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Ocean Forecasting Australia Model – version 3

OFAM3

PUBLICATIONS ● MODEL DESCRIPTION ● RESULTS

For access to data from the latest OFAM3 spinup run see http://www.marine.csiro.au/ofam1/

Publications

Oke, P. R., D. A. Griffin, A. Schiller, R. J. Matear, R. Fiedler, J. Mansbridge, M. Cahill, M. A. Chamberlain, K. Ridgway, 2012: Analysis of the variability in a near-global eddy-resolving ocean model, in preparation.

Model Description

Resolution

OFAM3 is a near-global (i.e., non-Arctic) eddy-resolving configuration of version 4p1d of the Modular Ocean Model (Griffies et al. 2004), developed principally for the purpose of hindcasting and forecasting upper ocean conditions in non-polar regions when used in conjunction with a data assimilation system. The model grid has 1/10^o grid spacing for all longitudes and between 75^oS and 75^oN (~ 8-11 km x 11 km) and is comprised of 3600x1500 grid points. The vertical model coordinate is z-star (Griffies et al. 2004), with 51 vertical levels, with 5-m resolution down to 40-m depth, and 10-m vertical resolution to 200-m depth.

Topography

The topography for OFAM3 is derived from GEBCO_08 (www.bodc.ac.uk/data/online_delivery/gebco/) for most of the world, a 30 arc-second topography released in 2008, and a 9 arc-second topography produced by Geoscience Australia. The minimum number of vertical levels in the model is three, so the minimum depth in the model is 15 m.

Forcing

OFAM3 is forced with 1.5^o-resolution, 3-hourly surface heat, freshwater, and momentum fluxes from ERA-interim (Dee et al. 2009). The surface heat flux is applied to the top model layer for components associated with the latent, sensible and long-wave heat flux. The penetrating short-wave heat flux is applied over multiple model-levels according to a single exponential decay law, with penetration depths based on SeaWIFS Kd-490. The model forcing also includes climatological, seasonal river flows estimated by Dai and Trenberth (2002; and Dai et al. (2009). River forcing is applied as a nudging term to salinity over the top 3 model layers at coastal grid points that are close to each river location, with a restoring time-scale of 180 days. Surface temperature is relaxed to monthly-averaged Reynolds SST (Reynolds et al. 2007) with a restoring time-scale of 10-days. The SST relaxation is applied as a surface heat flux that depends on the difference between the modelled and observed SST, and on the climatological mixed layer depth, with weaker restoring over shallower mixed layers. Surface salinity is also restored to climatological, monthly-averaged salinity from CARS (Ridgway and Dunn 2003) with a restoring time-scale of 180 days. Although OFAM3 is not global, with no Arctic Ocean, representation of the impact of variability in the Arctic is included by restoring the temperature and salinity over all depths within 1 degree of the northern boundary to monthly averaged fields from version 2.1.6 of the Simple Ocean Data Assimilation (SODA; Carton et al. 2000; Carton and Giese 2008; accessed in October 2010) between 1993 and 2008, using a restoring timescale of 30 days. After 2008, we restore to a seasonal climatology based on SODA. Meridional velocities at the northern and southern boundaries are zero, with a no-slip condition for zonal velocities. To avoid any significant drift in the deep ocean fields, the temperature and salinity are restored to climatology below 2000 m using a seasonal climatology (Ridgway and Dunn 2003). This deep-ocean restoring is applied with a restoring time-scale of 365 days.

Numerics

The time step if 540 s for model tracers is 540 s, and 6 s for sea-level and depth-integrated velocities. A staggered forward time-step is used for tracers and velocity (Griffies 2004; section 12.6). The model time-step is typically limited by vertical velocities at about 200 m depth. A third-order Adams-Bashforth scheme is used for velocity advection, and a third-order upwind biased scheme is used for tracers (Hundsdorfer and Trompert 1994), in conjunction with a flux limiter scheme (Sweby, 1984). A predictor-corrector time-filter is also applied to sea-level using a non-dimensional damping parameter of gamma=0.2, as recommended by Griffies (2004; section 12.7)

Mixing parameterisations

OFAM3 uses the hybrid mixed-layer model described by Chen et al. (1994). The background vertical diffusivity and viscosity are 1x10^-5 and 1x10^-4 m²s^-1, respectively. The enhanced vertical diffusivity and viscosity due to shear instabilities are 2.5x10^-3 and 5x10^-3 m²s^-1, respectively. Additional vertical mixing is applied to represent the mixing effects of tides according to Lee et al. (2006). The Munk-Anderson-P and Munk-Anderson-Sigma parameters for the Lee-scheme are 0.25 and 3.0, respectively. This additional mixing is applied over the water column and depends on the amplitude of the dominant tidal components. This results in stronger mixing in regions of large-amplitude tides, such as the north-west of Australia. Spatially resolved, but time invariant estimates of tidal amplitudes are obtained from a global inverse model (Egbert et al. 1994).

A convective adjustment is applied for every time step using fully explicit mixing when the water column becomes unstable. The explicit horizontal diffusion is zero. Horizontal viscosity is resolution- and state-dependent using a biharmonic Smagorinsky viscosity scheme (Griffies and Hallberg 2000), with an isotropic parameter of 3.0 and an anisotropic parameter of 3.0.

Volume Conservation

OFAM3 is configured to be volume-conserving. Thermal expansion is therefore not included in the model.

Initialisation and Integration

The model was initialised at rest, with zero sea-level, and with potential temperature and salinity from a global version of the CSIRO Atlas for Regional Seas (CARS, released in 2009; Ridgway and Dunn 2003). The model was spun-up for 13 years, spanning the period 1993-2005, with realistic time-varying forcing, as described above. After this initial spin-up, the model forcing was reset to 1993 with the spun-up temperature, salinity, sea-level, and velocity fields, and run for the period 1993-2011.

Results

Graphics archive of OFAM and BRAN experiments