Submesoscales are important to nutrient supply!

In related research, Phil Hosegood and Ricardo Torres have just published results from the Mauritanian upwelling systems that show submesoscale instabilities to be potentially crucial for nutrient supply to the euphotic zone. The results were obtained from a NERC-funded research cruise to the region off the north west African coast in 2009 where nutrients are continually upwelled to the surface by the offshore Ekman transport driven by the persistent northerly winds. The upwelled water moves offshore in mesoscale filaments, which measure more than 100 km across their inshore extent. The nutrients upwelled within the nearshore region stimulate huge plankton blooms which persist some distance offshore in the filaments.
The important result demonstrated in this paper, however, is that the concentration of nutrients does not drop as fast as it should given the observed primary production, suggesting an additional source of nutrients coming from somewhere. We demonstrated that these weren’t coming from turbulent entrainment across the mixed layer base, nor from Ekman pumping caused by divergent winds. Instead, the periphery of the filaments were susceptible to submesoscale instabilities, direct evidence for which was obtained from drifting ADCPs and tracer release experiments. A drifting ADCP was deployed in a submesoscale front and measured vertical velocities of >100 m/day as the drifter moved from cooler into warmer water (See Figure 1.). The evidence for strong subduction was further apparent from a tracer release at another submesoscale front; the tracer was subducted out of the mixed layer within 24 hours, providing further evidence for the efficacy of submesoscales in causing rapid subduction of waters from the surface to the underlying pycnocline.
The paper was published in Progress in Oceanography during October 2017. The full reference is:
Hosegood PJ, Nightingale P, Rees A, Widdicombe C, Woodward M, Clark D & Torres R, 2017. Nutrient Pumping by Submesoscale Circulations in the Mauritanian Upwelling System, Progress in Oceanography, doi: 10.1016/j.pocean.2017.10.004