Error in setting background viscosity

[dcherian]

MOM6/src/parameterizations/vertical/MOM_set_diffusivity.F90

Lines 311 to 315 in 9786710

	if (present(Kd_lay)) Kd_lay(:,:,:) = CS%Kd
	Kd_int(:,:,:) = CS%Kd
	if (present(Kd_extra_T)) Kd_extra_T(:,:,:) = 0.0
	if (present(Kd_extra_S)) Kd_extra_S(:,:,:) = 0.0
	if (associated(visc%Kv_slow)) visc%Kv_slow(:,:,:) = CS%Kv

Kd_lay and Kv_slow are set to KD and KV; effectively these are background values and the defaults are KD=2e-5, KV=1e-4

Later calculate_bkgnd_mixing calculates background values Kv_bkgnd and Kd_bkgnd (I have HORIZ_VARYING_BACKGROUND=TRUE) and assigns Kd_lay_2d and Kd_int_2d

MOM6/src/parameterizations/vertical/MOM_set_diffusivity.F90

Lines 409 to 410 in 9786710

	! Add background mixing
	call calculate_bkgnd_mixing(h, tv, N2_lay, Kd_lay_2d, Kd_int_2d, Kv_bkgnd, j, G, GV, US, CS%bkgnd_mixing_csp)

Kd_bkgnd is assigned to Kd_lay in there, overwriting the KD values set earlier.

Now back in MOM_set_diffusivity: Kv_bkgnd is added to Kv_slow (≡ KD):

MOM6/src/parameterizations/vertical/MOM_set_diffusivity.F90

Lines 411 to 414 in 9786710

	! Update Kv and 3-d diffusivity diagnostics.
	if (associated(visc%Kv_slow)) then ; do K=1,nz+1 ; do i=is,ie
	visc%Kv_slow(i,j,K) = visc%Kv_slow(i,j,K) + Kv_bkgnd(i,K)
	enddo ; enddo ; endif

This is NOT what's being done with diffusivity where Kd_lay is set by calculate_bkgnd_mixing.

It is wrong because we've added two different "background" viscosity formulations. We should be overwriting Kv_slow with Kv_bkgnd.

Setting KD=0, KV=0 makes things work like I expected.

EDIT: For the record, here's the image that shows the issue (mean profiles at 0N, 140W; averaged over years 46-68), the prandtl number (visc/diff) is ~200 below the EUC. We want it to be in the 1-10 range.

[gustavo-marques]

Thanks for reporting this.
cc'ing @alperaltuntas

[dcherian]

As update, after discussing with Alper and Gustavo, we decided to do something similar for diffusivity where we add the background field to whatever Kd_int is initialized to (KD in this case) , and then set KV=0, KD=0 by default in MOM_interface.

See #239 and ESCOMP/MOM_interface#134, ESCOMP/MOM_interface#135