, showing how the currents in the shallower waters of the continental shelf were going northwards while the currents off the continental shelf were going southwards. The increasing northward velocities that were forecast are also validated by the radar data.

Agi Gedeon, manager of the Western Australia node of IMOS (WAIMOS), was very close to the action as she was a volunteer marshal on the day. She spoke with a number of competitors who told her that knowledge of currents before the race is important in planning a strategy for keeping the swimming distance (19.7km in a straight line from Cottesloe Beach to Rottnest Island) to a minimum.

One of the competitors, Richard, who completed the race in 6 hours 51 minutes said "I am sure your current predictions helped as I stayed left of the course and although I cannot  scientifically back this up, my gut feeling is it benefited me."

In an amazing feat of human endurance, winner 24 year old Kane Radford completed the race in just under 4 hours 26 minutes. We suspect he made use of IMOS ocean current data!