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POLCOMS

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Overview

Simulation of the marine environment has become an important tool across a wide range of human activities, with applications in coastal engineering, offshore industries, fisheries management, marine pollution monitoring, weather forecasting and climate research to name but a few. Sustainable management of the ecological resources of coastal environments demands an ability to understand and predict the behaviour of the marine ecosystem. Thus, it is highly desirable to extend the capabilities of existing hydrodynamic models to include chemical, bio-geo-chemical and biological processes within the marine ecosystem.

The Proudman Oceanographic Laboratory Coastal Ocean Modeling System (POLCOMS) has been developed to tackle multi-disciplinary studies in coastal/shelf environments. The central core is a sophisticated 3-dimensional hydrodynamic model that provides realistic physical forcing to interact with, and transport, environmental parameters. Integrating from ocean to coast, or vice versa, biological production and the fate of contaminants can be determined. Figure 1 shows the concept of the coupled model.


Figure 1: Conceptual Diagram of the coupled hydrodynamic-ecosystem model
(Click for full size image)

The Hydrodynamic Model

The hydrodynamic model is a 4- dimensional finite difference model based on a latitude-longitude Arakawa B-grid in the horizontal and S-coordinates in the vertical. Conservative monotonic PPM advection routines are used to ensure strong frontal gradients. Vertical mixing is through turbulence closure (Mellor-Yamada level 2.5).

European Regional Seas Ecosystem Model (ERSEM)

ERSEM is a generic model that describes both the pelagic and benthic ecosystems and the coupling between them in terms of the significant bio-geo-chemical processes affecting the flow of carbon, nitrogen, phosphorous and silicon. It uses a functional group approach to describe the ecosystem whereby biota are grouped together according to their trophic level and sub-divided according to size and feeding method.


Figure 2: Surface to bed temperature difference for 1995 Julian Day 200
(Click for full size image)

Seasonal stratification and primary production

Seasonal stratification on the NW European shelf is controlled by tidal motion. Tides and stratification have a strong interplay with primary production. Modelled total primary production for 1995 shows good agreement with regional estimates.

Spring-Neap Modulation in Productivity

The time series of temperature, diatoms, flagellates and nitrate from station CS in the North Sea (figure 3), show that there is a clear spring-neap tidal cycle in productivity at the base of the pycnocline. Station CS is shown as a circle in the plot of surface-to-bed temperature difference in figure 2. An interesting feature to note is how the flagellate maxima are out of phase with respect to the diatom maxima.


Figure 3: Time series of temperature, diatoms, flagellates and nitrate from station CS
(Click for full size image)

The mid-water chlorophyll maxima and the fortnightly variability appear to be a shelf wide feature within the stratified area, as shown by the time series of primary production in the different stratified areas within the NW European shelf (figure 4).


Figure 4: Time series of primary production for four different stratified areas of the NW European shelf
(Click for full size image)

Climate Change

Simple climate change scenarios, modifying the storminess and the average air temperature based on HADCM3 results for the mid-21st century, show small but noticeable effects on the amplitude and timing of the spring bloom. Although patchy in response, small changes in climate can alter the amplitude of the spring bloom by nearly 50% and advance/delay its timing by up to 30 days.

Parallel Computing

The combination of a fine resolution grid, sophisticated numerical schemes and a complex sub-grid-scale ecological model presents a serious computational challenge that can only be met with the use of high- performance computer systems. The model has been structured to allow execution on parallel systems using two-dimensional horizontal partitioning of the domain and message passing between neighbouring sub-domains. Reordering of the dimensions of the main data arrays and of the loop nests has been shown to give a large performance improvement on modern RISC microprocessors. Absolute performance of 800 model days per day of CPU time has been achieved using 320 Cray T3E-1200 processors or 192 processors of an SGI Origin 3000 with 400 MHz R12000 processors. This performance allows us to run annual simulations of the fully coupled model overnight.

References

Ashworth M., Proctor R., Holt J. T., Allen J. I., and Blackford J. C.,
Coupled Marine Ecosystem Modelling on High-Performance Computers,
Proceedings of the Ninth ECMWF Workshop on the Use of Parallel Processors in Meteorology (2001) in press.

Allen J. I., Blackford J. C., Holt J. T., Proctor R., Ashworth M. and Siddorn J.,
A highly spatially resolved ecosystem model for the North West European Continental Shelf,
Sarsia (2001) in press.

Further Information

Acknowledgements

Mike Ashworth, CCLRC Daresbury Laboratory

Roger Proctor, Jason Holt, Proudman Oceanographic Laboratory

Icarus Allen, Jerry Blackford, Plymouth Marine Laboratory

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