Monday, 13 October 2008

Drifting around in the Indian Ocean


Late last week we deployed two drifters at 18.20S, 59.01E. The drifters were deployed in the deep channel north of Mauritius. A number of oceanographers have suggested that a strong current flows from east to west through this channel. Looking at a schematic of the general circulation of this region, you can see that the current flowing westwards is in fact a major tributary of the South Equatorial Current (SEC), which eventually on its travels passes to the north of Madagascar.



This survey is providing us with a wealth of hydrographic data it’s only a snap shot and what happens downstream of the survey or 2 months after the survey has been completed is not known. So drifters give us that missing information.

Once deployed they drift with the surface current collecting data such as temperature, current speed and direction, air temperature, air pressure and relay the information back to satellite.

History

For many years, ocean currents have been estimated by how they carry objects. For example, sailors measured the speed of their ship through the water using the ship log. They measured their absolute position by celestial navigation (in the good old days, pre-GPS!). The difference between the absolute speed and the speed through the water gave the speed of the currents. Very strong currents, such as the Agulhas Current made a big difference in how long it takes to travel. More recently, researchers began tracking objects while they were drifting. This tracking was first done visually either from a coastline or anchored ship, then using radio, and most recently using satellites. During the 1970s, when satellite tracking became possible, many competing drifter designs were proposed, built and deployed in various studies around the world.

How does a drifter work?
The modern drifter is a high-tech version of the "message in a bottle". It consists of a surface buoy and a subsurface drogue attached by a long, thin tether.

The buoy measures temperature and other properties, and has a transmitter to send the data to passing satellites. The drogue dominates the total area of the instrument and extends 20 m below the sea surface. Here is a picture of a drifter in a cold clockwise eddy in the southern ocean. You can see how the drifter rotates round and round in the eddy!!

Drifter Design
Drifters basically have a surface float and attached to that a long holey sock known as a drogue. The surface float is on average 40 cm in diameter. It contains: batteries; a transmitter, which relays all the information on a 6-hourly basis to satellite; a thermistor to measure sea surface temperature; a barometer to measure atmospheric pressure, wind speed and direction, as well as a GPS to measure position and the time taken for the drifter to move from one position to another – this way you can measure the surface current speed and direction.

The drogue is centered at 15 meters beneath the surface to “anchor” it to the surface – this prevents strong winds from pushing the drifter around. The outer surface of the drogue is made of nylon cloth. Throughout the drogue, rigid rings support the drogue's cylindrical shape. The drogue is a "holey-sock": each drogue section contains two opposing holes, which allow water to flush through and prevent the drogue from getting “wound up” in strong surface currents.

Once deployed, a drifter lives an average of 400 days before ceasing transmission. Occasionally, drifters are picked up by fishermen or lose their drogue and run aground.

Deploying a drifter
Drifters weigh about 25 kg each. Before deployment, the drogue and tether are bound with paper tape which dissolves in the water, and the tether is sometimes wrapped around a water-soluble cardboard tube to protect it from kinking.


The drifter is deployed by throwing it from the stern of a vessel, preferably from the lowest deck and within 10 m of the sea surface. After deployment, it may take up to an hour for the paper tape to dissolve and trapped air bubbles to be released, so that the drogue sinks to the target depth (15 m).

How much do they cost
Drifters range in price depending on the number of sensors you have. On average you can expect t pay about $2500 per drifter.

Keep posted for more cruise stories!


Does my bum look BIG in this?
by Isabelle Ansorge

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