Icarus Allen

Modelling the global coastal ocean

Shelf and coastal seas are regions of exceptionally high biological productivity, high rates of biogeochemical cycling and immense socio-economic importance. They are, however, poorly represented by the present generation of Earth system models, both in terms of resolution and process representation. Hence, these models cannot be used to elucidate the role of the coastal ocean in global biogeochemical cycles and the effects global change (both direct anthropogenic and climatic) are having on them. Here, we present a system for simulating all the coastal regions around the world (the Global Coastal Ocean Modelling System) in a systematic and practical fashion. It is based on automatically generating multiple nested model domains, using the Proudman Oceanographic Laboratory Coastal Ocean Modelling System coupled to the European Regional Seas Ecosystem Model. Preliminary results from the system are presented. These demonstrate the viability of the concept, and we discuss the prospects for using the system to explore key areas of global change in shelf seas, such as their role in the carbon cycle and climate change effects on fisheries.

Modelling the spatial and temporal variability of the Cretan Sea ecosystem

Abstract The ecosystem function of the oligotrophic Cretan Sea is explored through the development and application of a 3D ecological model. The simulation system comprises of two on-line coupled submodels: the 3D Princeton Ocean Model (POM) and the 1D European Regional Seas Ecosystem Model (ERSEM) adapted to the Cretan Sea. For the tuning and initialisation of the ecosystem parameters, the 1D version of the biogeochemical model is used. After a model spin up period of 10 years to reach a quasi-steady state, the results from an annual simulation are presented. A cost function is used as validation method for the comparison of model results with field data. The estimated annual primary and bacteria production are found to be in the range of the reported values. Simulation results are in good agreement with in situ data illustrating the role of the physical processes in determining the evolution and variability of the ecosystem.

Modelling the annual cycles of nutrients and phytoplankton in a Mediterranean lagoon (Gialova Greece)

Abstract Nutrient dynamics for phosphate, nitrate, ammonium and silicate have been simulated with the European Regional Seas Ecosystem Model in a Mediterranean lagoon. This generic model designed for the open sea can be usefully applied also to coastal lagoon ecosystems with minimum modifications. The annual cycles of the nutrients phosphate and silicate compare quite well with the observed ranges of variability. This does not hold for ammonium and nitrate where the increased concentrations could be attributed to external inputs from the land. Nutrient budgets calculated from the model results indicate some competition between phytoplankton and bacteria for nutrients, a common characteristic of lagoons. To further develop the model, an adaptation of the phytoplankton submodel to represent benthic primary production, as well as the modification of the benthic nutrient model to cope with anoxic events, are suggested.

Ocean net heat flux influences seasonal to interannual patterns of plankton abundance.

Changes in the net heat flux (NHF) into the ocean have profound impacts on global climate. We analyse a long-term plankton time-series and show that the NHF is a critical indicator of ecosystem dynamics. We show that phytoplankton abundance and diversity patterns are tightly bounded by the switches between negative and positive NHF over an annual cycle. Zooplankton increase before the transition to positive NHF in the spring but are constrained by the negative NHF switch in autumn. By contrast bacterial diversity is decoupled from either NHF switch, but is inversely correlated (r = −0.920) with the magnitude of the NHF. We show that the NHF is a robust mechanistic tool for predicting climate change indicators such as spring phytoplankton bloom timing and length of the growing season.

Solutions for ecosystem-level protection of ocean systems under climate change.

The Paris Conference of Parties (COP21) agreement renewed momentum for action against climate change, creating the space for solutions for conservation of the ocean addressing two of its largest threats: climate change and ocean acidification (CCOA). Recent arguments that ocean policies disregard a mature conservation research field and that protected areas cannot address climate change may be oversimplistic at this time when dynamic solutions for the management of changing oceans are needed. We propose a novel approach, based on spatial meta-analysis of climate impact models, to improve the positioning of marine protected areas to limit CCOA impacts. We do this by estimating the vulnerability of ocean ecosystems to CCOA in a spatially explicit manner and then co-mapping human activities such as the placement of renewable energy developments and the distribution of marine protected areas. We test this approach in the NE Atlantic considering also how CCOA impacts the base of the food web which supports protected species, an aspect often neglected in conservation studies. We found that, in this case, current regional conservation plans protect areas with low ecosystem-level vulnerability to CCOA, but disregard how species may redistribute to new, suitable and productive habitats. Under current plans, these areas remain open to commercial extraction and other uses. Here, and worldwide, ocean conservation strategies under CCOA must recognize the long-term importance of these habitat refuges, and studies such as this one are needed to identify them. Protecting these areas creates adaptive, climate-ready and ecosystem-level policy options for conservation, suitable for changing oceans.

Eddy resolved ecosystem modelling in the Irish Sea

A computationally efficient three-dimensional modelling system (Proudman Oceanographic Laboratory Coastal-Ocean Modelling System, POLCOMS) has been developed for the simulation of shelf-sea, ocean and coupled shelf-ocean processes. The system is equally suited for use on single processor workstations and massively parallel supercomputers, and particular features of its numerics are an arbitrary (terrain following) vertical coordinate system, a feature preserving advection scheme and accurate calculation of horizontal pressure gradients, even in the presence of steep topography. One of the roles of this system is to act as a host to ecosystem models, so that they can interact with as accurate a physical environment as is currently feasible. In this study, a hierarchy of nested models links the shelf-wide circulation and ecosystem, via a high resolution physics model of the whole Irish Sea, to the test domain: a region of the western Irish Sea. In this domain, ecosystem models are tested at a resolution of ~1.5km (c.f. the typical summer Rossby radius of 4km). Investigations in the physics-only model show the significance of advective processes (particularly shear diffusion and baroclinic eddies) in determining the vertical and horizontal temperature structure in this region. Here we investigate how a hierarchy of complexity (and computational load) from a 1D point model to a fully 3D eddy resolved model affects the distribution of phytoplankton (and primary production) and nutrients predicted by the European Regional Seas Ecosystem Model (ERSEM), a complex multi-compartment ecosystem model. We shall also show how the parallel programming features of the POLCOMS code allows large-scale simulations to be carried out on hundreds, and now on over a thousand, processors, approaching Teraflop/s performance levels. This is shown using a series of benchmark runs on the 1280 processor IBM POWER4 system operated by the UKs HPCx Consortium.