The impact of meltwater discharge from the Greenland ice sheet on the Atlantic nutrient supply to the northwest European shelf

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.

Historical increases in land‐derived nutrient inputs may alleviate effects of a changing physical climate on the oceanic carbon cycle

“…Second, however, Laufkötter et al (2013) and Matsumoto et al (2010) used a fully coupled Earth System Model that accounts for feedbacks between atmospheric and oceanic processes while our modelling framework relies on a prescribed atmospheric state. Third, the global response of hydrodynamical and biogeochemical processes to changes in the physical climate has been shown to strongly vary from one model to the other (Bopp et al, 2013;Kwiatkowski et al, 2017Kwiatkowski et al, , 2020Laufkötter et al, 2015;Mathis & Mikolajewicz, 2020;Matsumoto et al, 2010). Thus, the trend in the marine NPP is also strongly affected by the different architectures and formulations for hydrodynamical and biogeochemical processes in different models.…”

“…Local changes in ocean circulation seem to be of even larger importance in the coastal ocean, where hydrodynamical circulation While decadal variability of the ocean system currently prevents definite conclusions regarding the impacts of climate-induced changes on the biological productivity from observations alone, modelling studies have addressed these perturbations at global (Bopp et al, 2013;Laufkötter et al, 2013Laufkötter et al, , 2015Matsumoto et al, 2010) and regional scales (Fennel et al, 2006;Frischknecht et al, 2018;Holt et al, 2016Holt et al, , 2018Mathis et al, 2019). In Laufkötter et al (2013) to changes in the physical climate has been shown to strongly vary from one model to the other (Bopp et al, 2013;Kwiatkowski et al, 2017Kwiatkowski et al, , 2020Laufkötter et al, 2015;Mathis & Mikolajewicz, 2020;Matsumoto et al, 2010). Thus, the trend in the marine NPP is also strongly affected by the different architectures and formulations for hydrodynamical and biogeochemical processes in different models.…”

A framework for estuarine future sea-level scenarios: Response of the industrialised Elbe estuary to projected mean sea level rise and internal variability

“…The ensemble simulation dataset represents changes in ocean density and currents as well as meteorological conditions including atmospheric water transport due to the enhanced radiative forcing under climate change. The ensemble is created by starting the global model from different historical states of the years 1950-1959 of three previous simulations with the same model system (Mathis and Mikolajewicz, 2020). For the downscaling of the global simulations, its 6-hourly outputs serve as a forcing for the MPIOM-REMO regional climate modelling framework, which, in turn, provides the boundary forcing for the SCHISM simulations analysed in this study.…”

RCP8.5-projected changes in German Bight storm surge characteristics from regionalized ensemble simulations for the end of the twenty-first century

“…Each member of this ensemble covers the period 1950-2005 with the corresponding historical atmospheric pCO2 and their continuation from 2006 to 2099 under the RCP8.5 emission scenario. Members differ from each other only due to different initial states, taken from historical initial states of the years 1950-1959 of three previous simulations with the same model system, which were analyzed by Mathis and Mikolajewicz (2020). An additional control run with constant pre-industrial atmospheric pCO2 (PI) simulated the full period 1950-2099 to obtain information about model-internal variability and possible drifts.…”