Seasonal variability in mass, nutrients and DOC lateral transports off Northwest African Upwelling System

Abstract. The Coastal-Ocean Carbon Exchange in the Canary Region Project (COCA) arises in order to analyse and get to understand the impact of lateral export of nutrients and organic matter from the highly productive Coastal Upwelling System off NW Africa in the biogeochemical cycles during two different seasons. The circulation patterns off NW African Upwelling System are examined by applying an inverse model to two hydrographic datasets gathered in fall 2002 and spring 2003. The mass transports estimated by model are consistent with the thermal wind equation and the conservation of mass in a closed volume. Besides, the Ekman transport and the freshwater flux are also considered. These estimates show a seasonal variability in the circulation patterns at central levels, particularly in the southern boundary of the domain, where the Cape Verde Frontal Zone is located. In the beginning of fall, this circulation is deeper and northward with a net transport of 6 ± 3 Sv and, in the late spring, it is shallower and southward with a similar intensity. At intermediate levels important differences are also observed between the two seasons. In fall, the Antarctic Intermediate Waters reaches higher latitudes with 2 ± 2 Sv flowing northward. During spring, there is no significant northward flow of AAIW. However, there is a moderate westward mass transport which impacts both the lateral transports of inorganic nutrients and organic matter at intermediate layers and also the shallowest lateral transports of organic matter. Seasonal variability in circulation patterns are also reflected in lateral transports of inorganic nutrients and dissolved organic carbon. Therefore, the changes in the circulation patterns between the two seasons have allowed us to assess the variability in the contributions of SiO2, NO3, PO4 and DOC from the first to the second season. In fall, the transports are mainly northward from the south with −0.80 ± 0.34, −1.11 ± 0.47 and −0.07 ± 0.03 kmol s-1 of SiO2, NO3 and PO4, respectively. In spring, however, lateral transports off-shore are favoured with 0.75 ± 0.37, 1.34 ± 0.66 and 0.08 ± 0.04 kmol s-1 of SiO2, NO3 and PO4, respectively. This westward transport stimulates in turn an intensified westward DOC transport at shallow layers, specifically 0.50 ± 0.25 x 108 mol C day-1.

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