Microdrifters for Ocean Currents

Surface currents in the ocean are measured well over broad scales, by satellite altimetry and sporadic ocean drifters. However, these measurement methods do not resolve smaller scale processes including submesoscale eddies, filaments, and higher time varying processes like surface wind-driven events under variable forcing. One oceanographic case where these small scale processes are likely to be critical is in shelf-edge exchange. Over the UK shelf edge, continental shelf-deep water exchange processes are important for replenishing surface nutrients to fuel biological productivity. Around Greenland, shelf waters are very fresh, due to recent ice melt from the Greenland Ice Sheet and Arctic, and the cross-shelf exchange of these freshwaters can influence or even shutdown deep convection and the overturning circulation. At the shelf, there is often a boundary current following the slope which presents a barrier to cross-shelf exchange, except under the influence of surface wind-driven processes or until the boundary current becomes unstable and sheds eddies. These processes are opaque to coarse satellite measurements, and cannot be resolved by sparsely distributed individual drifters.
Traditional ocean drifters float near the surface of the oceans, following the currents. They regularly determine their position from GPS satellites, and transmit their position and other measured quantities (temperature, conductivity) via satellite communications. These drifters are about the size of a beach-ball, weigh 20 kg, and cost > £1000/unit. To investigate time-varying smaller scale processes, 100s of drifters would be needed, representing an enormous cost. We propose the development of micro-drifters - the size of a tennis ball, < 500 g and O(£200/unit) - to enable process-based oceanographic studies requiring large numbers of drifters.