How the curved flow around costal headlands causes upwelling.

What is it about?

The upwelling of nutrient-rich bottom waters supports life at the oceans’ surface. Secondary circulation can produce localized upwelling at headlands. Secondary circulation develops in the curved flow around headlands resulting in a loose helical flow pattern within the curved flow. The magnitude of secondary flow can be up to 20% of the depth average current. Moving vessel ADCP measurements were taken at Cape Saunders, Otago Peninsula, New Zealand. During peak floods of 1 ms−1 the measured strength of secondary flow is 0.2 ms−1. A region of secondary flow approximately 1500 m long and 800 m wide is observed downstream of the Cape. On the inshore edge this region, areas of vertical velocity inferred from horizontal ADCP measurements using mass continuity show a localized upwelling of up to 0.007 ms−1. Concurrent CTD measurements also show this tidally synchronized upwelling. Linear regression between upwelling from the CTD measurements and the inferred vertical velocity from the ADCP measurements show the data are well correlated, r= 0.65. The upwelling is 4 m per hour so in this location with a mean depth of 25 m the entire water column is replacing itself with deep waters at least once during a tidal cycle. On a global scale, the cumulative upwelling from headlands and islands due to secondary circulation could be a significant source of nutrient delivery to coastal surface waters.

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Why is it important?

There are tens of thousands of headlands around the world, through this mechanism the cumulative upwelling of nutrients to surface waters could be significant.