Northwest African upwelling and its effect on offshore organic carbon export to the deep sea

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Abstract
We combined the analysis of sediment trap data and satellite‐derived sea surface chlorophyll to quantify the amount of organic carbon export to the deep sea in the upwelling induced high production area off northwest Africa. In contrast to the generally global or basin‐wide adoption of export models, we used a regionally fitted empirical model. Furthermore, the application of our model was restricted to a dynamically defined region of high chlorophyll concentration in order to restrict the model application to an environment of more homogeneous export processes. We developed a correlation‐based approximation to estimate the surface source area for a sediment trap deployed from 11 June 1998 to 7 November 1999 at 21.25°N latitude and 20.64°W longitude off Cape Blanc. We also developed a regression model of chlorophyll and export of organic carbon to the 1000 m depth level. Carbon export was calculated for an area of high chlorophyll concentration (>1 mg m−3) adjacent to the coast on a daily basis. The resulting zone of high chlorophyll concentration was 20,000–800,000 km2 large and yielded a yearly export of 1.123 to 2.620 Tg organic carbon. The average organic carbon export within the area of high chlorophyll concentration was 20.6 mg m−2 d−1 comparable to 13.3 mg m−2 d−1 as found in the sediment trap results if normalized to the 1000 m level. We found strong interannual variability in export. The period autumn 1998 to summer 1999 was exceeding the mean of the other three comparable periods by a factor of 2.25. We believe that this approach of using more regionally fitted models can be successfully transferred even to different oceanographic regions by selecting appropriate definition criteria like chlorophyll concentration for the definition of an area to which it is applicable.

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