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Using satellites and biology to understand Southern Ocean physics

Theory and observations of a combined Ekman and geostrophic trajectory in the chlorophyll distribution downstream of South Georgia

Due to its remoteness from the continents, plant growth in the Southern Ocean is limited by iron supply, which arrives as dust or from continental margins. Certain sub-Antarctic islands, namely South Georgia, Crozet and Kerguelen Islands, release iron into the ocean and fertilise huge phytoplankton blooms downstream. These blooms are important both as the base of the foodweb and for the carbon that they capture and export to the deep oceans.

This paper shows that these blooms can also further our understanding of the physics of the Southern Ocean. The iron comes from a point source (South Georgia) and fertilises the phytoplankton, whose chlorophyll (effectively their greenness) is visible from space. Other satellites measure the sea-surface height and gravitational pull of the Earth very precisely, allowing ocean currents to be mapped – the oceanographer’s equivalent of calculating wind speeds from air pressure on a weather chart. Mapping the chlorophyll relative to the flow reveals a wind-driven flow, the Ekman Flux, that is well known in theory but very hard to observe directly. Theoretical arguments are developed in the paper that predict closely the spreading of the bloom under these combined effects. This enables a greater understanding of how the shape of the bloom might change in future.

This paper not only furthers our knowledge of Southern Ocean physics, and also some aspects of its biology, but also demonstrates the value in the range of satellite measurements currently being taken. There is plenty more understanding to be gained by examining and merging the data sources available.

Find link to the full paper in the NERC Open Research Archive

Authors

Hugh J. Venables and Michael P. Meredith

Publication

Geophysical Research Letters, Vol. 36, L23610, doi:10.1029/2009GL041371, 2009