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Hydrographic controls on the inter-annual variability of chlorophyll-a concentration estimates in the Atlantic Sector of the Southern Ocean

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Citation:
Borrione, I. and Schlitzer, R. (2011): Hydrographic controls on the inter-annual variability of chlorophyll-a concentration estimates in the Atlantic Sector of the Southern Ocean , Modeling and Synthesis of Southern Ocean Natural Iron Fertilization Workshop, Woods Hole Oceanographic Institution, Woods Hole - U.S.A. .
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Abstract:

Despite the high macro-nutrient content of waters transported by the Antarctic Circumpolar Current, most of the Southern Ocean waters fall in the High Nutrient Low Chlorophyll category. Exceptions are found downstream from islands or along coastal shelves, which are believed to be sources of the bio-limiting nutrient iron. Nevertheless, further top-down and bottom-up controls may play an important role in affecting the recurrence and distribution of phytoplankton blooms. Analyzing a 13-year long time series of satellite ocean color imagery for the Atlantic sector of the Southern Ocean, we have detected two well confined regions, in the southern Drake Passage and downstream from South Georgia, where inter-annual variability is surprisingly low. We thus combined satellite based sea surface height data and diverse langrangian measurements of circulation (Argo floats and surface drifters) to investigate existing connections between local hydrography and the observed chl-a patterns. Our results highlight how the occurrence and distribution of the observed pigment biomass is strongly controlled by surface circulation, which not only is responsible for the dispersal of chlorophyll-a patches but also that of micro-nutrients, possibly including iron. Our observations lead us to the conclusion that the topographic steering of the currents may also determine the stability of the observed patterns. Furthermore, we observe how both in the Southern Drake Passage, and downstream from South Georgia, higher chlorophyll-a concentrations are measured where a meander is formed. Combining our observations with those derived from other natural iron enrichment studies in the Southern Ocean (i.e. in the Crozet and Kerguelen Islands), we argue that downstream from a nutrient source the presence of a meander with greater elocities, yet embracing calmer waters, may act as a preconditioner to the observed higher chlorophyll-a concentrations.

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