Prospects for improving the representation of coastal and shelf seas in global ocean models

Abstract

Abstract. Accurately representing coastal and shelf seas in global ocean models represents one of the grand challenges of Earth system science. They are regions of immense societal importance through the goods and services they provide, hazards they pose and their role in global-scale processes and cycles, e.g. carbon fluxes and dense water formation. However, they are poorly represented in the current generation of global ocean models. In this contribution, we aim to briefly characterise the problem, and then to identify the important physical processes, and their scales, needed to address this issue in the context of the options available to resolve these scales globally and the evolving computational landscape.We find barotropic and topographic scales are well resolved by the current state-of-the-art model resolutions, e.g. nominal 1/12 • , and still reasonably well resolved at 1/4 • ; here, the focus is on process representation. We identify tides, vertical coordinates, river inflows and mixing schemes as four areas where modelling approaches can readily be transferred from regional to global modelling with substantial benefit. In terms of finer-scale processes, we find that a 1/12 • global model resolves the first baroclinic Rossby radius for only ∼ 8 % of regions < 500 m deep, but this increases to ∼ 70 % for a 1/72 • model, so resolving scales globally requires substantially finer resolution than the current state of the art.We quantify the benefit of improved resolution and process representation using 1/12 • global-and basin-scale northern North Atlantic nucleus for a European model of the ocean (NEMO) simulations; the latter includes tides and a k-ε vertical mixing scheme. These are compared with global stratification observations and 19 models from CMIP5. In terms of correlation and basin-wide rms error, the high-resolution models outperform all these CMIP5 models. The model with tides shows improved seasonal cycles compared to the highresolution model without tides. The benefits of resolution are particularly apparent in eastern boundary upwelling zones.To explore the balance between the size of a globally refined model and that of multiscale modelling options (e.g. finite element, finite volume or a two-way nesting approach), we consider a simple scale analysis and a conceptual grid refining approach. We put this analysis in the context of evolving computer systems, discussing model turnaround time, scalability and resource costs. Using a simple cost model compared to a reference configuration (taken to be a 1/4 • global model in 2011) and the increasing performance of the UK Research Councils' computer facility, we estimate an unstructured mesh multiscale approach, resolving process scales down to 1.5 km, would use a comparable share of the computer resource by 2021, the two-way nested multiscale approach by 2022, and a 1/72 • global model by 2026. However, we also note that a 1/12 • global model would not have a comparable computational cost to a 1 • global model in 2017 until 2027. Hence, we conc...

The Impact of Waves and Tides on Residual Sand Transport on a Sediment‐Poor, Energetic, and Macrotidal Continental Shelf

“…More details on wave-tide interactions in Delft3D are presented in section 3.2. Holt et al (2017) indicate that a resolution of 1.5 km is necessary to be eddy resolving over~70% of the coastal ocean areas globally, and 1.5 km was considered sufficient for resolving the internal Rossby radius on the Northwest European shelf in the development of the operational AMM15 model (Graham et al, 2018). A hydrodynamic resolution of 1 km was used in this study, on a curvilinear, spherical coordinate grid.…”

Resolving Shelf Break Exchange Around the European Northwest Shelf

“…However, if nutrient content differs between opposing currents of similar strength, this could lead to larger differences between the two configurations, despite there being a negligible net volume flux. Most global Earth system models use resolutions >7 km, lacking many processes acting at the shelf break (including tides; Holt et al, 2017). If such models are unable to parameterize shelf break processes effectively, then it is unreasonable to expect that they will accurately represent the downwelling circulation or its variability.…”

“…Despite their importance, shelf seas are still not well represented in many climate models (e.g., Holt et al, 2017). Downwelling circulation occurs around the European northwest shelf (NWS), with net on-shelf transport in the upper layer and off-shelf transport at the base of the water column.…”

Kilometric Scale Modeling of the North West European Shelf Seas: Exploring the Spatial and Temporal Variability of Internal Tides

“…Examples include boundary currents, internal tides, eddies, coastal‐upwelling jets and filaments, tidal straining, and benthic boundary layer processes such as Ekman drains and cascades (see Huthnance, for a review). This dynamical complexity arises as many of the dominant dynamical scales decrease with reducing water depth (e.g., barotropic and baroclinic Rossby radii; see e.g., Holt et al, ). The processes then have complex interactions with tidal phenomena and other bathymetric interactions whose amplitudes and significance generally increase as the water depth decreases.…”