What do we measure and how?
Phil preparing drifters
A 'holey sock' drogue deployed from the ship at night
Drifter buoys released
JR311 represents an amazing opportunity to measure in situ oceanographic processes responsible for shaping the surface mixed layer. However, we only have one ship and the RRS James Clark Ross can only be in one place at a time. For this reason we deployed a number of drifters during the cruise that follow for days and weeks the water in which the drifters were placed. The purpose of the drifters is to show us exactly where the water we observe at one place, for example the eddy that we saw being born during the cruise and in which you can see below the drifters going round and round the eddy core, ends up after the ocean has evolved over several weeks, long after we have left and gone home.
Due to the top 20 metres or so of the ocean being blown by the wind, a layer we refer to as the Ekman layer, we need to ensure that the drifters have enough drag below this layer so that they follow the currents within the upper ocean rather than simply being blown around by whatever the wind is doing at the time. To ensure that this happens we attach the surface buoys, within which are GPS receivers and Iridium satellite transceivers to send us the drifter’s position, to ‘holey sock’ drogues. The drogues are 10 metre long, 90 centimetres in diameter and attached to the buoy by a 35 metre long tether so that they are effectively following the water at a depth of 40 metres, below the influence of the wind. The key design consideration when building drifters is that the drag exerted by the drogue is more than 40 times bigger than the drag of the surface buoy. Much less than this and the trajectory of the drifters may reflect the wind-driven currents rather than the flows associated with the fronts that we are interested in.
At the time of writing this during JR311, we have deployed a number of drifters but the three released at the edge of the eddy during the second half of the cruise are performing particularly well. We placed them next to the strong front that surrounds the eddy core and so far they have covered over 600 km and circled the eddy twice in one week. Particularly amazing is that they are still within 5 km of each other, demonstrating how narrow the jet is that circles the eddy core and also how difficult it is for water to escape it to either side.