Weather by Malcolm RileyThe Bluelink project

The Bluelink project is a joint research venture between the Bureau of Meteorology, CSIRO and the Royal Australian Navy (RAN).
Diag 1. Sea Surface Temperature.The navy uses the data from Bluelink in its underwater operations. The sound speed of sonar increases through warmer and more saline water and as the ocean depth increases. The oceans have varying layers of temperature and salinity. To get the most out of your sonar these variations need to be included in the mix.
The navy also uses undersea current forecasts in their mine hunting operations. The Bluelink forecasts were used by the teams searching for the wreck of the Australian hospital ship Centaur that was torpedoed off Queensland during World War II and lies more than 2km below the surface.
Diag 2. Southern Bluefin fishing areas map.Bluelink forecasts are available on Bureau’s web site at www.bom.gov.au/oceanography/forecasts/index.shtml. The charts include forecasts of ocean currents, sea surface temperature, salinity and sea height.
The charts are seven day forecasts and update every Monday and Thursday, advancing in 24 hour time steps from the initial chart.
In the example (Diag 1) you can see the warmer waters (red) off the coast moving down to around Coffs Harbour. Some fish species are attracted by specific temperature ranges and currents.
The Australian Fisheries Management Authority use Bluelink data as part of their management program. The southern bluefin tuna tends to stay in waters that are 20°C or below (Diag 2). The management plan reflects this with areas of where commercial tuna fishing can be undertaken.
Diag 3. Sea level anomaly chart.Sea level anomaly charts can also be useful for fishing. In Diag 3, Blue eddies showing lower sea levels have a clockwise current and red eddies showing higher sea levels have anticlockwise currents associated with them.
When the water moves clockwise, coriolis tends to move the water outwards reducing lower sea levels in the centre of the eddy. In the middle of these outflow (blue) areas, nutrient rich water is being brought to the surface and moving out from the centre.
As the nutrient rich water moves away from the eddy the temperature rises and this gives the food chain a boost with microscopic life feeding on the nutrients. Small fish and shrimp then feed on the microscopic life. By the time the water reaches the outer edges of the eddy (where blue becomes white) larger fish would be feeding on the baitfish.
The eddies with higher sea level (red) also bring up nutrient rich water but at their edges. In Diag 3, areas where the blue and the red are close together could be a productive areas for fishing.
charts
The two charts above are for Monday 5 July (left) and Sunday 11 July (right) and they show some interesting features. The blue clockwise eddy that is to the east of Cape York is the remains of the clockwise sea current that was formed during tropical cyclone Ului.
This clockwise eddy formed to the northwest of Vanuatu in mid March and has been tracking westward through the Coral Sea for four months.
Another feature is the (up to) 60 centimetre change in the water level in the Gulf of Carpentaria. This is a massive volume of water that has been moved by southeasterly winds.
The Gulf is a bit of a ‘sea level engine’, the southeasterly trades blow the water out of the Gulf and the northwesterly monsoon blows it in.
In an event like the above, a coastal trapped wave (negative sea level anomaly) will propagate anticlockwise around the continent. The ‘wave’ will travel at around six knots and can travel around to South Australia.

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.