Abstract
AbstractSustaining biological export over the open ocean requires a physical supply of nutrients to the mixed layer and thermocline. The relative importance of diapycnal mixing, diapycnal advection, and isopycnal stirring by mesoscale eddies in providing this nutrient supply is explored using a field campaign in oligotrophic waters in the subtropical North Atlantic, consisting of transects over and off the mid‐Atlantic ridge. Eddy stirring rates are estimated from the excess temperature variance dissipation relative to the turbulent kinetic energy dissipation, and using eddy statistics from satellite observations combined with 9‐month‐long mooring data. The vertical nutrient fluxes by diapycnal mixing, diapycnal advection, and isopycnal mesoscale eddy stirring are assessed using nitrate measurements from observations or a climatology. Diapycnal mixing and advection provide a nutrient supply within the euphotic zone, but a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline. The eddy supply of nitrate is relatively small in the center of the subtropical gyre, reaching up to 0.06 mol N m−2yr−1, but is likely to be enhanced on the flanks of the gyre due to larger isopycnal slopes and lateral nitrate gradients. The nutrient supply to the euphotic zone is achieved via a multistage mechanism: a diapycnal transfer of nutrients by small‐scale turbulence to the euphotic zone, and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline.
Highlights
Key Points:Diapycnal loss of nutrients below the seasonal boundary layer is partly replenished by eddy stirring in the upper thermocline
Biological export of organic matter in the open ocean is usually viewed as being sustained by a vertical supply of nutrients to the euphotic zone from the upper thermocline
The observations analyzed here were obtained by a field study in some of the most oligotrophic waters of the North Atlantic subtropical gyre, between 24°N to 36°N, as part of the RidgeMix program to investigate the role of internal tides in diapycnal nutrient supply to the euphotic zone (Figure 2a)
Summary
Diapycnal loss of nutrients below the seasonal boundary layer is partly replenished by eddy stirring in the upper thermocline. Relay race of nutrient supply by eddy stirring to the upper thermocline passed on by diapycnal mixing and advection to the euphotic zone. Alberto Naveira Garabato, Alexander Forryan Funding acquisition: Jonathan Sharples Investigation: Carl P. Alberto Naveira Garabato, Clément Vic, Alexander Forryan Software: Carl P. Tuerena , Alberto Naveira Garabato , Clément Vic, Alexander Forryan, and Jonathan Sharples
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