The ACOM 2 Ocean Model

Andreas Schiller

CSIRO Marine Research

ACOM2 includes the following features:

• the Chen mixing scheme
• Atmospheric Boundary Layer Model (ocean-only integrations)
• a scheme for solar shortwave penetration
• Mediterranean and Red Sea outflows simulated by relaxation to Levitus data
• tidal mixing parameterisation in the Indonesian region
• careful representation of islands and straits in the Indonesian region
• viscosity in Lombok Strait tuned to give observed fraction of total Indonesian throughflow
  

The grid spacing is 2 degrees in the zonal direction. The meridional spacing is 0.5° within 8° of the equator, increasing gradually to 1.5° near the poles. There are 196 grid points from south to north and 182 grid points from west to east.

There are 25 levels in the vertical, with 12 in the top 185 metres. The maximum depth is 5,000 metres. The level thicknesses range from 15 metres near the surface to almost 1,000 metres near the bottom.

The bathymetry of the model represents a smoothed approximation to the high-resolution data set of Gates and Nelson (1975). To simulate the Indonesian Throughflow we modified the model's topography in that area, allowing for a transport of water masses through Lombok Strait and the Timor Sea. Because islands are computationally expensive in this version of the GFDL model only the Philipines, Kalimantan, Celebes, the Lombok-Flores islands, Australia (combined with New Guinea), New Zealand and Antarctica are separated from the remaining land points. There is no Bering or Torres Strait. The Drake Passage was artificially widened to aid throughflow of the Antarctic Circumpolar Current. See Schiller et al. (1998) for further information.

An Atmospheric Boundary Layer Model (ABLM) has been used to create surface fluxes for the ocean-only experiments described in this report. It is NOT used in the coupled version of ACOM2 or during the POAMA data assimilation cycle. The ABLM provides net surface heat fluxes (based on heat flux components) to the ocean model. The boundary layer model contains a prognostic advection/diffusion equation for air temperature in the atmospheric mixed layer. Within the boundary layer, air temperature is determined by a balance between surface fluxes, horizontal advection by imposed winds, entrainment from above the mixed layer, horizontal diffusion and radiative cooling. Use of the Atmospheric Boundary Layer Model (ABLM) leads to improved simulations of SST and its anomalies compared to simpler surface boundary parameterisations. The code was provided by R. Kleeman (BMRC, Melbourne). A detailed description is given in Kleeman and Power (1995).

The forcing of the ocean model in the operational version of POAMA does not use the ABLM. In coupled mode the ocean model uses daily average fields from the atmosphere model. These are: surface momentum fluxes, short-wave radiation, long-wave radiation, sensible heat flux, latent heat flux and fresh water flux. When the ocean model is run in the assimilation mode these fields are provided from an external source as six-hourly mean values. In the present system these come from the NCEP re-analysis when producing hind-analyses and from the Bureau of Meteorology operational NWP model (GASP) when producing real-time operational analyses.

The inflows of the Red Sea into the Indian Ocean and the Mediterranean Sea into the Atlantic Ocean significantly change the water mass structures of these oceans. Because the Red and Mediterranean Seas are not included in this model the inflows are simulated by applying a restoring of the model temperature and salinity to observed values (Levitus, 1982) at the lateral boundaries between the respective seas and oceans. The grid cells at which this is done are "hard-wired" into the model code. Should changes be made to the model grid, some care needs to be taken in choosing the exact grid cells at which this relaxation should take place. The model code is easily adaptable for different grids.

Vertical mixing and vertical friction in the model are parameterised by using the one-dimensional Chen et al. (1994) mixing scheme. Strong mixing is assumed to occur within a bulk mixed layer, as in Niiler and Kraus's (1977) model. Below the mixed layer, internal mixing is decribed by a gradient Richardson-number dependent mixing that uses a parameterisation based on observations by Peters, Gregg and Toole (1988) (Wilson, 2000). Their parameterisation shows a sharper drop-off towards higher Richardson-numbers, which particularly improves the model performance at lower latitudes. The hybrid structure of this mixing scheme allows its application to high latitudes (where mixing is strongly influenced by the bouyancy fluxes of heat and freshwater, and thus the Niiler-Kraus part is assumed to be dominating); it is also applicable to the equatorial ocean (where vertical mixing is predominantly determined by large vertical current shears, and thus the gradient Richardson-number part is assumed to be dominating). The Chen et al. mixing scheme uses the surface fluxes of heat, freshwater and momentum as input to compute the mixed-layer depth and to determine the vertical diffusion and viscosity coefficients within the whole water column. The maximum values for viscosity and diffusivity in the mixed layer are set to 266.0 x 10^-4m2/s and 199.0 x 10^-4m2/s, respectively. The background values (i.e. the smallest possible values for viscosity and diffusivity) are set to 0.2 x 10^-4m2/s and 0.01 x 10^-4m2/s, respectively. The level version of this mixing scheme has been implemented in the MOM model by Richard Kleeman at the Bureau of Meteorology Research Centre, Melbourne (Power et al., 1995). The gradient Richardson-number parameterisation of Peters, Gregg and Toole was made available to us by Steve Wilson, CSIRO Division of Atmospheric Research, Melbourne (Wilson, 2000). For a detailed discussion of the Chen et al. (1994) mixed-layer model in its level version and tests with observed data we refer to the CSIRO report of Godfrey and Schiller (1997).

Due to the large changes in grid sizes of the model (enhanced tropical grid and coarse resolution close to the poles), and in order to guarantee numerical stability, the zonal and meridional viscosity are dependent on latitude. The modifications are similar to changes which Power et al. (1995) applied to another version of the MOM(1) model. The meridional viscosity is set to 2.0 x 10^3 m2/s near the equator but then increases to 1.0 x 10^5 m^2/s at high latitudes while the zonal viscosity is constant (2.0 x 10^4 m^2/s). The viscosity in Lombok Strait is also increased to better simulate the observed transport through the Indonesian Archipelago. To achieve this the meridional viscosity in Lombok Strait is increased by a factor of 8. This reproduces the observed ratio of volume transport through Lombok Strait (20%) to that through the Timor Sea (80%).

The horizontal diffusivity is set to 1.0 x 10^3 m^2/s everywhere in the standard version of ACOM2. However, the standard version of ACOM2 produced an unacceptable level of noise in the east Pacific when run in coupled mode. POAMA version 1.0 (both assimilation and coupled model forecasts) therefore uses a higher value for the horizontal diffusivity (4.5x10^3 m^2/s) in order to reduce the noise to an acceptable level. (Note the preliminary version of POAMA version 0.1 uses the lower value of the diffusivity.)

As water flows from the Pacific to the Indian Ocean, the tidally induced vertical mixing in the Indonesian archipelago is able to change the water mass structure of the Indian Ocean significantly. An analysis by Ffield and Gordon (1996) suggests that the center of the tidal mixing effect on SST is located in the Banda Sea. To simulate this observed feature, the vertical mixing coefficients (diffusion and viscosity) in the Indonesian area were increased. The centre of the additional tidal mixing in the model is located in the Banda Sea and has a maximum value of 2 x 10^-4 m^2/s; it gradually decreases as the distance from the Banda Sea increases. The additional mixing is independent of time, that is, no attempt is made to resolve the timescales associated with its physical origin. This is legitimate as long as one is only concerned with its larger time scale effects on SST.

An observation-based climatology of turbidity (CZCS) is used to estimate the subsurface decay rate for the short-wave penetration.

The model produces netCDF output files. All versions produce time mean fields or snapshots of the following variables: Potential Temperature, Salinity, Velocity, Surface Net Heat Flux, Surface Freshwater Flux, Wind Stresses, Streamfunction (rigid lid option) or Surface height and vertical velocity at surface (implicit free surface option). Output data can be written as separate files or as concatenated files (see namelist output). In addition for the POAMA version 1.0 analyses, hind-casts and forecasts various sections (latitude/time and longitude time) as well as various profiles (depth/time section) are produced using daily values, also in NetCDF format.

The model code and some forcing fields are available from the ACOM web page at

http://www.marine.csiro.au/acom/index.htm.

 

List of references used in this site

 

 

For further information: email the POAMA group

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