4th Aug – Sailing low in the south


SST update

The northern western shelf, and western waters in general, remain warmer than average, while in the last couple of weeks we’ve seen more temperature variation return to the south. Back in May, the Great Australian Bight was colder than average after a series of cold fronts swept through, but there’s now a mix of warm and cold anomalies in the region.

Two regions have remained well above average for this time of year, and further warmed since the last post. Along WA’s southwest cape, temperatures have been around 2-3 degrees above climatology, while warm eddies off the east coast of Tasmania are keeping that area in the 80th percentile of temperatures for this time of year.

IMOS sea surface temperature percentiles, WA cape, 20 July (observed)
IMOS sea surface temperature percentiles, Tasmania, 1st August (observed)

Meanwhile, the temperature is forecast to cool along the South Australian coast this week as the sea level drops.

So low

Last week, OceanMAPS began forecasting low sea levels along the SA coast due to a low pressure system moving over southwest WA. Several locations are now expecting a tide level below the lowest astronomical tide, a relatively unusual event (once or twice a year). This could impact shipping and boating, as well as fisheries.

The low is sitting on the coast at the moment, generating a strong E-SE wind which is running parallel to the SA and southern WA coasts, and in the opposite direction to the usual nearshore current. (Note that if the forecast was to change and the low moved south over the coast, we could well see the opposite – high sea levels.)

Mean sea level pressure forecast, Bureau of Meteorology

Looking more closely at the wind forecast, we have easterly winds of 15-20 knots along the coast which will strengthen to 25-35 knots until Wednesday, when they will push more from the southeast. Generally, winds aligning with the coast in the cyclonic direction (clockwise) will drive a lower sea level.

ACCESS-G3’s wind forecast for today.

The nearshore current is reversing over the course of today to flow east, and will strengthen due to that persistent wind. OceanMAPS is forecasting the reverse current to extend to the shelf floor (50m depth), inshore of the Leeuwin current.

OceanMAPS forecast surface current, South Australian coast, 30 July – 4 August

The Southern Australian Regional Ocean Model (SAROM), the in-shore ocean data assimilating forecast system for the south coast of Australia, gives us further insight. SAROM’s graphs below illustrate the change in wind direction in Coffin Bay, and the resulting effect on current and sea surface height over the next three days, with the directional change expected all the way down (bottom current chart not shown).

The sea level will be at its lowest during Wednesday, before the wind direction changes on Thursday.

Harnessing the waves

As both the west and east coast of Australia has experienced in the last month, waves can be ferociously powerful. Now a boat that is partially wave-powered is being built in the Philippines. The hybrid is a new take on the traditional bangka, a three-hulled boat, with the prototype expected to be completed by the end of this year. Read more here.



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