#### Context The `ORCA2_ICE_OBS` SETTE test of both NEMO `main` and `branch_4.2` has failed when run in debug mode due to the use of an undefined variable. #### Analysis When `ldnightav` is `.FALSE.` in subroutine `obs_surf_opt` of module [`obs_oper`](src/OCE/OBS/obs_oper.F90), the undefined variable `idayend` is accessed during the evaluation of the condition in two `IF` constructs. #### Fix The affected `IF` constructs could be restructured into nested `IF` constructs that test `ldnightav` in the first instance.