Evaluating the physical and biogeochemical state of the global ocean component of UKESM1 in CMIP6 historical simulations

Abstract

Abstract. The ocean plays a key role in modulating the climate of the Earth system (ES). At the present time it is also a major sink both for the carbon dioxide (CO2) released by human activities and for the excess heat driven by the resulting atmospheric greenhouse effect. Understanding the ocean's role in these processes is critical for model projections of future change and its potential impacts on human societies. A necessary first step in assessing the credibility of such future projections is an evaluation of their performance against the present state of the ocean. Here we use a range of observational fields to validate the physical and biogeochemical performance of the ocean component of UKESM1, a new Earth system model (ESM) for CMIP6 built upon the HadGEM3-GC3.1 physical climate model. Analysis focuses on the realism of the ocean's physical state and circulation, its key elemental cycles, and its marine productivity. UKESM1 generally performs well across a broad spectrum of properties, but it exhibits a number of notable biases. Physically, these include a global warm bias inherited from model spin-up, excess northern sea ice but insufficient southern sea ice and sluggish interior circulation. Biogeochemical biases found include shallow remineralization of sinking organic matter, excessive iron stress in regions such as the equatorial Pacific, and generally lower surface alkalinity that results in decreased surface and interior dissolved inorganic carbon (DIC) concentrations. The mechanisms driving these biases are explored to identify consequences for the behaviour of UKESM1 under future climate change scenarios and avenues for model improvement. Finally, across key biogeochemical properties, UKESM1 improves in performance relative to its CMIP5 precursor and performs well alongside its fellow members of the CMIP6 ensemble.

Modelling submerged biofouled microplastics and their vertical trajectories

“…2.4. The biogeochemical performance of NEMO-MEDUSA has previously been extensively validated at low resolution in the studies of Yool et al (2013Yool et al ( , 2021, with traceability at higher resolution demonstrated in Yool et al (2015).…”

Persistent Uncertainties in Ocean Net Primary Production Climate Change Projections at Regional Scales Raise Challenges for Assessing Impacts on Ecosystem Services

“…Largely absent from discussions around changing NPP in response to changing nutrient supply is the role of modification to external nutrient inputs to the ocean. While there have been some efforts including dynamic coupling with aerosol nutrient supply at the sea surface (e.g., Yool et al, 2021), future work that accounts for additional external inputs may introduce additional uncertainty in NPP changes. For instance, the inclusion of dynamic Greenland and Antarctic ice sheets in Earth system models is rapidly becoming computationally permissible and the associated changes to freshwater fluxes due to climate change have the potential to influence projections of NPP via both physical and biogeochemical mechanisms (Hopwood et al, 2020).…”

The Antarctic contribution to 21st century sea-level rise predicted by the UK Earth System Model with an interactive ice sheet

“…For CMIP6, UKESM1.0 with neither interactive ice sheets nor ice shelf cavities in the ocean model was spun up to equilibrium with preindustrial forcing (Yool et al, 2021) before an initial condition ensemble of historical simulations was conducted, starting from different points in the preindustrial control run (Sellar et al 2019). While these historical simulations provided states ready for UKESM1.0 ScenarioMIP projections (Swaminathan et al, 2022), they are not directly usable for the ocean in scenario runs in UKESM1.0-ice, given the absence of ocean under the ice shelves.…”