The Hybrid Coordinate Ocean Model (HYCOM) combines the optimal features of isopycnic-coordinate and fixed-grid ocean circulation models in one framework [1]. The name “hybrid” is derived from the models ability to dynamically change its vertical layer distribution between isopycnic (ρ), z-level and bathymetry following σ-coordinates, regularly adjusting to an optimal vertical structure most suitable for a specific region of the ocean. The adaptive vertical grid conveniently resolves regions of vertical density gradients, such as the thermocline and surface fronts.
Marjolaine Rouault has been working on deriving ocean surface current velocities from the Advanced Synthetic Aperture Radar (ASAR; [2]), which is mounted on the Envisat satellite. The ASAR velocity data product has a resolution ranging from 4 – 8 km. It is a microwave sensor and its ability to “see” through clouds makes it a very powerful data set to use to map the Agulhas Current at high resolution, and hence also to validation models.
The figure above on the left shows the 2 year mean surface radial velocity component derived from ASAR. It is the radial component of the velocities because Envisat's ground track is -15° from North, and one can only derive current velocities from ASAR, when the current is perpendicular to the satellite ground track, which is luckily the case for the Agulhas. The black contour lines represent the 200, 500, 1000, 2000, and 4000 m isobaths.
One can see that in the southern part of the Agulhas, the current is strongly steered by the bathymetry. At the eastern edge of the Agulhas Bank, the core of the current closely follows the 1000 m isobath.
The above figure to the right shows the 2 year mean radial velocity component derived from HYCOM version 2.1. Two noticeable differences are evident:
The current velocities of the southern Agulhas Current simulated in HYCOM are markedly weaker (almost a factor of 3 difference) than those derived from ASAR.
At the eastern edge of the Agulhas Bank, the current follows the 2000 m isobath instead of the 1000 m isobath, as suggested by the ASAR observations.
In HYCOM, each vertical layer is assigned a reference density, which is the density it reverts to once when changing from fixed vertical coordinates to isopycnic coordinates. The reference densities in version 2.1 of HYCOM were chosen to mimic the reference densities of the parent model supplying the lateral boundary conditions, which range from 21.0 to 28.3 kg/m3. Plotting the vertical distribution of the layers in HYCOM 2.1 (not shown) it is evident that too many layers are found within the mixed layer and the upper 200 m, and only 7 layers remain to simulate the remainder of the water column, which suggests that the reference densities selected are inadequate for the Agulhas region.
Version 2.1 of HYCOM applies a relatively crude vertical interpolation scheme to the mixed layer, which causes enhanced, and artificial, diapycnal mixing within the mixed layer that may diffuse the core of the current.
Recently, we upgraded HYCOM to version 2.2, and some new and improved features include:
A new GISS mixed layer scheme
WENO-like PPM interpolation of the mixed layer
(WENO = Weighted Essentially Non-Oscilatory; PPM = Piecewise Parabolic Method)
Bottom layer KPP
Slow evolution of the barotropic mode, which allows us to double the barotropic time-step.
Then, to test the importance of the vertical layer distribution, I ran two experiments using version 2.2 of HYCOM:
expt01.0: Using the same reference densities as in HYCOM 2.1
expt01.1: Adjust the reference densities to a range more suitable for the Agulhas region. Based on potential density observations from the WOCE transect I6, a new density range from 23.6 to 27.6 kg/m3 was chosen. The vertical resolution resolution was increased between 23.5 and 26.8 kg/m3 to capture the salinity maximum in the Mozambique Channel at approximately 150 – 300 m, and between 27.1 and 27.7 kg/m3 to capture the salinity minimum in the South Atlantic at approximately 600 – 1200 m.
The below figures show the 2 year mean surface radial velocity component derived from expt01.0 and expt01.1 (left and right respectively). The model was run for 2 years only, so is not spun up, these are preliminary tests only.
However, these preliminary results indicate that implementing the new version of HYCOM increases the velocities of the simulated current. There is also some improvement in HYCOM's ability to simulate an Agulhas Current following the 1000 m isobath along the eastern edge of the Agulhas Bank, despite using the same reference densities as in version 2.1.
Qualitatively comparing HYCOM 2.2 expt01.0 and expt01.1, indicates that adjusting the reference densities has an effect on the position on the mean flow of the Agulhas Current. Expt01.1 seems to follow the 1000 m isobath more closely, although in general the current remains too wide compared to the ASAR derived observations. The below results highlight the importance of the vertical discretisation of the model grid when simulation the ocean.
These experiments with HYCOM 2.2 have been carried out using a 2nd order momentum advection scheme, and it has been shown, using HYCOM 2.1, that a 4th order momentum advection scheme has a significant impact on the solution [3].
The 4th order momentum advection scheme uses a super-slip condition at the coast. Presently I am testing a simulation using the free-slip condition at the coast, to see whether this has an impact on the simulation of the Agulhas.
References
Bleck, R.: An oceanic general circulation model framed in hybrid isopycnic-Cartesian coordinates, Ocean Modell. 37, 55–88, 2002
Rouault, M. J., A. Mouche, F. Collard, J. A. Johannessen and B. Chapron: Mapping the Agulhas Current from space: an assessment of ASAR surface current velocities, J. Geophys. Res., 115, C10026, doi:10.1029/2009JC006050
Backeberg, B. C., Bertino, L., and Johannessen, J. A.: Evaluating two numerical advection schemes in HYCOM for eddy-resolving modelling of the Agulhas Current, Ocean Sci. 5(2), 173–190, 2009
