POP

    The Parallel Ocean Program (POP) is a descendent of the Bryan-Cox-Semtner class of ocean models first developed by Kirk Bryan and Michael Cox at the NOAA Geophysical Fluid Dynamics Laboratory in Princeton, NJ, in the late 1960's.  POP had its immediate origins in a version of the model developed by Bert Semtner and Bob Chervin at NCAR. Experience with this version led to a number of changes resulting in what is now known as POP. Details of these changes can be found in articles by Smith et al. (1992), Dukowicz et al. (1993), and Dukowicz and Smith (1994).  The model has continued to develop to adapt to new machines, incorporate new numerical algorithms and introduce new physical parameterizations.
    The most important algorithmic modification in POP involved the treatment of the barotropic mode.  The barotropic streamfunction formulation in the standard Bryan-Cox-Semtner models requires an additional equation to be solved for each continent and island that penetrate the ocean surface.  This was computationaly costly even on parallel-vector-processor computers, which had fast memory access. To reduce the number of equations to solve with the barotropic streamfunction formulation, it was common practice to submerge islands, connect them to nearby continents with artificial land bridges, or merge an island chain into a single mass without gaps.  On distributed-memory parallel computers, these added equations were even more costly because each required gathering data from an arbitrarily large set of processors to perform a line-integral around each landmass.  This computational dilemma was overcome by a new formulation of the barotropic mode based on surface pressure.  The boundary condition for the surface pressure at a land-ocean interface point is local, which eliminates the non-local line-integral.  Consequently, the surface-pressure formulation permits any number of islands to be included at no additional computational cost, and all channels between islands can be treated as precisely as the resolution of the grid permits.  The surface-pressure formulation also allows more realistic, unsmoothed bottom topography to be used with no reduction in time step.  This alleviates the difficulty in the barotropic streamfunction formulation that the elliptic problem to be solved is ill-conditioned when bottom topography has large spatial gradients.  In addition, the original "rigid-lid" boundary condition was replaced by an implicit free-surface boundary condition that allows the air-sea interface to evolve freely and makes sea-surface height a prognostic variable.
    Another significant feature of POP is that the primitive equations were reformulated and discretized to allow the use of any locally orthogonal horizontal grid.  This provides alternatives to the standard latitude-longitude grid with its singularity at the North Pole.  This generalization made possible the development of the displaced-pole grid, which moves the singularity arising from convergence of meridians at the North Pole into an adjacent landmass such as North America, Russia or Greenland.  Such a displaced pole leaves a smooth, singularity-free grid in the Arctic Ocean. That grid joins smoothly at the equator with a standard Mercator grid in the Southern Hemisphere.  The most recent versions of the code also support a tripole grid in which two poles can be placed opposite each other in land masses near the North Pole to give more uniform grid spacing in the Arctic Ocean while maintaining all the advantages of the displaced pole grids.
    POP is written in Fortran90 and can be run on a variety of parallel and serial computer architectures. The most recent version of the code supports current clusters of shared-memory multi-processor nodes through the use of thread-based parallism (OpenMP) between processors on a node and message-passing (MPI or SHMEM) for communication between nodes.  The flexibility of mixing thread-based and message-passing programming models gives the user the option of choosing the best combination of styles to suit a given machine.
    In the period 1994-97, POP was used to perform high resolution global ocean simulations, running on the Thinking Machines CM5 computer then located at LANL's Advanced Computing Laboratory. Output from global ocean simulations are available at http://climate.acl.lanl.gov and http://neit.cgd.ucar.edu/oce/bryan/woce-poster.html (see also Maltrud et al., 1998; Smith et al., 2000 and Washington et al., 2000). Recently, computer resources have become available to undertake a 0.1 degree global simulation and this calculation is in process.
    POP and the Los Alamos elastic-viscous-plastic sea-ice dynamics model have been coupled to the NCAR Community Climate Model (CCM) atmospheric and land-surface models, to form the Parallel Climate Model (PCM).  This model is being used for climate research and global-warming studies (Washington et al., 2000).  POP and the full sea-ice model (CICE) have also been adopted as the ocean and sea ice components of the Community Climate System Model (CCSM) at NCAR.  POP and CICE are also being used in coupled model development efforts at Colorado State University.