The OCCAM papers page

This directory contains details of papers and OCCAM documentation. Additional information is also available in the form of posters. Portable Document Format (PDF) versions of many of the posters presented during the project are available from our posters page.

Brief details:

Humphries, U.W. and Webb, D.J.,2008:
On the Indonesian Throughflow in the OCCAM 1/4 degree ocean model.
Ocean Science, 4, 183-198.

Webb, D.J. and de Cuevas, B. A., 2007:
On the fast response of the Southern Ocean to changes in the zonal wind,
Ocean Science, 3, 417-427.

Jia, Y., Coward, A.C., de Cuevas, B.A., Webb, D.J. and Drijfhout, S.S., 2007:
A model analysis of the behavior of the Mediterranean Water in the North Atlantic.
Journal of Physical Oceanography, 37, (3), 764-786. (doi: 10.1175/JPO3020.1)

Levine, R.C. and Webb, D.J., 2007:
On available energy in the ocean and its application to the Barents Sea,
Ocean Science Discussions, 4, 897-931.

Webb, D.J., and de Cuevas, B.A. 2003:
The region of large sea surface height variability in the southeast Pacific Ocean.
Journal of Physical Oceanography, 33(5), 1044-1056.

Webb, D.J., and de Cuevas, B.A. 2002
An ocean resonance in the Indian sector of the Southern Ocean.
Geophysical Research Letters, 29(14), 9.1-9.4, doi:10.1029/2002GL015270.

Lee, Mei-Man, A.C. Coward and A.J. George Nurser, 2002:
Spurious diapycnal mixing of the deep waters in an eddy resolving global ocean model.
Journal of Physical Oceanography, 32(5), 1522-1535.
Preprint: PDF version or just the abstract

Webb, D.J., 2000:
Evidence of shallow zonal jets in the South Equatorial Current region of the Southwest Pacific.
Journal of Physical Oceanography, 30 (4), 706-720. 2000

Fox, A.D., K. Haines and B.A. de Cuevas 1999:
Satellite altimetry data assimilation in the OCCAM global ocean model.
Phys. Chem. Earth (A), Vol. 24, No. 4, 375-380.

D. J. Webb et al (1998): The first main run of the OCCAM Global Ocean Model
Southampton Oceanography Centre, Internal Document No. 34.

This report contains background information on the OCCAM global ocean model and the first main run of the model. It includes a discussion of the underlying model equations and information on the model grid, the representation of advection and diffusion, the timestepping scheme and the free surface scheme used by the model.

The report also contains information on how the model was initialised for the first main run, how it was forced, the changes that were introduced during the model run and the format of the archived data sets. The report is available as a Portable Document Format or PostScript file.

P. Saunders et al (1999):
Circulation of the Pacific Ocean seen in a global ocean model (OCCAM)
J. Geophys. Res., 104, C8, 18281-18299. (1999).

This paper compares observations made in the Pacific Ocean and the 8-12 year climatology of the OCCAM global model. Meridional mass and heat transports and their divergences are calculated, and zonal averages compared with the air-sea exchange values of Doney et al. [1998] with mixed success. The large scale distribution of sea surface height and near-surface currents are shown to be well reproduced by the model, but their variance is underestimated, severely so in areas of weak variability. Synoptic wind forcing enhances the latter, though only slightly, while higher model resolution improves agreement in areas of strong variability. Model mean transports in the East Australian Current, in the Vitiaz Strait and in the Kuroshio exceed measurements by 10, 0, 18 Sv, respectively, and variability is underestimated by factors between 2 and 4. Near 1000 m and in strong currents the flow is overestimated, but over most of the ocean, model output does not match either the magnitude or the spatial variability of the observations. In contrast, the inflow of bottom water and its spreading path throughout both South and North Pacific is well described. Good agreement is found with measured transport through the Samoan Passage, but at 32�S, east of the Tonga-Kermadec Ridge, the model yields only 60% of the observed value. The Sverdrup balance is shown to hold widely in the model interior. Utilizing the complete depth-integrated vorticity equation it is found that bottom pressure torques balance the advection of planetary vorticity in both surface and bottom intensified western boundary currents.

The report is available as a Portable Document Format or gzipped PostScript file.

Lee, Mei-Man, A.C. Coward and A.J. George Nurser, 2001:
Spurious diapycnal mixing of the deep waters in an eddy resolving global ocean model.
Recent idealized studies have shown that the advection scheme in high resolution z coordinate models may drive unrealistically high rates of diapycnal mixing. Our aim here is to see whether the diapycnal mixing associated with the advection scheme in a global eddy resolving (1/4° by 1/4° ) z level model is sufficiently strong to corrupt the thermohaline circulation. We diagnose the diapycnal fluxes by using the ideas of water mass transformation. In the Southern Ocean, the model deep and bottom waters drift rapidly away from the Levitus climatology, with dense isopycnals moving downwards at rates of up to 35 m/year ; . The strong upward flux (up to 50 Sv) through the dense isopycnals cannot be explained by the incorrect surface forcing (as a result of poor surface fluxes and no ice model) as most of the anomalous diapycnal fluxes are occurring in the deep ocean far from surface forcing. Hence, the excessive diapycnal flux is driven by diffusion in the model, both explicit and implicit. The `effective' diapycnic diffusivity driven by the numerical diffusion (associated with the horizontal advection scheme) is found to be the same order, 1-10 cm²/s , as that driven by the explicit horizontal diffusion. For strong vertical velocities (about 20 m/day ) as in models forced by high frequency winds, the vertical advection scheme also gives similar effective diffusivities. These effective diffusivities are considerably greater than suggested by observations. To alleviate these problems, we suggest that eddy resolving z level climate models will require (1) less diffusive horizontal advection schemes and (2) better vertical resolution throughout much of the water column.

de Cuevas, B.A., Webb, D.J., Coward,A.C., Richmond, C.S. & E. Rourke, 1999.
The UK Ocean Circulation and Advanced Modelling Project (OCCAM)
pp 325-335 in: High Performance Computing, Proceedings of HPCI Conference 1998, Manchester
12-14 January 1998, R.J. Allan, M.F. Guest, A.D. Simpson, D.S. Henty, D.A. Nicole (Eds) Plenum Press.
Fox, A.D., Haines, K., de Cuevas, B.A. & Coward, A.C., 1999.
Satellite Altimeter data assimilation in the OCCAM global model,
pp 365-369 in: Proceedings of HPCI Conference1998, Manchester 12-14 January 1998, R.J. Allan, M.F. Guest, A.D. Simpson, D.S. Henty, D.A. Nicole (Eds). Plenum Press.
Gwilliam, C.S. (1995): The OCCAM Global Ocean Model
In: Coming of Age (The Proceedings of the Sixth ECMWF Workshop on the use of Parallel Processors in Meteorology), World Scientific, pp: 446-454

Over the past two decades, numerical models have been widely used to investigate the circulation, chemistry and biology of the world's oceans, as well as their effect on climate. However, accurate models need very powerful computers and, for this reason, it is expected that a lot more use will be made of array processor computing. In this paper, work being undertaken by the Ocean Circulation and Climate Advanced Modelling (OCCAM) Project is presented. The conversion from the sequential GFDL Modular Ocean Model (MOM) to an efficient array processor, global ocean code is described.

Gwilliam, C.S, A. C. Coward, B. A. de Cuevas, D. J. Webb, E. Rourke, S. R. Thompson, K. Döös: The OCCAM Global Ocean Model
A paper presented at the 2nd UNAM-Cray Supercomputing Conference on Numerical Simulations in the Envionmental and Earth Sciences (June 1995). Accepted for publication in the refereed proceedings.

Climate change is not affected by the atmosphere alone - the world's oceans also play an important role, hence the need for global ocean models. In the past ocean modelling has been restricted to coarse resolutions and to limited areas, for example the Antarctic. The arrival of powerful array processors is allowing global studies at resolutions high enough to resolve eddies in the ocean. This paper describes one such project, OCCAM, and some of the scientific results and problems from an initial three year run.

D. J. Webb et al.(1992): The equation of state algorithms used by the FRAM model
Institute of Oceanographic Sciences, Internal Document No. 313.

This report contains background information on the equation of state used by both the FRAM and OCCAM model. The Eckart equation of state that had been used previously was found to produce significant errors at high latitudes. The later FRAM runs use the full EOS80 internation equation of state for sea water. The OCCAM model uses a polynomial approximation to EOS80 similar to that distributed with the MOM ocean model code. The report is available as a Portable Document Format or PostScript file.

A.C. Coward (1993): The equation of state algorithms used by the OCCAM model
Institute of Oceanographic Sciences, Internal Document No. 323.

This report contains further details on the equation of state used by the OCCAM model and the method used to calculate the polynomial coefficients. The report is available as a Portable Document Format or PostScript file.

Gwilliam, C.S.(1995): Modelling the Global Ocean Circulation on the T3D
In Parallel Computational Fluid Dynamics: Implementations and Results using Parallel Computers, Elsevier, pp:33-40.

The Ocean Circulation and Climate Advanced Modelling Project is developing a global ocean model suitable for climate studies, especially the investigation of global ocean characteristics. The model will also act as a test bed for improved schemes which model ocean physics. The large memory and time requirements for such a study has led to the use of array processors. In this paper an outline of the model and the parallel code is given, along with some initial results.

Richmond, C.S.(1996): Changes to the OCCAM code to allow for different sizes of difference stencils.
An unpublished document detailing changes made to the OCCAM code to allow different advection schemes, with different sizes of difference stencils to be used.

The original OCCAM code uses a centred difference advection scheme in the tracer and momentum equations. This is known to cause under and overshooting of the tracer values where large tracer gradients and/or large velocities occur. Improvements to this scheme include QUICK and variations on it. These schemes are variants of an upwind method and the stencil is five points wide in each of the advected planes. In a parallel context, this means that each processor requires information from 2 rows of another processor i.e. a processor needs 2 halo rows and not the 1 required by the centred difference scheme. To allow for different schemes to be used, and the possibility of higher order schemes as well, the OCCAM code has been rewritten to allow for any number of halo rows. The details of these changes are documented here.

Richmond, C.S.(1996): Optimisation of the OCCAM code.
An unpublished document detailing optimisation changes made to the OCCAM code.

The OCCAM code has been developed for array-based, parallel processing platforms. The code is run in production mode on 256 processors of the 512 processor Cray T3D at Edinburgh. During the day tests are also carried out on 128 processors. For efficient running of the model on the T3D it is necessary to look at how the processors are used. This includes investigating the use of the small, 1MW cache, and the message passing system. This report details changes to the code that have improved its performance. The base code is the OCCAM code with `split Quick' in the tracer and momentum equations.

Richmond, C.S.(1996): Grid Partitioning Code.
This tar file contains codes and a README postscript file for interactively developing partitions for the OCCAM grids.

Richmond, C.S.(1997): Changes to the OCCAM code for the MPI implementation.
An unpublished document detailing the alterations made to the OCCAM code to allow the Message Passing Interface (MPI) to be used. OCCAM now permits PVM, SHMEM and MPI.

Webb, D.J.(1978): Non-linear transfers between sea waves.
Hasslemann's equation describing the non-linear interactions between sea waves is studied numberically for a Pierson-Moskowitz spectrum. The apparent creation of order, due to the non-linear enhancement of the peak of the spectrum occurs as a by-produce of a large amount of disorder created at high wavenumbers.

It is also found that the physical process dominating the non-linear transfer is the scattering of waves near the peak of the spectrum by short waves in the tail of the spectrum. In this process the long waves become longer and the short waves shorter.

Occam page

File last amended: March 2009