Theory and observations of Ekman flux in the chlorophyll distribution downstream of South Georgia.

A large phytoplankton bloom occurs downstream of South Georgia, an island on the northern edge of the Scotia Sea, Atlantic sector of the Southern Ocean. This is due to natural iron fertilisation being advected downstream in the Antarctic Circumpolar Current. Mapping of SeaWiFS chl-a in dynamic height/longitude space reveals the trajectory of surface water under the dual influences of geostrophic flow and Ekman flux. A theoretical estimate of the trajectory shows good agreement with observations. The trajectory is dependent on the wind stress:mixed layer depth ratio, but independent of the latitudinal separation of dynamic height contours. The chl-a distribution suggests that iron reaches 20°W below the mixed layer, away from the effects of Ekman flux, but beyond the iron is confined to the surface layer. Chl-a reduces monotonically east of South Georgia with the signal detectable to 15°E, over 3500 km downstream, indicating the basin-scale biogeochemical influence of the iron supply.