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Discussion papers
https://doi.org/10.5194/os-2019-100
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-100
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Sep 2019

Submitted as: research article | 17 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

The impact of melt water discharge from the Greenland ice sheet on the Atlantic nutrient supply to the Northwest European Shelf

Moritz Mathis1,2 and Uwe Mikolajewicz1 Moritz Mathis and Uwe Mikolajewicz
  • 1Max-Planck-Institute for Meteorology, Bundesstr. 53, D-20146 Hamburg
  • 2Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Max-Planck-Str. 1, D-21502 Geesthacht

Abstract. Projected future shoaling of the wintertime mixed layer in the Northeast (NE) Atlantic has been shown to induce a regime shift in the main nutrient supply pathway from the Atlantic to the Northwest European Shelf (NWES) near the end of the 21st century. While reduced winter convection leads to a substantial decrease in the vertical nutrient supply and biological productivity in the open ocean, vertical mixing processes at the shelf break maintain a connection to the subpycnocline nutrient pool and thus productivity on the shelf. Here we investigate how meltwater discharge from the Greenland ice sheet (GIS) not yet taken into account impacts the mixed layer shoaling and the regime shift in terms of spatial distribution and temporal variability. To this end we have downscaled sensitivity experiments by a global earth system model for various GIS melting rates with a regionally coupled ocean-atmosphere climate system model. The model results indicate that increasing GIS meltwater discharge leads to a general intensification of the regime shift. Atlantic subpycnocline water masses mixed up at the shelf break become richer in nutrients and thus limit the projected nutrient decline on the shelf. Moreover, the stronger vertical nutrient gradient through the pycnocline results in an enhanced interannual variability of on-shelf nutrient fluxes which, however, do not significantly increase variations in nutrient concentrations and primary production on the shelf. Moreover, due to the impact of the GIS meltwater discharge on the NE Atlantic mixed layer depth, the regime shift becomes initiated earlier in the century by about 1–2 decades, depending on the discharge rate. The effect on the onset timing, though, is found to be strongly damped by the weakening of the Atlantic meridional overturning circulation. A GIS melting rate that is even 10 times higher than expected for emission scenario RCP8.5 would lead to an onset of the regime shift not until the 2070s.

Moritz Mathis and Uwe Mikolajewicz
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Moritz Mathis and Uwe Mikolajewicz
Moritz Mathis and Uwe Mikolajewicz
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Short summary
We show that under strong global warming, meltwater discharge form the Greenland ice sheet leads to a weakening of both the expected nutrient decline on the Northwest European Shelf and the reduction of the biological primary production. Due to the impact of the meltwater discharge on the meridional overturning circulation, Atlantic water masses of deeper levels become richer in nutrients and thus increase the nutrient concentration of the water masses transferred to open shelf areas.
We show that under strong global warming, meltwater discharge form the Greenland ice sheet leads...
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