# 8.5. Activating variables

There are some variables in the SICOPOLIS code, for example `c_slide`

, the basal sliding coefficient, that are constant in time and reinitialized at each time step in the subroutines in `src/subroutines/general/calc_vxy_m.F90`

. This reinitialization has an “erasing” effect on the the dependency of cost functions to such variables, resulting in identically zero adjoint fields. From a physics standpoint, it is pretty clear that `c_slide`

should have affect various kinds of cost functions.

The workaround to this issue is to complete the following steps (the code snippets are based on the `c_slide`

example):

Declare a dummy variable in

`src/subroutines/general/sico_variables_m.F90`

.

real(dp), dimension(0:JMAX,0:IMAX) :: xxc_slide

Initialize the dummy variable to 0. at the end of

`src/subroutines/{domain}/sico_init_m.F90`

. Here`{domain}`

can either be`grl`

or`ant`

.

xxc_slide = 0.

Add the dummy variable to the actual variable. In the case of

`c_slide`

, add it at the point where it’s initialization is complete.

c_slide = c_slide + xxc_slide

Evaluate sensitivities to the dummy variable i.e. the

`{ind_vars}`

should contain`xxc_slide`

for a non-trivial senstivity with respect to`c_slide`

.

This procedure has the effect of continuity of the variable `xxc_slide`

in time, preventing the “erasing” effect caused by the reinitialization of `c_slide`

to the same value for each time step. Also, since `xxc_slide`

is initialized to 0, the results in the normal simulations are unaffected, because for the purposes of the non-linear forward model, all we did was add zeros to `c_slide`

.