Irreversible nitrate fluxes due to turbulent mixing in a coastal upwelling system Public Deposited

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  • Detailed and repeated measurements of nitrate across the Oregon shelf, made coincident with turbulence measurements, reveal the importance of cross-isopycnal mixing via turbulence in providing nitrate to the upper water column. Spatial distributions of vertical gradients and turbulent fluxes in the Oregon coastal ocean reveal variability that could not have been resolved with traditional sampling approaches. Nitrate-rich near-bottom waters are drawn up the shelf during upwelling and back down the shelf during downwelling. Continuous turbulent mixing in the bottom boundary layer increases nitrate on intermediate isopycnals by several μM toward the shelf. This increased nitrate is consistent with measured turbulent flux of nitrate of O(0.1 μmol m⁻² s⁻¹) for a period of 3 days, roughly the timescale for changes in the wind and hence to the upwelling circulation. Our flux estimates are consistent with high-biomass, nutrient-replete, incubation-based primary productivity estimates in this area and suggest that turbulent vertical nitrate transport plays a significant role in supporting the high productivity seen here. Offshore of 30 m depth, we estimate the irreversible transport of nitrate from nitrate-rich near-bottom waters to be about 25% of the rate at which it is provided by upwelling. The total irreversible transport is greater by some unknown amount when we consider the transport inshore of 30 m depth. We suggest that the onshore transport of offshore water, whether deep water during upwelling or surface water during relaxation, forces the juxtaposition of strong gradients and mixing regimes such that turbulent vertical fluxes are consistently elevated in the shoreward reaches of the coastal ocean.
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  • Hales , B., J. N. Moum, P. Covert, and A. Perlin (2005), Irreversible nitrate fluxes due to turbulent mixing in a coastal upwelling system, J. Geophys. Res., 110, C10S11.
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