1996 Sydney to Hobart Yacht Race

Latest NOAA AVHRR sea surface temperature images Welcome to a special service of the latest satellite sea surface temperature images for the Sydney to Hobart yacht race region. Click on this image for the larger version. Comments on the satellite image and likely current patterns The satellite images are formed from the heat radiated up from the earth's surface to an infrared sensor aboard the US NOAA satellites. The signals are transmitted to Hobart and then corrected for distortions and for the rotation of the earth beneath the satellite. Temperatures are calculated with a formula and we assign colours for the different temperatures. The temperature scale is at the bottom of the image. Note that sometimes our temperatures may be slightly incorrect and it is necessary for individual boats to "intercalibrate" the images with their onboard thermometers. We mark the clouds white. There is a 2°latitude-longitude grid and the 200m depth contour (shelf edge) is shown as a thin grey line. The selected satellite image, which is a composite from 21 and 22 December, shows a classic pattern of a meander of the East Australian Current (EAC) and strong anticlockwise eddy as was first described by CSIRO oceanographer Bruce Hamon 35 years ago. The EAC runs southward just beyond the shelf edge from the top of the image to 34°S. It then separates out to sea to reach 36°S, 153°E, where it turns to run up 154°E to 34°E before leaving the picture to the east. The position and shape of this meander will change by race time. The current speed will be greatest in the warm core of the EAC (temperature 24°C) and we have estimated it to be a good 3 knots. The current speed decreases outward away from the core, but could be 2 knots at the middle of the continental shelf (in the upper part of this image), decreasing to near-zero values at the coast. However, near the coast other factors become important, such as currents resulting from local and remote wind forcing - but up to a knot at the most and changing from north south with a period of about a week. Just south of 36°S there is a very small anticlockwise eddy that has grown from unstable flow around the meander of the EAC. Current speeds in it may reach 1 knot, but by race time it may become merged with the bigger eddy, 'A', centred at 38°S, 151°E. Eddy A with temperature 17°C is being encircled by a streamer of warmer water (about 19°C). Work over a couple of decades has shown that eddies like Eddy A have edge speeds of up to 4 knots when they move in and press against the shelf edge. The current speed drops off very quickly outside the eddy. It decreases more or less linearly from the edge in towards the centre. Rotation times are about 5 days at the edge and 2 days in closer to the centre. The eddies have lifetimes of as long as one and a half years. Their centres wander as much as 20 miles per day, often taking them out to sea and then back again. Their encounters with the shelf edge distort them into ellipses and it is this that results in the strongest currents (ie. conservation of angular momentum is important). There is a second large anticlockwise eddy ('B') that is located at roughly 39°30'S, 150°E (southwest of eddy A). Warm water from the southwest quadrant of eddy A flows westward on 39°S, southward along the shelf edge of Bass Strait, then into eddy B. We guess the maximum currents in this flow to be about 1 knot. From the south of eddy B down to Tasman Island the currents are often somewhat random, due to small scale eddies and meanders, with speeds only up to 1 knot. Some pattern to these currents may become obvious in the images between now and race time. CSIRO Marine Research traditionally provides these images commercially for mariners, shipping, professional and recreational fishermen. However for a period until early January, up to four images daily will appear on this page for the region between Sydney and Hobart. CSIRO Marine Research can provide satellite coverage from Shark Bay in Western Australia to New Zealand and from Cairns in Queensland to Antarctica.   For more information contact Kim Badcock Phone: 03 6232 5398 Fax: 03 6232 5123 Email: Kim.Badcock@ml.csiro.au