Evolution of the Atlantic nutrient supply to the NWES under future climate conditions

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 Gröger et al., 2013). The main driver has been attributed to a reduction in the oceanic nutrient import from the adjacent northeast (NE) Atlantic. Under present-day conditions, up to 80 %–90% of the nutrient inventory on the shelf is advected from the open ocean via cross-shelf break transport of nutrient-rich Atlantic water masses. 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 (Gröger et al., 2013). The warming and freshening of the upper North Atlantic projected by global climate models induces a weakening of the buoyancy-driven convection and thus a reduction of the wintertime MLD and upper-ocean nutrient concentration (e.g., Alexander et al., 2018). A central question thus is, how these changes in the North Atlantic would affect the nutrient supply to the NWES and thereby its biologically rich ecosystem.

We use the high-resolution regionally coupled ocean-atmosphere climate system model MPIOM/HAMOCC/REMO  (see model setup) to downscale an ensemble of global climate projections according to emission scenarios RCP4.5 and RCP8.5

We find that for a critical ML shoaling under scenario RCP8.5, upper North Atlantic water masses lose their dominant influence on shaping biogeochemical conditions on the NWES. This is due to a regime shift in the main supply pathway of Atlantic nutrients to the shelf. In the open ocean, the ML shoaling leads to a reduction of the vertical nutrient supply from intermediate depths to the euphotic zone. At the shelf break, however, various mixing processes maintain a connection to the Atlantic subpycnocline nutrient pool, e.g., due to the interaction of tidal currents with the abrupt change in the topography, internal waves, and instabilities of the slope current. Nutrient-enriched water masses mixed up near the shelf break spread to the NWES and lead to a weaker nutrient decline in open shelf areas, compared to the adjacent upper NE Atlantic (Fig. 1a, b). As a consequence, the projected weakening of the biological productivity on the shelf is not as strong as would have resulted from a sole influence of upper NE Atlantic water masses. Finally, nutrient concentrations in the upper NE Atlantic reach lower levels than on the shelf, leading to a reversal of the ocean–shelf nutrient gradient and to the development of a nutrient front along the continental margin.

In simulations where we add different amounts of meltwater from the Greenland ice sheet (GIS) to the North Atlantic, we have found that increasing GIS meltwater discharge leads to an intensification of this future ocean-shelf nutrient gradient (Fig. 1c, d).  As shown by us and other groups, the effect of the GIS meltwater on the haline-induced reduction of surface buoyancy decreases North Atlantic MLDs and weakens the Atlantic meridional overturning circulation (AMOC) or even leads to its collapse. Our simulations indicate that the meltwater induced slowdown of the North Atlantic deep circulation resulting from the AMOC weakening induces an increase in subpycnocline nutrient concentrations. These nutrient-enriched water masses are mixed up near the shelf break and transported to open shelf areas, intensifying the future ocean-shelf nutrient front.

Furthermore, during the future shallow-ML regime, the on-shelf nutrient transport is subject to a rapid increase in interannual to multidecadal variability (Fig. 2a). As near the end of the 21st century the ML in the NE Atlantic becomes as shallow as the shelf edge, upper-ocean conditions are more sensitive to variations in the atmospheric forcing. In particular, a positive MLD anomaly (deepening) leads to an erosion of warm and saline subpycnocline water masses and initiates a positive feedback on the surface heat flux, the upper-ocean buoyancy and the MLD. The resulting enhanced variability of the on-shelf nutrient transport affects the variability in pre-bloom nutrient concentrations and annual primary production on the NWES.

The impact of GIS meltwater discharge on this variability increase is indicated at the shelf break (Fig. 2b, c), leading to both a higher variability of upper-ocean nutrient concentrations and an earlier onset of the future shallow-ML regime. The additional impact on the variability of the NWES productivity, however, is low.

The increasing influence of Atlantic subpycnocline water masses on the future NWES physical and biogeochemical state affects not only the supply of oceanic nutrients to the shelf. Any conservative constituent of the on-shelf transport potentially has an impact on the NWES marine ecosystem, such as salinity, temperature, oxygen and alkalinity. The parent global model used to derive the forcing data for our simulations (MPI-ESM-LR), however, is not able to simulate the regime shift in the on-shelf transport mechanism due to its coarse grid resolution and the neglect of tides. By contrast, the dominance of Atlantic upper-ocean conditions on shaping shelf conditions is also seen in the GIS meltwater hosing experiments.

Read more in:

Mathis, M., Elizalde, A., and Mikolajewicz, U.: The future regime of Atlantic nutrient supply to the Northwest European Shelf, J. Mar. Syst., 189, 98–115, 2019 (link)

Mathis, M., and Mikolajewicz, U.: The impact of meltwater discharge from the Greenland ice sheet on the Atlantic nutrient supply to the northwest European shelf, Ocean Sci., 16, 167–193, 2020 (link)

References:

Gröger, M., Maier-Reimer, E., Mikolajewicz, U., Moll, A., and Sein, D.: NW European shelf under climate warming: implications for open ocean – shelf exchange, primary production, and carbon absorption. Biogeosciences 10, 3767-3792, https://doi.org/10.5194/bg-10-3767-2013, 2013.

Alexander, M. A., Scott, J. D., Friedland, K. D., Mills, K. E., Nye, J. A., Pershing, A. J., and Thomas, A. C.: Projected sea surface temperatures over the 21st century: Changes in the mean, variability and extremes for large marine ecosystem regions of Northern Oceans, Elem. Sci. Anth., 6, http://doi.org/10.1525/elementa.191, 2018