Shear enhanced nutrient supply at the Mesoscale

Abstract

Phytoplankton live almost exclusively in the sunlit waters of the euphotic zone. However, in addition to sunlight, phytoplankton require a regular supply of nutrients to grow. In the open ocean such nutrients are abundant in the dark waters below the euphotic zone. Hence, to a large extent it is the physical mechanisms driving the transfer of nutrient rich water into the euphotic zone which dictate patterns of phytoplankton growth. Using a combination of observation and high resolution computer modelling this thesis investigates whether shear associated with mesoscale features leads to locally enhanced turbulent mixing and a shear-enhanced nutrient supply. Measurements of turbulent diffusivity and nutrient concentrations have been made in a region containing an eddy dipole, a strong mesoscale feature, consisting of a cyclonic eddy and an anti-cyclonically rotating mode-water eddy. The effect of this strong mesoscale feature on vertical turbulent mixing is assessed by investigating whether variations in vertical shear associated with the mesoscale feature enhance the observed vertical turbulent mixing. Using these observations of turbulent diffusivity, augmented by further measurements from two other ocean regions, a new parametrization of shear-enhanced vertical turbulent mixing is developed. The new shear-enhanced mixing parametrization is implemented in a high-resolution computer model of a mode-water eddy. This model is then used to examine the effect of interactions between the eddy and the wind on vertical nutrient fluxes. The shear enhancement to nutrient supply by mesoscale circulation is found to be potentially of much greater significance than has previously been considered. Modelling suggests that when forced by high variability winds mode-water eddies appear to be capable of locally enhancing the vertical turbulent nutrient flux by up to an order of magnitude. The work in this thesis suggests that vertical turbulent flux may well be underestimated as a stimulus to new production.

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