Abstract
Abstract. This study considers ocean–atmosphere influences on marine productivity over the shelf near Cape St. Francis, South Africa. Multiday estimates of chlorophyll fluorescence in the period 2006–2017 with an area outlined by 34.5–33.75∘ S and 24–26.5∘ E provide the basis for evaluation using data from high-resolution reanalyses. Correlations with the mean annual cycle of chlorophyll fluorescence were significant for salinity, linking marine productivity and the coastal hydrology. A strengthened Agulhas Current induces cyclonic shear that lifts water at the shelf edge. Composite high-chlorophyll-fluorescence events were dominated by a large-scale midlatitude atmospheric ridge of high pressure. The resultant easterly winds caused offshore transport and the upwelling of cool nutrient-rich water in multiday events at the beginning and end of austral summer. Environmental controls on interannual fluctuations of the commercial fishery were also explored. Southwestward currents and diminished heat fluxes favored the squid catch, while anchovy and sardine catches were linked with upper northerly wind, consistent with large-scale weather patterns that underpin coastal upwelling and river discharge. Productivity lags a few days behind cyclonic wind and current shear and the upstream coastal hydrology, which shares a common atmospheric driver.
Highlights
The southern coast of South Africa is swept by the prevailing warm Agulhas Current (Lutjeharms et al, 2000)
The relative role of local and remote atmosphere and ocean forcing on marine productivity over the shelf near Cape St
The 8 d MODIS chlorophyll fluorescence and HYCOM salinity in the period 2006–2017 were the primary descriptors in an index area outlined by 34.5–33.75◦ S and 24–26.5◦ E
Summary
The southern coast of South Africa is swept by the prevailing warm Agulhas Current (Lutjeharms et al, 2000). Rivers of the Eastern Cape discharge fresh, silty water a rate of up to 100 m3 s−1 during infrequent floods in the Gamtoos, Sondags (Sundays), and Great Fish river valleys (van Bladeren et al, 2007). Their plumes spread along the shore and into the large bays (Scharler and Baird, 2005). The inshore bays are sheltered and develop stable layers during summer that aggregate phytoplankton In this area, known as the eastern Agulhas Bank, a commercial fishery aimed at sardines (67 kt yr−1), squid (3 kt yr−1), anchovy, roman, mackerel, etc.
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