BAMOS
Mar 2019
Research corner
with Damien Irving
Eddies in the Southern Ocean, from a visualisation done by Dr Isabella Rosso and the team at NCI’s VizLab.
Image: Isabella Rosso
requires detailed knowledge of the grid configuration (i.e. you
need to know that it’s a C-grid in the first place), which isn’t
readily accessible for most CMIP5 models.
In the end, the “zigzag” solution that Outten et al. (2018) came
up with was somewhat similar to the Nummelin et al. (2017)
scalar work-around described above. In their method, grid cells
were selected along a line of single latitude. A zonal boundary
was then identified from the edges of these grid cells, and the
fluxes across this boundary were summed to give the meridional
transport at the respective latitude. At latitudes close to the
model grid poles where the grid is curved, the identified cells at
a single latitude were not on the same row, thus the transport
across the boundary included heat transport in both the y and x
directions. The process was repeated at each latitude to obtain
the complete meridional ocean heat transport.
Where possible, others get around the vector quantity issue by
converting to scalar quantities. For instance, when dealing with
water velocity it is possible to use Helmholtz decomposition
to write each vector in terms of the streamfunction and
velocity potential. These scalar quantities can be remapped to
a rectilinear grid, and then gradients can be calculated on the
new grid to recover the eastward and northward components
of the water velocity.
Conclusion
Once you move beyond the simple case of remapping a
scalar quantity (where conservation isn’t important) from
an ocean curvilinear to rectilinear grid, it’s important to be
aware of the limitations of “off the shelf” software packages.
For scalar quantities where conservation of the grid sum/total
is important, an easy solution can be to set missing values to
zero before remapping. For vector quantities it is sometimes
possible to convert to and from relevant scalar quantities,
but in many cases an easy solution is lacking. If grid angles
and configurations were archived/documented by modelling
groups, it would be possible to make use of existing software
packages (and/or write new packages) that ingest that
information in order to perform the vector remapping.
References
Nummelin, A., Li, C., and Hezel, P.J., 2017. Connecting
ocean heat transport changes from the midlatitudes to the
Arctic Ocean, Geophysical Research Letters, 44, 1899–1908.
doi:10.1002/2016GL071333
Outten, S., Esau, I., and Otterå, O.H., 2018. Bjerknes compensation
in the CMIP5 climate models, Journal of Climate, 31, 8745–8760.
doi:10.1175/JCLI-D-18-0058.1
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