missing extra limitation of eddy diffusivity coefficient in ORCA1

[deshayes]

In case GM parameterization is activated (key_traldf_eiv), the coefficient for lateral diffusivity is based on the growth rate of baroclinic instability, with 2 limitations applied :
(1) decreased to zero at the Equator
(2) the Rossby radius used in the growth rate computation is bounded by 2km (min) and 40km (max).

In addition to these two limitations, there is a possibility to apply a maximum value to the eddy diffusivity coefficient, and this value is expected to be set in the namelist via rn_aeiv_0. However, this value is actually used in the code only for ORCA05. In case of ORCA2, the namelist value of rn_aeiv_0 is not used, but a maximum value of 1000 m2/s is applied to the eddy diffusivity coefficient. In case of ORCA1, there is no additional limitation to the eddy diffusivity coefficient beyond the 2 mentioned above, and rn_aeiv_0 is not used.

The attached ldfeiv_modJD.F90 (extracted from nemo_v3_6_STABLE rev 6353) has been modified so that the namelist parameter rn_aeiv_0 is used and applied as maximum value of the eddy diffusivity coefficient in case of ORCA1 configuration. I have not modified the case for ORCA2 configuration, which still ignores namelist parameter rn_aeiv_0 but applies a hard-coded 1000 m2/s maximum value to the eddy diffusivity coefficient, as done before.

[cetlod]

Done see changeset ​http://forge.ipsl.jussieu.fr/nemo/changeset/6631