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 UK's HPCx Consortium.