Quantifying the Role of Mesoscale-Driven Processes of Nitrate Supply Within an Iceland Basin Eddy Dipole

Abstract

This study quantitatively assesses the mesoscale spatial variability in local nitrate transport and the net nitrate flux associated with an open ocean eddy dipole, using novel data from the SUV?6 ultra?violet (UV) nitrate sensor integrated with SeaSoar. The innovative, combined system provides high?resolution (2?3 km horizontal, ~ 1 m vertical) nitrate data with 0.2 ?M accuracy concomitantly with temperature, salinity and dissolved oxygen. This is the first time coincident physical data and measurements of nitrate concentration have been collected at this resolution in a three?dimensional volume encompassing a mesoscale feature, in order to infer the net nitrate flux at specific depth levels. This approach represents a powerful new tool for quantifying the mesoscale, and potentially submesoscale, contribution to vertical nutrient flux to the euphotic layer.

Data were collected during a multi?disciplinary research cruise aboard the NERC research ship RRS Discovery to the Iceland Basin in July/August 2007. Three quasi?synoptic spatial surveys show a southeastward flowing jet with counter?rotating eddies on either side. The anti?cyclonic component is a mode water eddy, characterised by a homogenous core (~ 35.5 psu and 12 °C) centred at ~ 600 m. The cyclonic eddy is characterised by a surface (10 m) temperature and salinity signature of ~ 13.5 °C and 35.21 psu, respectively, and maximum observed vertical displacement of isopycnals of over 200 m. The strong vertical velocities in the region are concentrated primarily in the central jet between the eddies, as fast flowing water is forced up over raised isopycnals associated with the large potential vorticity anomaly of the mode water eddy. Additionally, upward (downward) vertical velocity is diagnosed ahead of the cyclonic (mode water) eddy in the direction of propagation.

Maximum local vertical advection of nitrate at the base of the euphotic zone due to the mesoscale is indicated to be 1.15 mol N m?2 yr?1. This value is an order of magnitude higher than local rates of nitrate supply due to turbulent mixing and exceeds maximum local nitrate uptake rates. However, findings indicate that estimating the net mesoscale nitrate flux into the euphotic zone associated with the eddy dipole is considerably more complex. The results obtained are heavily biased by the existence of a non?zero mean vertical velocity for the region, arising from incomplete and/or misrepresentative sampling. The implications of this for the interpretation of the current data set, and future similar data sets, are assessed in full. A comprehensive quantitative assessment of other potential sources of error associated with the analysis is also presented.

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Identifiers

ORCID for Alberto C. Naveira Garabato: orcid.org/0000-0001-6071-605X

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