Abstract
Abstract. Continental shelf regions in the ocean play an important role in the global cycling of carbon and nutrients, but their responses to global change are understudied. Global Earth system models (ESMs), as essential tools for building understanding of ocean biogeochemistry, are used extensively and routinely for projections of future climate states; however, their relatively coarse spatial resolution is likely not appropriate for accurately representing the complex patterns of circulation and elemental fluxes on the shelves along ocean margins. Here, we compared 29 ESMs used in the Intergovernmental Panel on Climate Change (IPCC)'s Assessment Reports (ARs) 5 and 6 and a regional biogeochemical model for the northwest North Atlantic (NWA) shelf to assess their ability to reproduce surface observations of temperature, salinity, nitrate and chlorophyll. The NWA region is biologically productive, influenced by the large-scale Gulf Stream and Labrador Current systems and particularly sensitive to climatically induced changes in large-scale circulation. Most ESMs compare relatively poorly to observed surface nitrate and chlorophyll and show differences with observed surface temperature and salinity that suggest spatial mismatches in their large-scale current systems. Model-simulated nitrate and chlorophyll compare better with available observations in AR6 than in AR5, but none of the models perform equally well for all four parameters. The ensemble means of all ESMs, and of the five best-performing ESMs, strongly underestimate observed chlorophyll and nitrate. The regional model has a much higher spatial resolution and reproduces the observations significantly better than any of the ESMs. It also simulates reasonably well vertically resolved observations from gliders and bi-monthly ship-based monitoring observations. A ranking of the ESMs indicates that only one ESM has good and consistent performance for all variables. An additional evaluation of the ESMs along the regional model boundaries shows larger variability but is generally consistent with the ranking on the shelf. Overall, 11 ESMs were deemed satisfactory for use in the NWA, either directly or for regional downscaling.
Highlights
Elemental fluxes along ocean margins, which are areas of complex physical and biogeochemical interactions, are important components of the global cycles of carbon (C) and nitrogen (N)
Models and model ensembles are first compared with observations to assess their ability to reproduce the annual cycles of surface temperature, salinity, chlorophyll and nitrate in the NWA region
The Earth system models (ESMs) reasonably reproduce the annual cycle of surface temperature, but the annual cycles of salinity, chlorophyll and nitrate are not simulated well in any of them
Summary
Elemental fluxes along ocean margins, which are areas of complex physical and biogeochemical interactions, are important components of the global cycles of carbon (C) and nitrogen (N). Many shelf regions are thought to be a significant sink for atmospheric CO2 (Cai et al, 2006; Chen et al, 2013; Laruelle et al, 2018), including the eastern margin of North America (Fennel et al, 2019, and references therein), there are significant discrepancies in available estimates. Despite their importance, the response of ocean margins to climate change is understudied relative to the open ocean.
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