tracer_advect_meridional Subroutine

   pure subroutine tracer_advect_meridional(grid, metrics, this, ms, dt, bc)
      !! Meridional half of the direction-split tracer advection.
      !! Reads the post-zonal `h_layer` (since
      !! `continuity_apply_zonal` has already updated h
      !! in the split flow) and the just-computed
      !! `mass_flux_y_layer` from
      !! `continuity_meridional_flux`.
      !!
      !! Same OBC_CLAMPED ghost-cell override as the zonal helper —
      !! see `tracer_advect_zonal` for the rationale.
      type(hgrid_t), intent(in) :: grid
      type(ocean_metrics_t), intent(in) :: metrics
      type(continuity_t), intent(inout) :: this
      type(multilayer_state_t), intent(inout) :: ms
      real(wp), intent(in) :: dt
      type(ocean_bc_state_t), intent(in), optional :: bc

      integer :: it, j_s_ghost, j_n_ghost, i, j, k
      real(wp) :: clamped_tr

      if (.not. allocated(ms%tracers)) return

      j_s_ghost = grid%nghost
      j_n_ghost = grid%nghost + grid%ny_phys + 1

      do it = 1, size(ms%tracers)
         if (.not. ms%tracers(it)%do_horizontal_advection) cycle
         if (present(bc)) then
            if (bc%south%bc_type == OBC_CLAMPED .and. bc%has_south .and. &
                allocated(bc%south%clamped_tracer) .and. &
                it <= size(bc%south%clamped_tracer)) then
               clamped_tr = bc%south%clamped_tracer(it)
               do concurrent(k=1:ms%nz_ml, i=1:grid%nx_total, j=1:grid%nghost)
                  ms%tracers(it)%hTr(i, j, k) = clamped_tr*ms%h_layer(i, j, k)
               end do
            end if
            if (bc%north%bc_type == OBC_CLAMPED .and. bc%has_north .and. &
                allocated(bc%north%clamped_tracer) .and. &
                it <= size(bc%north%clamped_tracer)) then
               clamped_tr = bc%north%clamped_tracer(it)
               do concurrent(k=1:ms%nz_ml, i=1:grid%nx_total, &
                             j=grid%nghost + grid%ny_phys + 1:grid%ny_total)
                  ms%tracers(it)%hTr(i, j, k) = clamped_tr*ms%h_layer(i, j, k)
               end do
            end if
         end if
         ! Budget dispatch: pass the matching budget accumulator for whichever
         ! tracer opted into a budget slot (heat/salt); age/passive tracers
         ! (budget_id = NONE) get no budget arg (absent ⇒ inert).
         select case (ms%tracers(it)%budget_id)
         case (TRACER_BUDGET_HEAT)
            call tracer_advect_meridional_one_impl( &
               grid%nx_total, grid%ny_total, ms%nz_ml, &
               dt, metrics%iareaT, metrics%wet_T, &
               ms%h_layer, &
               ms%tracers(it)%hTr, &
               ms%mass_flux_y_layer, &
               this%h_face_left_y%data, &
               this%h_face_right_y%data, &
               budget_adv=ms%heat_budget_horiz_adv)
         case (TRACER_BUDGET_SALT)
            call tracer_advect_meridional_one_impl( &
               grid%nx_total, grid%ny_total, ms%nz_ml, &
               dt, metrics%iareaT, metrics%wet_T, &
               ms%h_layer, &
               ms%tracers(it)%hTr, &
               ms%mass_flux_y_layer, &
               this%h_face_left_y%data, &
               this%h_face_right_y%data, &
               budget_adv=ms%salt_budget_horiz_adv)
         case default
            call tracer_advect_meridional_one_impl( &
               grid%nx_total, grid%ny_total, ms%nz_ml, &
               dt, metrics%iareaT, metrics%wet_T, &
               ms%h_layer, &
               ms%tracers(it)%hTr, &
               ms%mass_flux_y_layer, &
               this%h_face_left_y%data, &
               this%h_face_right_y%data)
         end select
      end do
   end subroutine tracer_advect_meridional