Abstract
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 further 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. 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. 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 Representative Concentration Pathway (RCP) 8.5 would not lead to an onset of the regime shift until the 2070s.
Highlights
Previous climate change impact studies identified a general weakening of the biological productivity of the outer northwest European shelf (NWES) as a regional response to a globally warming climate (e.g., Holt et al, 2012, 2016; Gröger et al, 2013; Wakelin et al, 2015; Schrum et al, 2016; Mathis et al, 2018)
The general freshening and warming of the upper ocean in the subpolar North Atlantic during emission scenario RCP8.5 leads to a decrease in sea surface salinity, a strengthening of the permanent stratification (Fig. A2a, b) and a reduction of the wintertime MLD in the NE Atlantic (Fig. 5a, b)
The considered Greenland ice sheet (GIS) melting rates cover a wide range projected by ice sheet models (Burkett et al, 2014)
Summary
Previous climate change impact studies identified a general weakening of the biological productivity of the outer northwest European shelf (NWES) as a regional response to a globally warming climate (e.g., Holt et al, 2012, 2016; Gröger et al, 2013; Wakelin et al, 2015; Schrum et al, 2016; Mathis et al, 2018). The nutrient concentrations of the water masses flushing the shelf are primarily controlled by the maximum depth of the wintertime mixed layer (MLD) west of the shelf break (e.g., Holliday, 2003; Williams et al, 2011; Holt et al, 2012; Gröger et al, 2013; Mathis et al, 2018). The warming and freshening of the upper North Atlantic projected by global climate models induces a weakening of the buoyancy-driven convection and a reduction of the wintertime MLD and upper-ocean nutrient concentration (e.g., Yool et al, 2015; Fu et al, 2016; Alexander et al, 2018). Previous downscaling simulations mainly based on the Special Report on Emissions Scenarios (SRES) emission scenario A1B indicated that a nutrient decline in the upper NE Atlantic would lead to a similar nutrient decline on the NWES, limiting net primary
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