#include "cppdefs.h"
      MODULE tl_pre_step3d_mod
#if defined TANGENT && defined SOLVE3D
# define NEUMANN
!
!svn $Id$
!================================================== Hernan G. Arango ===
!  Copyright (c) 2002-2020 The ROMS/TOMS Group       Andrew M. Moore   !
!    Licensed under a MIT/X style license                              !
!    See License_ROMS.txt                                              !
!=======================================================================
!                                                                      !
!  This subroutine initialize computations for new time step of the    !
!  tangent linear 3D primitive variables.                              !
!                                                                      !
!  Both n-1 and n-2 time-step contributions of the  Adams/Bashforth    !
!  scheme are added here to u and v at time index "nnew", since the    !
!  right-hand-side  arrays ru and rv at  n-2  will be overwriten in    !
!  subsequent calls to routines within the 3D engine.                  !
!                                                                      !
!  It also computes the time  "n"  vertical viscosity and diffusion    !
!  contributions of the Crank-Nicholson implicit scheme because the    !
!  thicknesses "Hz" will be overwriten at the end of the  2D engine    !
!  (barotropic mode) computations.                                     !
!                                                                      !
!  The actual time step will be carried out in routines "step3d_uv"    !
!  and "step3d_t".                                                     !
!                                                                      !
!  BASIC STATE variables needed: AKt, AKv, Huon, Hvom, Hz, Tsrc, W,    !
!                                bustr, bvstr, ghats, srflx, sustr,    !
!                                svstr,  t, z_r, z_w                   !
!                                                                      !
!=======================================================================
!
      implicit none
!
      PRIVATE
      PUBLIC  :: tl_pre_step3d
!
      CONTAINS
!
!***********************************************************************
      SUBROUTINE tl_pre_step3d (ng, tile)
!***********************************************************************
!
      USE mod_param
# ifdef DIAGNOSTICS
!!    USE mod_diags
# endif
      USE mod_forces
      USE mod_grid
      USE mod_mixing
      USE mod_ocean
      USE mod_stepping
# ifdef ICE_MODEL
      USE mod_ice
# endif
!
!  Imported variable declarations.
!
      integer, intent(in) :: ng, tile
!
!  Local variable declarations.
!
# include "tile.h"
!
# ifdef PROFILE
      CALL wclock_on (ng, iTLM, 22, __LINE__, __FILE__)
# endif
      CALL tl_pre_step3d_tile (ng, tile,                                &
     &                         LBi, UBi, LBj, UBj,                      &
     &                         IminS, ImaxS, JminS, JmaxS,              &
     &                         nrhs(ng), nstp(ng), nnew(ng),            &
# ifdef MASKING
     &                         GRID(ng) % rmask,                        &
     &                         GRID(ng) % umask,                        &
     &                         GRID(ng) % vmask,                        &
#  if defined SOLAR_SOURCE && defined WET_DRY
     &                         GRID(ng) % rmask_wet,                    &
#  endif
# endif
     &                         GRID(ng) % pm,                           &
     &                         GRID(ng) % pn,                           &
     &                         GRID(ng) % Hz,                           &
     &                         GRID(ng) % tl_Hz,                        &
     &                         GRID(ng) % Huon,                         &
     &                         GRID(ng) % tl_Huon,                      &
     &                         GRID(ng) % Hvom,                         &
     &                         GRID(ng) % tl_Hvom,                      &
     &                         GRID(ng) % z_r,                          &
     &                         GRID(ng) % tl_z_r,                       &
     &                         GRID(ng) % z_w,                          &
     &                         GRID(ng) % tl_z_w,                       &
     &                         FORCES(ng) % tl_btflx,                   &
     &                         FORCES(ng) % tl_bustr,                   &
     &                         FORCES(ng) % tl_bvstr,                   &
     &                         FORCES(ng) % tl_stflx,                   &
     &                         FORCES(ng) % tl_sustr,                   &
     &                         FORCES(ng) % tl_svstr,                   &
# ifdef SOLAR_SOURCE
     &                         FORCES(ng) % srflx,                      &
# endif
     &                         MIXING(ng) % Akt,                        &
     &                         MIXING(ng) % tl_Akt,                     &
     &                         MIXING(ng) % Akv,                        &
     &                         MIXING(ng) % tl_Akv,                     &
# ifdef LMD_NONLOCAL_NOT_YET
     &                         MIXING(ng) % tl_ghats,                   &
# endif
     &                         OCEAN(ng) % W,                           &
     &                         OCEAN(ng) % tl_W,                        &
     &                         OCEAN(ng) % tl_ru,                       &
     &                         OCEAN(ng) % tl_rv,                       &
# ifdef DIAGNOSTICS_TS
!!   &                         DIAGS(ng) % DiaTwrk,                     &
# endif
# ifdef DIAGNOSTICS_UV
!!   &                         DIAGS(ng) % DiaU3wrk,                    &
!!   &                         DIAGS(ng) % DiaV3wrk,                    &
!!   &                         DIAGS(ng) % DiaRU,                       &
!!   &                         DIAGS(ng) % DiaRV,                       &
# endif
# ifdef ICE_MODEL
     &                         ICE(ng) % rfaci,                         &
# endif
     &                         OCEAN(ng) % t,                           &
     &                         OCEAN(ng) % tl_t,                        &
     &                         OCEAN(ng) % u,                           &
     &                         OCEAN(ng) % tl_u,                        &
     &                         OCEAN(ng) % v,                           &
     &                         OCEAN(ng) % tl_v)
# ifdef PROFILE
      CALL wclock_off (ng, iTLM, 22, __LINE__, __FILE__)
# endif

      RETURN
      END SUBROUTINE tl_pre_step3d
!
!***********************************************************************
      SUBROUTINE tl_pre_step3d_tile (ng, tile,                          &
     &                               LBi, UBi, LBj, UBj,                &
     &                               IminS, ImaxS, JminS, JmaxS,        &
     &                               nrhs, nstp, nnew,                  &
# ifdef MASKING
     &                               rmask, umask, vmask,               &
#  if defined SOLAR_SOURCE && defined WET_DRY
     &                               rmask_wet,                         &
#  endif
# endif
     &                               pm, pn,                            &
     &                               Hz, tl_Hz,                         &
     &                               Huon, tl_Huon,                     &
     &                               Hvom, tl_Hvom,                     &
     &                               z_r, tl_z_r,                       &
     &                               z_w, tl_z_w,                       &
     &                               tl_btflx, tl_bustr, tl_bvstr,      &
     &                               tl_stflx, tl_sustr, tl_svstr,      &
# ifdef SOLAR_SOURCE
     &                               srflx,                             &
# endif
     &                               Akt, tl_Akt,                       &
     &                               Akv, tl_Akv,                       &
# ifdef LMD_NONLOCAL_NOT_YET
     &                               tl_ghats,                          &
# endif
     &                               W, tl_W,                           &
     &                               tl_ru, tl_rv,                      &
# ifdef DIAGNOSTICS_TS
!!   &                               DiaTwrk,                           &
# endif
# ifdef DIAGNOSTICS_UV
!!   &                               DiaU3wrk, DiaV3wrk,                &
!!   &                               DiaRU, DiaRV,                      &
# endif
# ifdef ICE_MODEL
     &                               rfaci,                             &
# endif
     &                               t, tl_t,                           &
     &                               u, tl_u,                           &
     &                               v, tl_v)
!***********************************************************************
!
      USE mod_param
      USE mod_scalars
      USE mod_sources
!
      USE exchange_3d_mod, ONLY : exchange_r3d_tile
# ifdef DISTRIBUTE
      USE mp_exchange_mod, ONLY : mp_exchange4d
# endif
      USE tl_t3dbc_mod, ONLY : tl_t3dbc_tile
!
!  Imported variable declarations.
!
      integer, intent(in) :: ng, tile
      integer, intent(in) :: LBi, UBi, LBj, UBj
      integer, intent(in) :: IminS, ImaxS, JminS, JmaxS
      integer, intent(in) :: nrhs, nstp, nnew
!
# ifdef ASSUMED_SHAPE
#  ifdef MASKING
      real(r8), intent(in) :: rmask(LBi:,LBj:)
      real(r8), intent(in) :: umask(LBi:,LBj:)
      real(r8), intent(in) :: vmask(LBi:,LBj:)
#   if defined SOLAR_SOURCE && defined WET_DRY
      real(r8), intent(in) :: rmask_wet(LBi:,LBj:)
#   endif
#  endif
      real(r8), intent(in) :: pm(LBi:,LBj:)
      real(r8), intent(in) :: pn(LBi:,LBj:)
      real(r8), intent(in) :: Hz(LBi:,LBj:,:)
      real(r8), intent(in) :: Huon(LBi:,LBj:,:)
      real(r8), intent(in) :: Hvom(LBi:,LBj:,:)
      real(r8), intent(in) :: z_r(LBi:,LBj:,:)
      real(r8), intent(in) :: z_w(LBi:,LBj:,0:)
#  ifdef SOLAR_SOURCE
      real(r8), intent(in) :: srflx(LBi:,LBj:)
#  endif
#  ifdef SUN
      real(r8), intent(in) :: Akt(LBi:UBi,LBj:UBj,0:N(ng),NAT)
#  else
      real(r8), intent(in) :: Akt(LBi:,LBj:,0:,:)
#  endif
      real(r8), intent(in) :: Akv(LBi:,LBj:,0:)
      real(r8), intent(in) :: W(LBi:,LBj:,0:)
#  ifdef ICE_MODEL
      real(r8), intent(in) :: rfaci(LBi:,LBj:)
#  endif
#  ifdef SUN
      real(r8), intent(in) :: t(LBi:UBi,LBj:UBj,N(ng),3,NT(ng))
#  else
      real(r8), intent(in) :: t(LBi:,LBj:,:,:,:)
#  endif
      real(r8), intent(in) :: u(LBi:,LBj:,:,:)
      real(r8), intent(in) :: v(LBi:,LBj:,:,:)

      real(r8), intent(in) :: tl_Hz(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_Huon(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_Hvom(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_z_r(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_z_w(LBi:,LBj:,0:)
      real(r8), intent(in) :: tl_btflx(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_bustr(LBi:,LBj:)
      real(r8), intent(in) :: tl_bvstr(LBi:,LBj:)
      real(r8), intent(in) :: tl_stflx(LBi:,LBj:,:)
      real(r8), intent(in) :: tl_sustr(LBi:,LBj:)
      real(r8), intent(in) :: tl_svstr(LBi:,LBj:)
      real(r8), intent(in) :: tl_ru(LBi:,LBj:,0:,:)
      real(r8), intent(in) :: tl_rv(LBi:,LBj:,0:,:)
#  ifdef LMD_NONLOCAL_NOT_YET
      real(r8), intent(in) :: tl_ghats(LBi:,LBj:,0:,:)
#  endif
#  ifdef SUN
      real(r8), intent(in) :: tl_Akt(LBi:UBi,LBj:UBj,0:N(ng),NAT)
#  else
      real(r8), intent(in) :: tl_Akt(LBi:,LBj:,0:,:)
#  endif
      real(r8), intent(in) :: tl_Akv(LBi:,LBj:,0:)
      real(r8), intent(in) :: tl_W(LBi:,LBj:,0:)

#  ifdef DIAGNOSTICS_TS
!!    real(r8), intent(inout) :: DiaTwrk(LBi:,LBj:,:,:,:)
#  endif
#  ifdef DIAGNOSTICS_UV
!!    real(r8), intent(inout) :: DiaU3wrk(LBi:,LBj:,:,:)
!!    real(r8), intent(inout) :: DiaV3wrk(LBi:,LBj:,:,:)
!!    real(r8), intent(inout) :: DiaRU(LBi:,LBj:,:,:,:)
!!    real(r8), intent(inout) :: DiaRV(LBi:,LBj:,:,:,:)
#  endif
#  ifdef SUN
      real(r8), intent(inout) :: tl_t(LBi:UBi,LBj:UBj,N(ng),3,NT(ng))
#  else
      real(r8), intent(inout) :: tl_t(LBi:,LBj:,:,:,:)
#  endif
      real(r8), intent(inout) :: tl_u(LBi:,LBj:,:,:)
      real(r8), intent(inout) :: tl_v(LBi:,LBj:,:,:)

# else

#  ifdef MASKING
      real(r8), intent(in) :: rmask(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: umask(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: vmask(LBi:UBi,LBj:UBj)
#   if defined SOLAR_SOURCE && defined WET_DRY
      real(r8), intent(in) :: rmask_wet(LBi:UBi,LBj:UBj)
#   endif
#  endif
      real(r8), intent(in) :: pm(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: pn(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: Hz(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: Huon(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: Hvom(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: z_r(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: z_w(LBi:UBi,LBj:UBj,0:N(ng))
#  ifdef SOLAR_SOURCE
      real(r8), intent(in) :: srflx(LBi:UBi,LBj:UBj)
#  endif
      real(r8), intent(in) :: Akt(LBi:UBi,LBj:UBj,0:N(ng),NAT)
      real(r8), intent(in) :: Akv(LBi:UBi,LBj:UBj,0:N(ng))
      real(r8), intent(in) :: W(LBi:UBi,LBj:UBj,0:N(ng))
#  ifdef ICE_MODEL
      real(r8), intent(in) :: rfaci(LBi:UBi,LBj:UBj)
#  endif
      real(r8), intent(in) :: t(LBi:UBi,LBj:UBj,N(ng),3,NT(ng))
      real(r8), intent(in) :: u(LBi:UBi,LBj:UBj,N(ng),2)
      real(r8), intent(in) :: v(LBi:UBi,LBj:UBj,N(ng),2)

      real(r8), intent(in) :: tl_Hz(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: tl_Huon(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: tl_Hvom(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: tl_z_r(LBi:UBi,LBj:UBj,N(ng))
      real(r8), intent(in) :: tl_z_w(LBi:UBi,LBj:UBj,0:N(ng))
      real(r8), intent(in) :: tl_btflx(LBi:UBi,LBj:UBj,NT(ng))
      real(r8), intent(in) :: tl_bustr(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: tl_bvstr(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: tl_stflx(LBi:UBi,LBj:UBj,NT(ng))
      real(r8), intent(in) :: tl_sustr(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: tl_svstr(LBi:UBi,LBj:UBj)
      real(r8), intent(in) :: tl_ru(LBi:UBi,LBj:UBj,0:N(ng),2)
      real(r8), intent(in) :: tl_rv(LBi:UBi,LBj:UBj,0:N(ng),2)
#  ifdef LMD_NONLOCAL_NOT_YET
      real(r8), intent(in) :: ghats(LBi:UBi,LBj:UBj,0:N(ng),NAT)
#  endif
      real(r8), intent(in) :: tl_Akt(LBi:UBi,LBj:UBj,0:N(ng),NAT)
      real(r8), intent(in) :: tl_Akv(LBi:UBi,LBj:UBj,0:N(ng))
      real(r8), intent(in) :: tl_W(LBi:UBi,LBj:UBj,0:N(ng))

#  ifdef DIAGNOSTICS_TS
!!    real(r8), intent(inout) :: DiaTwrk(LBi:UBi,LBj:UBj,N(ng),NT(ng),  &
!!   &                                   NDT)
#  endif
#  ifdef DIAGNOSTICS_UV
!!    real(r8), intent(inout) :: DiaU3wrk(LBi:UBi,LBj:UBj,N(ng),NDM3d)
!!    real(r8), intent(inout) :: DiaV3wrk(LBi:UBi,LBj:UBj,N(ng),NDM3d)
!!    real(r8), intent(inout) :: DiaRU(LBi:UBi,LBj:UBj,N(ng),2,NDrhs)
!!    real(r8), intent(inout) :: DiaRV(LBi:UBi,LBj:UBj,N(ng),2,NDrhs)
#  endif
      real(r8), intent(inout) :: tl_t(LBi:UBi,LBj:UBj,N(ng),3,NT(ng))
      real(r8), intent(inout) :: tl_u(LBi:UBi,LBj:UBj,N(ng),2)
      real(r8), intent(inout) :: tl_v(LBi:UBi,LBj:UBj,N(ng),2)
# endif
!
!  Local variable declarations.
!
      integer :: Isrc, Jsrc
      integer :: i, ic, indx, is, itrc, j, k, ltrc
# if defined AGE_MEAN && defined T_PASSIVE
      integer :: iage
# endif
# if defined DIAGNOSTICS_TS || defined DIAGNOSTICS_UV
      integer :: idiag
# endif
      real(r8), parameter :: eps = 1.0E-16_r8

      real(r8) :: cff, cff1, cff2, cff3, cff4
      real(r8) :: tl_cff, tl_cff1, tl_cff2, tl_cff3, tl_cff4
      real(r8) :: Gamma

      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: CF
      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: DC
      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: FC

      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: tl_CF
      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: tl_DC
      real(r8), dimension(IminS:ImaxS,0:N(ng)) :: tl_FC

# ifdef SOLAR_SOURCE
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS,0:N(ng)) :: tl_swdk
# endif

      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: FE
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: FX
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: curv
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: grad

      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: tl_FE
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: tl_FX
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: tl_curv
      real(r8), dimension(IminS:ImaxS,JminS:JmaxS) :: tl_grad

# include "set_bounds.h"

# ifndef TS_FIXED
!
!=======================================================================
!  Tracer equation(s).
!=======================================================================

#  ifdef SOLAR_SOURCE
!
!  Compute fraction of the solar shortwave radiation, "swdk"
!  (at vertical W-points) penetrating water column.
!
      DO k=1,N(ng)-1
        DO j=Jstr,Jend
          DO i=Istr,Iend
            FX(i,j)=z_w(i,j,N(ng))-z_w(i,j,k)
            tl_FX(i,j)=tl_z_w(i,j,N(ng))-tl_z_w(i,j,k)
          END DO
        END DO
!>      CALL lmd_swfrac_tile (ng, tile,                                 &
!>   &                        LBi, UBi, LBj, UBj,                       &
!>   &                        IminS, ImaxS, JminS, JmaxS,               &
!>   &                        -1.0_r8, FX, FE)
!>
        CALL tl_lmd_swfrac_tile (ng, tile,                              &
     &                           LBi, UBi, LBj, UBj,                    &
     &                           IminS, ImaxS, JminS, JmaxS,            &
# ifdef ICE_MODEL
     &                           rfaci,                                 &
# endif
     &                           -1.0_r8, FX, tl_FX, tl_FE)
        DO j=Jstr,Jend
          DO i=Istr,Iend
!>          swdk(i,j,k)=FE(i,j)
!>
            tl_swdk(i,j,k)=tl_FE(i,j)
          END DO
        END DO
      END DO
#  endif
!
!-----------------------------------------------------------------------
!  Compute intermediate tracer at n+1/2 time-step, t(i,j,k,3,itrc).
!-----------------------------------------------------------------------
!
!  Compute time rate of change of intermediate tracer due to
!  horizontal advection.
!
      T_LOOP1 : DO itrc=1,NT(ng)
        K_LOOP : DO k=1,N(ng)
!
          HADV_FLUX : IF (tl_Hadvection(itrc,ng)%CENTERED2) THEN
!
!  Second-order, centered differences horizontal advective fluxes.
!
            DO j=Jstr,Jend
              DO i=Istr,Iend+1
!>              FX(i,j)=Huon(i,j,k)*                                    &
!>   &                  0.5_r8*(t(i-1,j,k,nstp,itrc)+                   &
!>   &                          t(i  ,j,k,nstp,itrc))
!>
                tl_FX(i,j)=0.5_r8*                                      &
     &                     (tl_Huon(i,j,k)*                             &
     &                      (t(i-1,j,k,nstp,itrc)+                      &
     &                       t(i  ,j,k,nstp,itrc))+                     &
     &                      Huon(i,j,k)*                                &
     &                      (tl_t(i-1,j,k,nstp,itrc)+                   &
     &                       tl_t(i  ,j,k,nstp,itrc)))
              END DO
            END DO
            DO j=Jstr,Jend+1
              DO i=Istr,Iend
!>              FE(i,j)=Hvom(i,j,k)*                                    &
!>   &                  0.5_r8*(t(i,j-1,k,nstp,itrc)+                   &
!>   &                          t(i,j  ,k,nstp,itrc))
!>
                tl_FE(i,j)=0.5_r8*                                      &
     &                     (tl_Hvom(i,j,k)*                             &
     &                      (t(i,j-1,k,nstp,itrc)+                      &
     &                       t(i,j  ,k,nstp,itrc))+                     &
     &                      Hvom(i,j,k)*                                &
     &                      (tl_t(i,j-1,k,nstp,itrc)+                   &
     &                       tl_t(i,j  ,k,nstp,itrc)))
              END DO
            END DO
!
          ELSE IF ((tl_Hadvection(itrc,ng)%MPDATA).or.                  &
     &             (tl_Hadvection(itrc,ng)%HSIMT)) THEN
!
!  First-order, upstream differences horizontal advective fluxes.
!
            DO j=Jstr,Jend
              DO i=Istr,Iend+1
                cff1=MAX(Huon(i,j,k),0.0_r8)
                cff2=MIN(Huon(i,j,k),0.0_r8)
                tl_cff1=(0.5_r8+SIGN(0.5_r8, Huon(i,j,k)))*             &
     &                  tl_Huon(i,j,k)
                tl_cff2=(0.5_r8+SIGN(0.5_r8,-Huon(i,j,k)))*             &
     &                  tl_Huon(i,j,k)
!>              FX(i,j)=cff1*t(i-1,j,k,nstp,itrc)+                      &
!>   &                  cff2*t(i  ,j,k,nstp,itrc)
!>
                tl_FX(i,j)=tl_cff1*t(i-1,j,k,nstp,itrc)+                &
     &                     cff1*tl_t(i-1,j,k,nstp,itrc)+                &
     &                     tl_cff2*t(i  ,j,k,nstp,itrc)+                &
     &                     cff2*tl_t(i  ,j,k,nstp,itrc)
              END DO
            END DO
            DO j=Jstr,Jend+1
              DO i=Istr,Iend
                cff1=MAX(Hvom(i,j,k),0.0_r8)
                cff2=MIN(Hvom(i,j,k),0.0_r8)
                tl_cff1=(0.5_r8+SIGN(0.5_r8, Hvom(i,j,k)))*             &
     &                  tl_Hvom(i,j,k)
                tl_cff2=(0.5_r8+SIGN(0.5_r8,-Hvom(i,j,k)))*             &
     &                  tl_Hvom(i,j,k)
!>              FE(i,j)=cff1*t(i,j-1,k,nstp,itrc)+                      &
!>   &                  cff2*t(i,j  ,k,nstp,itrc)
!>
                tl_FE(i,j)=tl_cff1*t(i,j-1,k,nstp,itrc)+                &
     &                     cff1*tl_t(i,j-1,k,nstp,itrc)+                &
     &                     tl_cff2*t(i,j  ,k,nstp,itrc)+                &
     &                     cff2*tl_t(i,j  ,k,nstp,itrc)
              END DO
            END DO
!
          ELSE IF ((tl_Hadvection(itrc,ng)%AKIMA4).or.                  &
     &             (tl_Hadvection(itrc,ng)%CENTERED4).or.               &
     &             (tl_Hadvection(itrc,ng)%SPLIT_U3).or.                &
     &             (tl_Hadvection(itrc,ng)%UPSTREAM3)) THEN
!
!  Fourth-order Akima, fourth-order centered differences, or third-order
!  upstream-biased horizontal advective fluxes.
!
            DO j=Jstr,Jend
              DO i=Istrm1,Iendp2
                FX(i,j)=t(i  ,j,k,nstp,itrc)-                           &
     &                  t(i-1,j,k,nstp,itrc)
                tl_FX(i,j)=tl_t(i  ,j,k,nstp,itrc)-                     &
     &                     tl_t(i-1,j,k,nstp,itrc)
#  ifdef MASKING
                FX(i,j)=FX(i,j)*umask(i,j)
                tl_FX(i,j)=tl_FX(i,j)*umask(i,j)
#  endif
              END DO
            END DO
            IF (.not.(CompositeGrid(iwest,ng).or.EWperiodic(ng))) THEN
              IF (DOMAIN(ng)%Western_Edge(tile)) THEN
                DO j=Jstr,Jend
                  FX(Istr-1,j)=FX(Istr,j)
                  tl_FX(Istr-1,j)=tl_FX(Istr,j)
                END DO
              END IF
            END IF
            IF (.not.(CompositeGrid(ieast,ng).or.EWperiodic(ng))) THEN
              IF (DOMAIN(ng)%Eastern_Edge(tile)) THEN
                DO j=Jstr,Jend
                  FX(Iend+2,j)=FX(Iend+1,j)
                  tl_FX(Iend+2,j)=tl_FX(Iend+1,j)
                END DO
              END IF
            END IF
!
            DO j=Jstr,Jend
              DO i=Istr-1,Iend+1
                IF (tl_Hadvection(itrc,ng)%UPSTREAM3) THEN
                  curv(i,j)=FX(i+1,j)-FX(i,j)
                  tl_curv(i,j)=tl_FX(i+1,j)-tl_FX(i,j)
                ELSE IF (tl_Hadvection(itrc,ng)%AKIMA4) THEN
                  cff=2.0_r8*FX(i+1,j)*FX(i,j)
                  tl_cff=2.0_r8*(tl_FX(i+1,j)*FX(i,j)+                  &
     &                           FX(i+1,j)*tl_FX(i,j))
                  IF (cff.gt.eps) THEN
                    grad(i,j)=cff/(FX(i+1,j)+FX(i,j))
                    tl_grad(i,j)=((FX(i+1,j)+FX(i,j))*tl_cff-           &
     &                            cff*(tl_FX(i+1,j)+tl_FX(i,j)))/       &
     &                           ((FX(i+1,j)+FX(i,j))*                  &
     &                            (FX(i+1,j)+FX(i,j)))
                  ELSE
                    grad(i,j)=0.0_r8
                    tl_grad(i,j)=0.0_r8
                  END IF
                ELSE IF ((tl_Hadvection(itrc,ng)%CENTERED4).or.         &
     &                   (tl_Hadvection(itrc,ng)%SPLIT_U3)) THEN
                  grad(i,j)=0.5_r8*(FX(i+1,j)+FX(i,j))
                  tl_grad(i,j)=0.5_r8*(tl_FX(i+1,j)+tl_FX(i,j))
                END IF
              END DO
            END DO
!
            cff1=1.0_r8/6.0_r8
            cff2=1.0_r8/3.0_r8
            DO j=Jstr,Jend
              DO i=Istr,Iend+1
                IF (tl_Hadvection(itrc,ng)%UPSTREAM3) THEN
!>                FX(i,j)=Huon(i,j,k)*0.5_r8*                           &
!>   &                    (t(i-1,j,k,nstp,itrc)+                        &
!>   &                     t(i  ,j,k,nstp,itrc))-                       &
!>   &                    cff1*(curv(i-1,j)*MAX(Huon(i,j,k),0.0_r8)+    &
!>   &                          curv(i  ,j)*MIN(Huon(i,j,k),0.0_r8))
!>
                  tl_FX(i,j)=0.5_r8*                                    &
     &                       (tl_Huon(i,j,k)*                           &
     &                        (t(i-1,j,k,nstp,itrc)+                    &
     &                         t(i  ,j,k,nstp,itrc))+                   &
     &                        Huon(i,j,k)*                              &
     &                        (tl_t(i-1,j,k,nstp,itrc)+                 &
     &                         tl_t(i  ,j,k,nstp,itrc)))-               &
     &                       cff1*                                      &
     &                       (tl_curv(i-1,j)*MAX(Huon(i,j,k),0.0_r8)+   &
     &                        curv(i-1,j)*                              &
     &                        (0.5_r8+SIGN(0.5_r8, Huon(i,j,k)))*       &
     &                        tl_Huon(i,j,k)+                           &
     &                        tl_curv(i  ,j)*MIN(Huon(i,j,k),0.0_r8)+   &
     &                        curv(i  ,j)*                              &
     &                        (0.5_r8+SIGN(0.5_r8,-Huon(i,j,k)))*       &
     &                        tl_Huon(i,j,k))
                ELSE IF ((tl_Hadvection(itrc,ng)%AKIMA4).or.            &
     &                   (tl_Hadvection(itrc,ng)%CENTERED4).or.         &
     &                   (tl_Hadvection(itrc,ng)%SPLIT_U3)) THEN
!>                FX(i,j)=Huon(i,j,k)*0.5_r8*                           &
!>   &                    (t(i-1,j,k,nstp,itrc)+                        &
!>   &                     t(i  ,j,k,nstp,itrc)-                        &
!>   &                     cff2*(grad(i  ,j)-                           &
!>   &                           grad(i-1,j)))
!>
                  tl_FX(i,j)=0.5_r8*                                    &
     &                       (tl_Huon(i,j,k)*                           &
     &                        (t(i-1,j,k,nstp,itrc)+                    &
     &                         t(i  ,j,k,nstp,itrc)-                    &
     &                         cff2*(grad(i  ,j)-                       &
     &                               grad(i-1,j)))+                     &
     &                        Huon(i,j,k)*                              &
     &                        (tl_t(i-1,j,k,nstp,itrc)+                 &
     &                         tl_t(i  ,j,k,nstp,itrc)-                 &
     &                        cff2*(tl_grad(i  ,j)-                     &
     &                              tl_grad(i-1,j))))
                END IF
              END DO
            END DO
!
            DO j=Jstrm1,Jendp2
              DO i=Istr,Iend
                FE(i,j)=t(i,j  ,k,nstp,itrc)-                           &
     &                  t(i,j-1,k,nstp,itrc)
                tl_FE(i,j)=tl_t(i,j  ,k,nstp,itrc)-                     &
     &                     tl_t(i,j-1,k,nstp,itrc)
#  ifdef MASKING
                FE(i,j)=FE(i,j)*vmask(i,j)
                tl_FE(i,j)=tl_FE(i,j)*vmask(i,j)
#  endif
              END DO
            END DO
            IF (.not.(CompositeGrid(isouth,ng).or.NSperiodic(ng))) THEN
              IF (DOMAIN(ng)%Southern_Edge(tile)) THEN
                DO i=Istr,Iend
                  FE(i,Jstr-1)=FE(i,Jstr)
                  tl_FE(i,Jstr-1)=tl_FE(i,Jstr)
                END DO
              END IF
            END IF
            IF (.not.(CompositeGrid(inorth,ng).or.NSperiodic(ng))) THEN
              IF (DOMAIN(ng)%Northern_Edge(tile)) THEN
                DO i=Istr,Iend
                  FE(i,Jend+2)=FE(i,Jend+1)
                  tl_FE(i,Jend+2)=tl_FE(i,Jend+1)
                END DO
              END IF
            END IF
!
            DO j=Jstr-1,Jend+1
              DO i=Istr,Iend
                IF (tl_Hadvection(itrc,ng)%UPSTREAM3) THEN
                  curv(i,j)=FE(i,j+1)-FE(i,j)
                  tl_curv(i,j)=tl_FE(i,j+1)-tl_FE(i,j)
                ELSE IF (tl_Hadvection(itrc,ng)%AKIMA4) THEN
                  cff=2.0_r8*FE(i,j+1)*FE(i,j)
                  tl_cff=2.0_r8*(tl_FE(i,j+1)*FE(i,j)+                  &
     &                           FE(i,j+1)*tl_FE(i,j))
                  IF (cff.gt.eps) THEN
                    grad(i,j)=cff/(FE(i,j+1)+FE(i,j))
                    tl_grad(i,j)=((FE(i,j+1)+FE(i,j))*tl_cff-           &
     &                            cff*(tl_FE(i,j+1)+tl_FE(i,j)))/       &
     &                           ((FE(i,j+1)+FE(i,j))*                  &
     &                            (FE(i,j+1)+FE(i,j)))
                  ELSE
                    grad(i,j)=0.0_r8
                    tl_grad(i,j)=0.0_r8
                  END IF
                ELSE IF ((tl_Hadvection(itrc,ng)%CENTERED4).or.         &
     &                   (tl_Hadvection(itrc,ng)%SPLIT_U3)) THEN
                  grad(i,j)=0.5_r8*(FE(i,j+1)+FE(i,j))
                  tl_grad(i,j)=0.5_r8*(tl_FE(i,j+1)+tl_FE(i,j))
                END IF
              END DO
            END DO
!
            cff1=1.0_r8/6.0_r8
            cff2=1.0_r8/3.0_r8
            DO j=Jstr,Jend+1
              DO i=Istr,Iend
                IF (tl_Hadvection(itrc,ng)%UPSTREAM3) THEN
!>                FE(i,j)=Hvom(i,j,k)*0.5_r8*                           &
!>   &                    (t(i,j-1,k,nstp,itrc)+                        &
!>   &                     t(i,j  ,k,nstp,itrc))-                       &
!>   &                    cff1*(curv(i,j-1)*MAX(Hvom(i,j,k),0.0_r8)+    &
!>   &                          curv(i,j  )*MIN(Hvom(i,j,k),0.0_r8))
!>
                  tl_FE(i,j)=0.5_r8*                                    &
     &                       (tl_Hvom(i,j,k)*                           &
     &                        (t(i,j-1,k,nstp,itrc)+                    &
     &                         t(i,j  ,k,nstp,itrc))+                   &
     &                        Hvom(i,j,k)*                              &
     &                        (tl_t(i,j-1,k,nstp,itrc)+                 &
     &                         tl_t(i,j  ,k,nstp,itrc)))-               &
     &                        cff1*                                     &
     &                        (tl_curv(i,j-1)*MAX(Hvom(i,j,k),0.0_r8)+  &
     &                         curv(i,j-1)*                             &
     &                         (0.5_r8+SIGN(0.5_r8, Hvom(i,j,k)))*      &
     &                         tl_Hvom(i,j,k)+                          &
     &                         tl_curv(i,j  )*MIN(Hvom(i,j,k),0.0_r8)+  &
     &                         curv(i,j  )*                             &
     &                         (0.5_r8+SIGN(0.5_r8,-Hvom(i,j,k)))*      &
     &                         tl_Hvom(i,j,k))
                ELSE IF ((tl_Hadvection(itrc,ng)%AKIMA4).or.            &
     &                   (tl_Hadvection(itrc,ng)%CENTERED4).or.         &
     &                   (tl_Hadvection(itrc,ng)%SPLIT_U3)) THEN
!>                FE(i,j)=Hvom(i,j,k)*0.5_r8*                           &
!>   &                    (t(i,j-1,k,nstp,itrc)+                        &
!>   &                     t(i,j  ,k,nstp,itrc)-                        &
!>   &                     cff2*(grad(i,j  )-                           &
!>   &                           grad(i,j-1)))
!>
                  tl_FE(i,j)=0.5_r8*                                    &
     &                       (tl_Hvom(i,j,k)*                           &
     &                        (t(i,j-1,k,nstp,itrc)+                    &
     &                         t(i,j  ,k,nstp,itrc)-                    &
     &                         cff2*(grad(i,j  )-                       &
     &                               grad(i,j-1)))+                     &
     &                        Hvom(i,j,k)*                              &
     &                        (tl_t(i,j-1,k,nstp,itrc)+                 &
     &                         tl_t(i,j  ,k,nstp,itrc)-                 &
     &                         cff2*(tl_grad(i,j  )-                    &
     &                               tl_grad(i,j-1))))
                END IF
              END DO
            END DO
          END IF HADV_FLUX
!
!  Apply tracers point sources to the horizontal advection terms,
!  if any.
!
          IF (LuvSrc(ng)) THEN
            DO is=1,Nsrc(ng)
              Isrc=SOURCES(ng)%Isrc(is)
              Jsrc=SOURCES(ng)%Jsrc(is)
              IF (((Istr.le.Isrc).and.(Isrc.le.Iend+1)).and.            &
     &            ((Jstr.le.Jsrc).and.(Jsrc.le.Jend+1))) THEN
                IF (INT(SOURCES(ng)%Dsrc(is)).eq.0) THEN
                  IF (LtracerSrc(itrc,ng)) THEN
!>                  FX(Isrc,Jsrc)=Huon(Isrc,Jsrc,k)*                    &
!>   &                            SOURCES(ng)%Tsrc(is,k,itrc)
!>
                    tl_FX(Isrc,Jsrc)=tl_Huon(Isrc,Jsrc,k)*              &
     &                               SOURCES(ng)%Tsrc(is,k,itrc)
                  ELSE
!>                  FX(Isrc,Jsrc)=0.0_r8
!>
                    tl_FX(Isrc,Jsrc)=0.0_r8
                  END IF
                ELSE
                  IF (LtracerSrc(itrc,ng)) THEN
!>                  FE(Isrc,Jsrc)=Hvom(Isrc,Jsrc,k)*                    &
!>   &                            SOURCES(ng)%Tsrc(is,k,itrc)
!>
                    tl_FE(Isrc,Jsrc)=tl_Hvom(Isrc,Jsrc,k)*              &
     &                               SOURCES(ng)%Tsrc(is,k,itrc)
                  ELSE
!>                  FE(Isrc,Jsrc)=0.0_r8
!>
                    tl_FE(Isrc,Jsrc)=0.0_r8
                  END IF
                END IF
              END IF
            END DO
          END IF
!
!  Time-step horizontal advection (m Tunits).
!
          IF ((tl_Hadvection(itrc,ng)%MPDATA).or.                       &
     &        (tl_Hadvection(itrc,ng)%HSIMT)) THEN
            Gamma=0.5_r8
          ELSE
            Gamma=1.0_r8/6.0_r8
          END IF
#  if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
          IF (iic(ng).eq.ntfirst(ng).and.SOinitial(ng)) THEN
#  else
          IF (iic(ng).eq.ntfirst(ng)) THEN
#  endif
            cff=0.5_r8*dt(ng)
            cff1=1.0_r8
            cff2=0.0_r8
          ELSE
            cff=(1.0_r8-Gamma)*dt(ng)
            cff1=0.5_r8+Gamma
            cff2=0.5_r8-Gamma
          END IF
          DO j=Jstr,Jend
            DO i=Istr,Iend
!>            t(i,j,k,3,itrc)=Hz(i,j,k)*(cff1*t(i,j,k,nstp,itrc)+       &
!>   &                                   cff2*t(i,j,k,nnew,itrc))-      &
!>   &                        cff*pm(i,j)*pn(i,j)*                      &
!>   &                        (FX(i+1,j)-FX(i,j)+                       &
!>   &                         FE(i,j+1)-FE(i,j))
!>
              tl_t(i,j,k,3,itrc)=tl_Hz(i,j,k)*                          &
     &                           (cff1*t(i,j,k,nstp,itrc)+              &
     &                            cff2*t(i,j,k,nnew,itrc))+             &
     &                           Hz(i,j,k)*                             &
     &                           (cff1*tl_t(i,j,k,nstp,itrc)+           &
     &                            cff2*tl_t(i,j,k,nnew,itrc))-          &
     &                           cff*pm(i,j)*pn(i,j)*                   &
     &                           (tl_FX(i+1,j)-tl_FX(i,j)+              &
     &                            tl_FE(i,j+1)-tl_FE(i,j))
            END DO
          END DO
        END DO K_LOOP
      END DO T_LOOP1

#  if defined AGE_MEAN && defined T_PASSIVE
!
!  If inert passive tracer and Mean Age, compute age concentration (even
!  inert index) forced by the right-hand-side term that is concentration
!  of an associated conservative passive tracer (odd inert index).
!  Implemented and tested by W.G. Zhang and J. Wilkin. (m Tunits)
!
      DO itrc=1,NPT,2
        IF (.not.((tl_Hadvection(inert(itrc),ng)%MPDATA).or.            &
     &            (tl_Hadvection(inert(itrc),ng)%HSIMT))) THEN
          IF (iic(ng).eq.ntfirst(ng)) THEN
            cff=0.5_r8*dt(ng)
          ELSE
            Gamma=1.0_r8/6.0_r8
            cff=(1.0_r8-Gamma)*dt(ng)
          END IF
          iage=inert(itrc+1)                   ! even inert tracer index
          DO k=1,N(ng)
            DO j=Jstr,Jend
              DO i=Istr,Iend
!>              t(i,j,k,3,iage)=t(i,j,k,3,iage)+                        &
!>   &                          cff*Hz(i,j,k)*                          &
!>   &                          t(i,j,k,nnew,inert(itrc))
!>
                tl_t(i,j,k,3,iage)=tl_t(i,j,k,3,iage)+                  &
     &                             cff*                                 &
     &                             (Hz(i,j,k)*                          &
     &                              tl_t(i,j,k,nnew,inert(itrc))+       &
     &                              tl_Hz(i,j,k)*                       &
     &                              t(i,j,k,nnew,inert(itrc)))
              END DO
            END DO
          END DO
        END IF
      END DO
#  endif
!
!-----------------------------------------------------------------------
!  Compute time rate of change of intermediate tracer due to vertical
!  advection.
!-----------------------------------------------------------------------
!
      J_LOOP1 : DO j=Jstr,Jend
        T_LOOP2 : DO itrc=1,NT(ng)
!
          VADV_FLUX : IF (tl_Vadvection(itrc,ng)%SPLINES) THEN
!
!  Build conservative parabolic splines for the BASIC STATE vertical
!  derivatives "FC" of the tracer.
!
            DO i=Istr,Iend
#  ifdef NEUMANN
              FC(i,0)=1.5_r8*t(i,j,1,nstp,itrc)
              CF(i,1)=0.5_r8
#  else
              FC(i,0)=2.0_r8*t(i,j,1,nstp,itrc)
              CF(i,1)=1.0_r8
#  endif
            END DO
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                cff=1.0_r8/(2.0_r8*Hz(i,j,k)+                           &
     &                      Hz(i,j,k+1)*(2.0_r8-CF(i,k)))
                CF(i,k+1)=cff*Hz(i,j,k)
                FC(i,k)=cff*(3.0_r8*(Hz(i,j,k  )*t(i,j,k+1,nstp,itrc)+  &
     &                               Hz(i,j,k+1)*t(i,j,k  ,nstp,itrc))- &
     &                       Hz(i,j,k+1)*FC(i,k-1))
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef NEUMANN
              FC(i,N(ng))=(3.0_r8*t(i,j,N(ng),nstp,itrc)-               &
     &                     FC(i,N(ng)-1))/(2.0_r8-CF(i,N(ng)))
#  else
              FC(i,N(ng))=(2.0_r8*t(i,j,N(ng),nstp,itrc)-               &
     &                     FC(i,N(ng)-1))/(1.0_r8-CF(i,N(ng)))
#  endif
            END DO
            DO k=N(ng)-1,0,-1
              DO i=Istr,Iend
                FC(i,k)=FC(i,k)-CF(i,k+1)*FC(i,k+1)
              END DO
            END DO
!
!  Now the tangent linear spline code.
!
            DO i=Istr,Iend
#  ifdef NEUMANN
!>            FC(i,0)=1.5_r8*t(i,j,1,nstp,itrc)
!>
              tl_FC(i,0)=1.5_r8*tl_t(i,j,1,nstp,itrc)
              CF(i,1)=0.5_r8
#  else
!>            FC(i,0)=2.0_r8*t(i,j,1,nstp,itrc)
!>
              tl_FC(i,0)=2.0_r8*tl_t(i,j,1,nstp,itrc)
              CF(i,1)=1.0_r8
#  endif
            END DO
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                cff=1.0_r8/(2.0_r8*Hz(i,j,k)+                           &
     &                      Hz(i,j,k+1)*(2.0_r8-CF(i,k)))
                CF(i,k+1)=cff*Hz(i,j,k)
                tl_FC(i,k)=cff*                                         &
     &                     (3.0_r8*(Hz(i,j,k  )*                        &
     &                              tl_t(i,j,k+1,nstp,itrc)+            &
     &                              Hz(i,j,k+1)*                        &
     &                              tl_t(i,j,k  ,nstp,itrc)+            &
     &                              tl_Hz(i,j,k  )*                     &
     &                              t(i,j,k+1,nstp,itrc)+               &
     &                              tl_Hz(i,j,k+1)*                     &
     &                              t(i,j,k  ,nstp,itrc))-              &
     &                              (tl_Hz(i,j,k+1)*FC(i,k-1)+          &
     &                               2.0_r8*(tl_Hz(i,j,k  )+            &
     &                                       tl_Hz(i,j,k+1))*FC(i,k)+   &
     &                               tl_Hz(i,j,k  )*FC(i,k+1))-         &
     &                               Hz(i,j,k+1)*tl_FC(i,k-1))
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef NEUMANN
!>            FC(i,N(ng))=(3.0_r8*t(i,j,N(ng),nstp,itrc)-               &
!>   &                     FC(i,N(ng)-1))/(2.0_r8-CF(i,N(ng)))
!>
              tl_FC(i,N(ng))=(3.0_r8*tl_t(i,j,N(ng),nstp,itrc)-         &
     &                        tl_FC(i,N(ng)-1))/                        &
     &                       (2.0_r8-CF(i,N(ng)))
#  else
!>            FC(i,N(ng))=(2.0_r8*t(i,j,N(ng),nstp,itrc)-               &
!>   &                     FC(i,N(ng)-1))/(1.0_r8-CF(i,N(ng)))
!>
              tl_FC(i,N(ng))=(2.0_r8*tl_t(i,j,N(ng),nstp,itrc)-         &
     &                        tl_FC(i,N(ng)-1))/                        &
     &                       (1.0_r8-CF(i,N(ng)))
#  endif
            END DO
            DO k=N(ng)-1,0,-1
              DO i=Istr,Iend
!>              FC(i,k)=FC(i,k)-CF(i,k+1)*FC(i,k+1)
!>
                tl_FC(i,k)=tl_FC(i,k)-CF(i,k+1)*tl_FC(i,k+1)
!>              FC(i,k+1)=W(i,j,k+1)*FC(i,k+1)
!>
                tl_FC(i,k+1)=tl_W(i,j,k+1)*FC(i,k+1)+                   &
     &                       W(i,j,k+1)*tl_FC(i,k+1)
              END DO
            END DO
            DO i=Istr,Iend
!>            FC(i,N(ng))=0.0_r8
!>
              tl_FC(i,N(ng))=0.0_r8
!>            FC(i,0)=0.0_r8
!>
              tl_FC(i,0)=0.0_r8
            END DO
!
!  Now complete the computation of the flux array FC.
!
            DO k=N(ng)-1,0,-1
              DO i=Istr,Iend
                FC(i,k+1)=W(i,j,k+1)*FC(i,k+1)
              END DO
            END DO
            DO i=Istr,Iend
              FC(i,N(ng))=0.0_r8
              FC(i,0)=0.0_r8
            END DO
!
          ELSE IF (tl_Vadvection(itrc,ng)%AKIMA4) THEN
!
!  Fourth-order, Akima vertical advective flux.
!
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                FC(i,k)=t(i,j,k+1,nstp,itrc)-                           &
     &                  t(i,j,k  ,nstp,itrc)
                tl_FC(i,k)=tl_t(i,j,k+1,nstp,itrc)-                     &
     &                     tl_t(i,j,k  ,nstp,itrc)
              END DO
            END DO
            DO i=Istr,Iend
              FC(i,0)=FC(i,1)
              tl_FC(i,0)=tl_FC(i,1)
              FC(i,N(ng))=FC(i,N(ng)-1)
              tl_FC(i,N(ng))=tl_FC(i,N(ng)-1)
            END DO
            DO k=1,N(ng)
              DO i=Istr,Iend
                cff=2.0_r8*FC(i,k)*FC(i,k-1)
                tl_cff=2.0_r8*(tl_FC(i,k)*FC(i,k-1)+                    &
     &                         FC(i,k)*tl_FC(i,k-1))
                IF (cff.gt.eps) THEN
                  CF(i,k)=cff/(FC(i,k)+FC(i,k-1))
                  tl_CF(i,k)=((FC(i,k)+FC(i,k-1))*tl_cff-               &
     &                        cff*(tl_FC(i,k)+tl_FC(i,k-1)))/           &
     &                       ((FC(i,k)+FC(i,k-1))*(FC(i,k)+FC(i,k-1)))
                ELSE
                  CF(i,k)=0.0_r8
                  tl_CF(i,k)=0.0_r8
                END IF
              END DO
            END DO
            cff1=1.0_r8/3.0_r8
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                FC(i,k)=W(i,j,k)*                                       &
     &                  0.5_r8*(t(i,j,k  ,nstp,itrc)+                   &
     &                          t(i,j,k+1,nstp,itrc)-                   &
     &                          cff1*(CF(i,k+1)-CF(i,k)))
                tl_FC(i,k)=0.5_r8*                                      &
     &                     (tl_W(i,j,k)*                                &
     &                      (t(i,j,k  ,nstp,itrc)+                      &
     &                       t(i,j,k+1,nstp,itrc)-                      &
     &                       cff1*(CF(i,k+1)-CF(i,k)))+                 &
     &                      W(i,j,k)*                                   &
     &                      (tl_t(i,j,k  ,nstp,itrc)+                   &
     &                       tl_t(i,j,k+1,nstp,itrc)-                   &
     &                       cff1*(tl_CF(i,k+1)-tl_CF(i,k))))
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef SED_MORPH
              FC(i,0)=W(i,j,0)*t(i,j,1,nstp,itrc)
              tl_FC(i,0)=tl_W(i,j,0)*t(i,j,1,nstp,itrc)+                &
     &                   W(i,j,0)*tl_t(i,j,1,nstp,itrc)
#  else
              FC(i,0)=0.0_r8
              tl_FC(i,0)=0.0_r8
#  endif
              FC(i,N(ng))=0.0_r8
              tl_FC(i,N(ng))=0.0_r8
            END DO
!
          ELSE IF (tl_Vadvection(itrc,ng)%CENTERED2) THEN
!
!  Second-order, central differences vertical advective flux.
!
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                FC(i,k)=W(i,j,k)*                                       &
     &                  0.5_r8*(t(i,j,k  ,nstp,itrc)+                   &
     &                          t(i,j,k+1,nstp,itrc))
                tl_FC(i,k)=0.5_r8*                                      &
     &                     (tl_W(i,j,k)*                                &
     &                      (t(i,j,k  ,nstp,itrc)+                      &
     &                       t(i,j,k+1,nstp,itrc))+                     &
     &                      W(i,j,k)*                                   &
     &                      (tl_t(i,j,k  ,nstp,itrc)+                   &
     &                       tl_t(i,j,k+1,nstp,itrc)))
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef SED_MORPH
              FC(i,0)=W(i,j,0)*t(i,j,1,nstp,itrc)
              tl_FC(i,0)=tl_W(i,j,0)*t(i,j,1,nstp,itrc)                 &
     &                   W(i,j,0)*tl_t(i,j,1,nstp,itrc)
#  else
              FC(i,0)=0.0_r8
              tl_FC(i,0)=0.0_r8
#  endif
              FC(i,N(ng))=0.0_r8
              tl_FC(i,N(ng))=0.0_r8
            END DO
!
          ELSE IF ((tl_Vadvection(itrc,ng)%MPDATA).or.                  &
     &             (tl_Vadvection(itrc,ng)%HSIMT)) THEN
!
!  First_order, upstream differences vertical advective flux.
!
            DO k=1,N(ng)-1
              DO i=Istr,Iend
                cff1=MAX(W(i,j,k),0.0_r8)
                cff2=MIN(W(i,j,k),0.0_r8)
                tl_cff1=(0.5_r8+SIGN(0.5_r8, W(i,j,k)))*tl_W(i,j,k)
                tl_cff2=(0.5_r8+SIGN(0.5_r8,-W(i,j,k)))*tl_W(i,j,k)
!>              FC(i,k)=cff1*t(i,j,k  ,nstp,itrc)+                      &
!>   &                  cff2*t(i,j,k+1,nstp,itrc)
!>
                tl_FC(i,k)=tl_cff1*t(i,j,k  ,nstp,itrc)+                &
     &                     cff1*tl_t(i,j,k  ,nstp,itrc)+                &
     &                     tl_cff2*t(i,j,k+1,nstp,itrc)+                &
     &                     cff2*tl_t(i,j,k+1,nstp,itrc)
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef SED_MORPH
!>            FC(i,0)=W(i,j,0)*t(i,j,1,nstp,itrc)
!>
              tl_FC(i,0)=tl_W(i,j,0)*t(i,j,1,nstp,itrc)+                &
     &                   W(i,j,0)*tl_t(i,j,1,nstp,itrc)
   else
!>            FC(i,0)=0.0_r8
!>
              tl_FC(i,0)=0.0_r8
#  endif
!>            FC(i,N(ng))=0.0_r8
!>
              tl_FC(i,N(ng))=0.0_r8
            END DO
!
          ELSE IF ((tl_Vadvection(itrc,ng)%CENTERED4).or.               &
     &             (tl_Vadvection(itrc,ng)%SPLIT_U3)) THEN
!
!  Fourth-order, central differences vertical advective flux.
!
            cff1=0.5_r8
            cff2=7.0_r8/12.0_r8
            cff3=1.0_r8/12.0_r8
            DO k=2,N(ng)-2
              DO i=Istr,Iend
                FC(i,k)=W(i,j,k)*                                       &
     &                  (cff2*(t(i,j,k  ,nstp,itrc)+                    &
     &                         t(i,j,k+1,nstp,itrc))-                   &
     &                   cff3*(t(i,j,k-1,nstp,itrc)+                    &
     &                         t(i,j,k+2,nstp,itrc)))
                tl_FC(i,k)=tl_W(i,j,k)*                                 &
     &                     (cff2*(t(i,j,k  ,nstp,itrc)+                 &
     &                            t(i,j,k+1,nstp,itrc))-                &
     &                      cff3*(t(i,j,k-1,nstp,itrc)+                 &
     &                            t(i,j,k+2,nstp,itrc)))+               &
     &                     W(i,j,k)*                                    &
     &                     (cff2*(tl_t(i,j,k  ,nstp,itrc)+              &
     &                            tl_t(i,j,k+1,nstp,itrc))-             &
     &                      cff3*(tl_t(i,j,k-1,nstp,itrc)+              &
     &                            tl_t(i,j,k+2,nstp,itrc)))
              END DO
            END DO
            DO i=Istr,Iend
#  ifdef SED_MORPH
              FC(i,0)=W(i,j,0)*2.0_r8*                                  &
     &                (cff2*t(i,j,1,nstp,itrc)-                         &
     &                 cff3*t(i,j,2,nstp,itrc))
              tl_FC(i,0)=2.0_r8*(tl_W(i,j,0)*                           &
     &                           (cff2*t(i,j,1,nstp,itrc)-              &
     &                            cff3*t(i,j,2,nstp,itrc))+             &
     &                           W(i,j,0)*                              &
     &                           (cff2*tl_t(i,j,1,nstp,itrc)-           &
     &                            cff3*tl_t(i,j,2,nstp,itrc)))
#  else
              FC(i,0)=0.0_r8
              tl_FC(i,0)=0.0_r8
#  endif
              FC(i,1)=W(i,j,1)*                                         &
     &                (cff1*t(i,j,1,nstp,itrc)+                         &
     &                 cff2*t(i,j,2,nstp,itrc)-                         &
     &                 cff3*t(i,j,3,nstp,itrc))
              tl_FC(i,1)=tl_W(i,j,1)*                                   &
     &                   (cff1*t(i,j,1,nstp,itrc)+                      &
     &                    cff2*t(i,j,2,nstp,itrc)-                      &
     &                    cff3*t(i,j,3,nstp,itrc))+                     &
     &                   W(i,j,1)*                                      &
     &                   (cff1*tl_t(i,j,1,nstp,itrc)+                   &
     &                    cff2*tl_t(i,j,2,nstp,itrc)-                   &
     &                    cff3*tl_t(i,j,3,nstp,itrc))
              FC(i,N(ng)-1)=W(i,j,N(ng)-1)*                             &
     &                      (cff1*t(i,j,N(ng)  ,nstp,itrc)+             &
     &                       cff2*t(i,j,N(ng)-1,nstp,itrc)-             &
     &                       cff3*t(i,j,N(ng)-2,nstp,itrc))
              tl_FC(i,N(ng)-1)=tl_W(i,j,N(ng)-1)*                       &
     &                         (cff1*t(i,j,N(ng)  ,nstp,itrc)+          &
     &                          cff2*t(i,j,N(ng)-1,nstp,itrc)-          &
     &                          cff3*t(i,j,N(ng)-2,nstp,itrc))+         &
     &                         W(i,j,N(ng)-1)*                          &
     &                         (cff1*tl_t(i,j,N(ng)  ,nstp,itrc)+       &
     &                          cff2*tl_t(i,j,N(ng)-1,nstp,itrc)-       &
     &                          cff3*tl_t(i,j,N(ng)-2,nstp,itrc))
              FC(i,N(ng))=0.0_r8
              tl_FC(i,N(ng))=0.0_r8
            END DO
          END IF VADV_FLUX
!
!  Apply mass point sources (volume vertical influx), if any.
!  (Notice the j-loop is pipelined above).
!
          IF (LwSrc(ng)) THEN
            DO is=1,Nsrc(ng)
              Isrc=SOURCES(ng)%Isrc(is)
              Jsrc=SOURCES(ng)%Jsrc(is)
              IF (LtracerSrc(itrc,ng).and.                              &
     &            ((Istr.le.Isrc).and.(Isrc.le.Iend+1)).and.            &
     &            (j.eq.Jsrc)) THEN
                DO k=1,N(ng)-1
!>                FC(Isrc,k)=FC(Isrc,k)+0.5_r8*                         &
!>   &                       (SOURCES(ng)%Qsrc(is,k  )*                 &
!>   &                        SOURCES(ng)%Tsrc(is,k  ,itrc)+            &
!>   &                        SOURCES(ng)%Qsrc(is,k+1)*                 &
!>   &                        SOURCES(ng)%Tsrc(is,k+1,itrc))
!>
                  tl_FC(Isrc,k)=tl_FC(Isrc,k)+0.0_r8
                END DO
              END IF
            END DO
          END IF
!
!  Compute artificial continuity equation and load it into private
!  array DC (1/m). It is imposed to preserve tracer constancy. It is
!  not the same for all the tracer advection schemes because of the
!  Gamma value.
!
          IF ((Vadvection(itrc,ng)%MPDATA).or.                          &
     &        (Vadvection(itrc,ng)%HSIMT)) THEN
            Gamma=0.5_r8
          ELSE
            Gamma=1.0_r8/6.0_r8
          END IF
#  if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
          IF (iic(ng).eq.ntfirst(ng).and.SOinitial(ng)) THEN
#  else
          IF (iic(ng).eq.ntfirst(ng)) THEN
#  endif
            cff=0.5_r8*dt(ng)
          ELSE
            cff=(1.0_r8-Gamma)*dt(ng)
          END IF
          DO k=1,N(ng)
            DO i=Istr,Iend
              DC(i,k)=1.0_r8/(Hz(i,j,k)-                                &
     &                        cff*pm(i,j)*pn(i,j)*                      &
     &                        (Huon(i+1,j,k)-Huon(i,j,k)+               &
     &                         Hvom(i,j+1,k)-Hvom(i,j,k)+               &
     &                        (W(i,j,k)-W(i,j,k-1))))
              tl_DC(i,k)=-DC(i,k)*DC(i,k)*                              &
     &                    (tl_Hz(i,j,k)-                                &
     &                     cff*pm(i,j)*pn(i,j)*                         &
     &                     (tl_Huon(i+1,j,k)-tl_Huon(i,j,k)+            &
     &                      tl_Hvom(i,j+1,k)-tl_Hvom(i,j,k)+            &
     &                     (tl_W(i,j,k)-tl_W(i,j,k-1))))
            END DO
          END DO
!
! Time-step vertical advection of tracers (Tunits). Impose artificial
! continuity equation.
!
! WARNING: t(:,:,:,3,itrc) at this point should be in units of
! =======  m Tunits. But, t(:,:,:,3,itrc) is read from a BASIC
!          STATE file and is in Tunits, so we need to multiply
!          by level thickness (Hz).
!
          DO k=1,N(ng)
            DO i=Istr,Iend
              cff1=cff*pm(i,j)*pn(i,j)
!>            t(i,j,k,3,itrc)=DC(i,k)*                                  &
!>   &                        (t(i,j,k,3,itrc)-                         &
!>   &                         cff1*(FC(i,k)-FC(i,k-1)))
!>
              tl_t(i,j,k,3,itrc)=tl_DC(i,k)*                            &
     &                           (t(i,j,k,3,itrc)*Hz(i,j,k)-            &
     &                            cff1*(FC(i,k)-FC(i,k-1)))+            &
     &                           DC(i,k)*                               &
     &                           (tl_t(i,j,k,3,itrc)-                   &
     &                            cff1*(tl_FC(i,k)-tl_FC(i,k-1)))
            END DO
          END DO
        END DO T_LOOP2
      END DO J_LOOP1
!
!-----------------------------------------------------------------------
!  Start computation of tracers at n+1 time-step, t(i,j,k,nnew,itrc).
!-----------------------------------------------------------------------
!
!  Compute vertical diffusive fluxes "FC" of the tracer fields at
!  current time step n, and at horizontal RHO-points and vertical
!  W-points.  Notice that the vertical diffusion coefficients for
!  passive tracers is the same as that for salinity (ltrc=NAT).
!
      DO j=Jstr,Jend
        cff3=dt(ng)*(1.0_r8-lambda)
        DO itrc=1,NT(ng)
          ltrc=MIN(NAT,itrc)
          DO k=1,N(ng)-1
            DO i=Istr,Iend
              cff=1.0_r8/(z_r(i,j,k+1)-z_r(i,j,k))
              tl_cff=-cff*cff*(tl_z_r(i,j,k+1)-tl_z_r(i,j,k))
!>            FC(i,k)=cff3*cff*Akt(i,j,k,ltrc)*                         &
!>   &                (t(i,j,k+1,nstp,itrc)-                            &
!>   &                 t(i,j,k  ,nstp,itrc))
!>
              tl_FC(i,k)=cff3*                                          &
     &                   (cff*(tl_Akt(i,j,k,ltrc)*                      &
     &                         (t(i,j,k+1,nstp,itrc)-                   &
     &                          t(i,j,k  ,nstp,itrc))+                  &
     &                         Akt(i,j,k,ltrc)*                         &
     &                         (tl_t(i,j,k+1,nstp,itrc)-                &
     &                          tl_t(i,j,k  ,nstp,itrc)))+              &
     &                    tl_cff*(Akt(i,j,k,ltrc)*                      &
     &                            (t(i,j,k+1,nstp,itrc)-                &
     &                             t(i,j,k,nstp,itrc))))
            END DO
          END DO

#  ifdef LMD_NONLOCAL_NOT_YET
!
!  Add in the nonlocal transport flux for unstable (convective)
!  forcing conditions into matrix FC when using the Large et al.
!  KPP scheme. The nonlocal transport is only applied to active
!  tracers.
!
          IF (itrc.le.NAT) THEN
            DO k=1,N(ng)-1
              DO i=Istr,Iend
!>              FC(i,k)=FC(i,k)-                                        &
!>   &                  dt(ng)*Akt(i,j,k,itrc)*ghats(i,j,k,itrc)
!>
                tl_FC(i,k)=tl_FC(i,k)-                                  &
     &                     dt(ng)*(tl_Akt(i,j,k,itrc)*                  &
     &                             ghats(i,j,k,itrc)+                   &
     &                             Akt(i,j,k,itrc)*                     &
     &                             tl_ghats(i,j,k,itrc))
              END DO
            END DO
          END IF
#  endif
#  ifdef SOLAR_SOURCE
!
!  Add in incoming solar radiation at interior W-points using decay
!  decay penetration function based on Jerlov water type.
!
          IF (itrc.eq.itemp) THEN
            DO k=1,N(ng)-1
              DO i=Istr,Iend
!>              FC(i,k)=FC(i,k)+                                        &
!>   &                  dt(ng)*srflx(i,j)*                              &
#   ifdef WET_DRY
!>   &                  rmask_wet(i,j)*                                 &
#   endif
!>   &                  swdk(i,j,k)
!>
                tl_FC(i,k)=tl_FC(i,k)+                                  &
     &                     dt(ng)*srflx(i,j)*                           &
#   ifdef WET_DRY_NOT_YET
     &                     rmask_wet(i,j)*                              &
#   endif
     &                     tl_swdk(i,j,k)
              END DO
            END DO
          END IF
#  endif
!
!  Apply bottom and surface tracer flux conditions.
!
          DO i=Istr,Iend
!>          FC(i,0)=dt(ng)*btflx(i,j,itrc)
!>
            tl_FC(i,0)=dt(ng)*tl_btflx(i,j,itrc)
!>          FC(i,N(ng))=dt(ng)*stflx(i,j,itrc)
!>
            tl_FC(i,N(ng))=dt(ng)*tl_stflx(i,j,itrc)
          END DO
!
!  Compute new tracer field (m Tunits).
!
          DO k=1,N(ng)
            DO i=Istr,Iend
              cff1=Hz(i,j,k)*t(i,j,k,nstp,itrc)
              tl_cff1=tl_Hz(i,j,k)*t(i,j,k,nstp,itrc)+                  &
     &                Hz(i,j,k)*tl_t(i,j,k,nstp,itrc)
              cff2=FC(i,k)-FC(i,k-1)
              tl_cff2=tl_FC(i,k)-tl_FC(i,k-1)
!>            t(i,j,k,nnew,itrc)=cff1+cff2
!>
              tl_t(i,j,k,nnew,itrc)=tl_cff1+tl_cff2
#  ifdef DIAGNOSTICS_TS
!!            DiaTwrk(i,j,k,itrc,iTrate)=cff1
!!            DiaTwrk(i,j,k,itrc,iTvdif)=cff2
#  endif
            END DO
          END DO
        END DO
      END DO
# endif /* !TS_FIXED */
!
!=======================================================================
!  3D momentum equation in the XI-direction.
!=======================================================================
!
!  Compute U-component viscous vertical momentum fluxes "FC" at
!  current time-step n, and at horizontal U-points and vertical
!  W-points.
!
      J_LOOP2: DO j=Jstr,Jend
        cff3=dt(ng)*(1.0_r8-lambda)
        DO k=1,N(ng)-1
          DO i=IstrU,Iend
            cff=1.0_r8/(z_r(i,j,k+1)+z_r(i-1,j,k+1)-                    &
     &                  z_r(i,j,k  )-z_r(i-1,j,k  ))
            tl_cff=-cff*cff*(tl_z_r(i,j,k+1)+tl_z_r(i-1,j,k+1)-         &
     &                       tl_z_r(i,j,k  )-tl_z_r(i-1,j,k  ))
!>          FC(i,k)=cff3*cff*(u(i,j,k+1,nstp)-u(i,j,k,nstp))*           &
!>   &                       (Akv(i,j,k)+Akv(i-1,j,k))
!>
            tl_FC(i,k)=cff3*                                            &
     &                 (cff*((tl_u(i,j,k+1,nstp)-tl_u(i,j,k,nstp))*     &
     &                       (Akv(i,j,k)+Akv(i-1,j,k))+                 &
     &                       (u(i,j,k+1,nstp)-u(i,j,k,nstp))*           &
     &                       (tl_Akv(i,j,k)+tl_Akv(i-1,j,k)))+          &
     &                  tl_cff*(u(i,j,k+1,nstp)-u(i,j,k,nstp))*         &
     &                         (Akv(i,j,k)+Akv(i-1,j,k)))
          END DO
        END DO
!
!  Apply bottom and surface stresses, if so is prescribed.
!
        DO i=IstrU,Iend
# ifdef BODYFORCE
!>        FC(i,0)=0.0_r8
!>
          tl_FC(i,0)=0.0_r8
!>        FC(i,N(ng))=0.0_r8
!>
          tl_FC(i,N(ng))=0.0_r8
# else
!>        FC(i,0)=dt(ng)*bustr(i,j)
!>
          tl_FC(i,0)=dt(ng)*tl_bustr(i,j)
!>        FC(i,N(ng))=dt(ng)*sustr(i,j)
!>
          tl_FC(i,N(ng))=dt(ng)*tl_sustr(i,j)
# endif
        END DO
!
!  Compute new U-momentum (m m/s).
!
        cff=dt(ng)*0.25_r8
        DO i=IstrU,Iend
          DC(i,0)=cff*(pm(i,j)+pm(i-1,j))*(pn(i,j)+pn(i-1,j))
        END DO
        indx=3-nrhs
# if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
        IF (iic(ng).eq.ntfirst(ng).and.SOinitial(ng)) THEN
# else
        IF (iic(ng).eq.ntfirst(ng)) THEN
# endif
          DO k=1,N(ng)
            DO i=IstrU,Iend
!>            cff1=u(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i-1,j,k))
!>
              tl_cff1=0.5_r8*(tl_u(i,j,k,nstp)*                         &
     &                        (Hz(i,j,k)+Hz(i-1,j,k))+                  &
     &                        u(i,j,k,nstp)*                            &
     &                        (tl_Hz(i,j,k)+tl_Hz(i-1,j,k)))
!>            cff2=FC(i,k)-FC(i,k-1)
!>
              tl_cff2=tl_FC(i,k)-tl_FC(i,k-1)
!>            u(i,j,k,nnew)=cff1+cff2
!>
              tl_u(i,j,k,nnew)=tl_cff1+tl_cff2
# ifdef DIAGNOSTICS_UV
!!            DO idiag=1,M3pgrd
!!              DiaU3wrk(i,j,k,idiag)=0.0_r8
!!            END DO
!!            DiaU3wrk(i,j,k,M3vvis)=cff2
!!            DiaU3wrk(i,j,k,M3rate)=cff1
# endif
            END DO
          END DO
# if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
        ELSE IF (iic(ng).eq.(ntfirst(ng)+1).and.SOinitial(ng)) THEN
# else
        ELSE IF (iic(ng).eq.(ntfirst(ng)+1)) THEN
# endif
          DO k=1,N(ng)
            DO i=IstrU,Iend
!>            cff1=u(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i-1,j,k))
!>
              tl_cff1=0.5_r8*(tl_u(i,j,k,nstp)*                         &
     &                        (Hz(i,j,k)+Hz(i-1,j,k))+                  &
     &                        u(i,j,k,nstp)*                            &
     &                        (tl_Hz(i,j,k)+tl_Hz(i-1,j,k)))
!>            cff2=FC(i,k)-FC(i,k-1)
!>
              tl_cff2=tl_FC(i,k)-tl_FC(i,k-1)
              cff3=0.5_r8*DC(i,0)
!>            u(i,j,k,nnew)=cff1-                                       &
!>   &                      cff3*ru(i,j,k,indx)+                        &
!>   &                      cff2
!>
              tl_u(i,j,k,nnew)=tl_cff1-                                 &
     &                         cff3*tl_ru(i,j,k,indx)+                  &
     &                         tl_cff2
# ifdef DIAGNOSTICS_UV
!!            DO idiag=1,M3pgrd
!!              DiaU3wrk(i,j,k,idiag)=-cff3*DiaRU(i,j,k,indx,idiag)
!!            END DO
!!            DiaU3wrk(i,j,k,M3vvis)=cff2
#  ifdef BODYFORCE
!!            DiaU3wrk(i,j,k,M3vvis)=DiaU3wrk(i,j,k,M3vvis)-            &
!!   &                               cff3*DiaRU(i,j,k,indx,M3vvis)
#  endif
!!            DiaU3wrk(i,j,k,M3rate)=cff1
# endif
            END DO
          END DO
        ELSE
          cff1= 5.0_r8/12.0_r8
          cff2=16.0_r8/12.0_r8
          DO k=1,N(ng)
            DO i=IstrU,Iend
!>            cff3=u(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i-1,j,k))
!>
              tl_cff3=0.5_r8*(tl_u(i,j,k,nstp)*                         &
     &                        (Hz(i,j,k)+Hz(i-1,j,k))+                  &
     &                        u(i,j,k,nstp)*                            &
     &                        (tl_Hz(i,j,k)+tl_Hz(i-1,j,k)))
!>            cff4=FC(i,k)-FC(i,k-1)
!>
              tl_cff4=tl_FC(i,k)-tl_FC(i,k-1)
!>            u(i,j,k,nnew)=cff3+                                       &
!>   &                      DC(i,0)*(cff1*ru(i,j,k,nrhs)-               &
!>   &                               cff2*ru(i,j,k,indx))+              &
!>   &                      cff4
!>
              tl_u(i,j,k,nnew)=tl_cff3+                                 &
     &                         DC(i,0)*(cff1*tl_ru(i,j,k,nrhs)-         &
     &                                  cff2*tl_ru(i,j,k,indx))+        &
     &                         tl_cff4
# ifdef DIAGNOSTICS_UV
!!            DO idiag=1,M3pgrd
!!              DiaU3wrk(i,j,k,idiag)=DC(i,0)*                          &
!!   &                                (cff1*DiaRU(i,j,k,nrhs,idiag)-    &
!!   &                                 cff2*DiaRU(i,j,k,indx,idiag))
!!            END DO
!!            DiaU3wrk(i,j,k,M3vvis)=cff4
#  ifdef BODYFORCE
!!            DiaU3wrk(i,j,k,M3vvis)=DiaU3wrk(i,j,k,M3vvis)+            &
!!   &                               DC(i,0)*                           &
!!   &                               (cff1*DiaRU(i,j,k,nrhs,M3vvis)-    &
!!   &                                cff2*DiaRU(i,j,k,indx,M3vvis))
#  endif
!!            DiaU3wrk(i,j,k,M3rate)=cff3
# endif
            END DO
          END DO
        END IF
!
!=======================================================================
!  3D momentum equation in the ETA-direction.
!=======================================================================
!
!  Compute V-component viscous vertical momentum fluxes "FC" at
!  current time-step n, and at horizontal V-points and vertical
!  W-points.
!
        IF (j.ge.JstrV) THEN
          cff3=dt(ng)*(1.0_r8-lambda)
          DO k=1,N(ng)-1
            DO i=Istr,Iend
              cff=1.0_r8/(z_r(i,j,k+1)+z_r(i,j-1,k+1)-                  &
     &                    z_r(i,j,k  )-z_r(i,j-1,k  ))
              tl_cff=-cff*cff*(tl_z_r(i,j,k+1)+tl_z_r(i,j-1,k+1)-       &
     &                         tl_z_r(i,j,k  )-tl_z_r(i,j-1,k  ))
!>            FC(i,k)=cff3*cff*(v(i,j,k+1,nstp)-v(i,j,k,nstp))*         &
!>   &                         (Akv(i,j,k)+Akv(i,j-1,k))
!>
              tl_FC(i,k)=cff3*                                          &
     &                   (cff*((tl_v(i,j,k+1,nstp)-tl_v(i,j,k,nstp))*   &
     &                         (Akv(i,j,k)+Akv(i,j-1,k))+               &
     &                         (v(i,j,k+1,nstp)-v(i,j,k,nstp))*         &
     &                         (tl_Akv(i,j,k)+tl_Akv(i,j-1,k)))+        &
     &                    tl_cff*(v(i,j,k+1,nstp)-v(i,j,k,nstp))*       &
     &                           (Akv(i,j,k)+Akv(i,j-1,k)))
            END DO
          END DO
!
!  Apply bottom and surface stresses, if so is prescribed.
!
          DO i=Istr,Iend
# ifdef BODYFORCE
!>          FC(i,0)=0.0_r8
!>
            tl_FC(i,0)=0.0_r8
!>          FC(i,N(ng))=0.0_r8
!>
            tl_FC(i,N(ng))=0.0_r8
# else
!>          FC(i,0)=dt(ng)*bvstr(i,j)
!>
            tl_FC(i,0)=dt(ng)*tl_bvstr(i,j)
!>          FC(i,N(ng))=dt(ng)*svstr(i,j)
!>
            tl_FC(i,N(ng))=dt(ng)*tl_svstr(i,j)
# endif
          END DO
!
!  Compute new V-momentum (m m/s).
!
          cff=dt(ng)*0.25_r8
          DO i=Istr,Iend
            DC(i,0)=cff*(pm(i,j)+pm(i,j-1))*(pn(i,j)+pn(i,j-1))
          END DO
# if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
          IF (iic(ng).eq.ntfirst(ng).and.SOinitial(ng)) THEN
# else
          IF (iic(ng).eq.ntfirst(ng)) THEN
# endif
            DO k=1,N(ng)
              DO i=Istr,Iend
!>              cff1=v(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i,j-1,k))
!>
                tl_cff1=0.5_r8*(tl_v(i,j,k,nstp)*                       &
     &                          (Hz(i,j,k)+Hz(i,j-1,k))+                &
     &                          v(i,j,k,nstp)*                          &
     &                          (tl_Hz(i,j,k)+tl_Hz(i,j-1,k)))
!>              cff2=FC(i,k)-FC(i,k-1)
!>
                tl_cff2=tl_FC(i,k)-tl_FC(i,k-1)
!>              v(i,j,k,nnew)=cff1+cff2
!>
                tl_v(i,j,k,nnew)=tl_cff1+tl_cff2
# ifdef DIAGNOSTICS_UV
!!              DO idiag=1,M3pgrd
!!                DiaV3wrk(i,j,k,idiag)=0.0_r8
!!              END DO
!!              DiaV3wrk(i,j,k,M3vvis)=cff2
!!              DiaV3wrk(i,j,k,M3rate)=cff1
# endif
              END DO
            END DO
# if defined STOCHASTIC_OPT && !defined STOCH_OPT_WHITE
          ELSE IF (iic(ng).eq.(ntfirst(ng)+1).and.SOinitial(ng)) THEN
# else
          ELSE IF (iic(ng).eq.(ntfirst(ng)+1)) THEN
# endif
            DO k=1,N(ng)
              DO i=Istr,Iend
!>              cff1=v(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i,j-1,k))
!>
                tl_cff1=0.5_r8*(tl_v(i,j,k,nstp)*                       &
     &                          (Hz(i,j,k)+Hz(i,j-1,k))+                &
     &                          v(i,j,k,nstp)*                          &
     &                          (tl_Hz(i,j,k)+tl_Hz(i,j-1,k)))
!>              cff2=FC(i,k)-FC(i,k-1)
!>
                tl_cff2=tl_FC(i,k)-tl_FC(i,k-1)
                cff3=0.5_r8*DC(i,0)
!>              v(i,j,k,nnew)=cff1-                                     &
!>   &                        cff3*rv(i,j,k,indx)+                      &
!>   &                        cff2
!>
                tl_v(i,j,k,nnew)=tl_cff1-                               &
     &                           cff3*tl_rv(i,j,k,indx)+                &
     &                           tl_cff2
# ifdef DIAGNOSTICS_UV
!!              DO idiag=1,M3pgrd
!!                DiaV3wrk(i,j,k,idiag)=-cff3*DiaRV(i,j,k,indx,idiag)
!!              END DO
!!              DiaV3wrk(i,j,k,M3vvis)=cff2
#  ifdef BODYFORCE
!!              DiaV3wrk(i,j,k,M3vvis)=DiaV3wrk(i,j,k,M3vvis)-          &
!!   &                                 cff3*DiaRV(i,j,k,indx,M3vvis)
#  endif
!!              DiaV3wrk(i,j,k,M3rate)=cff1
# endif
              END DO
            END DO
          ELSE
            cff1= 5.0_r8/12.0_r8
            cff2=16.0_r8/12.0_r8
            DO k=1,N(ng)
              DO i=Istr,Iend
!>              cff3=v(i,j,k,nstp)*0.5_r8*(Hz(i,j,k)+Hz(i,j-1,k))
!>
                tl_cff3=0.5_r8*(tl_v(i,j,k,nstp)*                       &
     &                          (Hz(i,j,k)+Hz(i,j-1,k))+                &
     &                          v(i,j,k,nstp)*                          &
     &                          (tl_Hz(i,j,k)+tl_Hz(i,j-1,k)))
!>              cff4=FC(i,k)-FC(i,k-1)
!>
                tl_cff4=tl_FC(i,k)-tl_FC(i,k-1)
!>              v(i,j,k,nnew)=cff3+                                     &
!>   &                        DC(i,0)*(cff1*rv(i,j,k,nrhs)-             &
!>   &                                 cff2*rv(i,j,k,indx))+            &
!>   &                        cff4
!>
                tl_v(i,j,k,nnew)=tl_cff3+                               &
     &                           DC(i,0)*(cff1*tl_rv(i,j,k,nrhs)-       &
     &                                    cff2*tl_rv(i,j,k,indx))+      &
     &                           tl_cff4
# ifdef DIAGNOSTICS_UV
!!              DO idiag=1,M3pgrd
!!                DiaV3wrk(i,j,k,idiag)=DC(i,0)*                        &
!!   &                                  (cff1*DiaRV(i,j,k,nrhs,idiag)-  &
!!   &                                   cff2*DiaRV(i,j,k,indx,idiag))
!!              END DO
!!              DiaV3wrk(i,j,k,M3vvis)=cff4
#  ifdef BODYFORCE
!!              DiaV3wrk(i,j,k,M3vvis)=DiaV3wrk(i,j,k,M3vvis)+          &
!!   &                                 DC(i,0)*                         &
!!   &                                 (cff1*DiaRV(i,j,k,nrhs,M3vvis)-  &
!!   &                                  cff2*DiaRV(i,j,k,indx,M3vvis))
#  endif
!!              DiaV3wrk(i,j,k,M3rate)=cff3
# endif
              END DO
            END DO
          END IF
        END IF
      END DO J_LOOP2

# ifndef TS_FIXED
!
!=======================================================================
!  Apply intermediate tracers lateral boundary conditions.
!=======================================================================
!
      ic=0
      DO itrc=1,NT(ng)
        IF (LtracerCLM(itrc,ng).and.LnudgeTCLM(itrc,ng)) THEN
          ic=ic+1
        END IF
!>      CALL t3dbc_tile (ng, tile, itrc, ic,                            &
!>   &                   LBi, UBi, LBj, UBj, N(ng), NT(ng),             &
!>   &                   IminS, ImaxS, JminS, JmaxS,                    &
!>   &                   nstp, 3,                                       &
!>   &                   t)
!>
        CALL tl_t3dbc_tile (ng, tile, itrc, ic,                         &
     &                      LBi, UBi, LBj, UBj, N(ng), NT(ng),          &
     &                      IminS, ImaxS, JminS, JmaxS,                 &
     &                      nstp, 3,                                    &
     &                      tl_t)

        IF (EWperiodic(ng).or.NSperiodic(ng)) THEN
!>        CALL exchange_r3d_tile (ng, tile,                             &
!>   &                            LBi, UBi, LBj, UBj, 1, N(ng),         &
!>   &                            t(:,:,:,3,itrc))
!>
          CALL exchange_r3d_tile (ng, tile,                             &
     &                            LBi, UBi, LBj, UBj, 1, N(ng),         &
     &                            tl_t(:,:,:,3,itrc))
        END IF
      END DO

#  ifdef DISTRIBUTE
!>    CALL mp_exchange4d (ng, tile, iNLM, 1,                            &
!>   &                    LBi, UBi, LBj, UBj, 1, N(ng), 1, NT(ng),      &
!>   &                    NghostPoints,                                 &
!>   &                    EWperiodic(ng), NSperiodic(ng),               &
!>   &                    t(:,:,:,3,:))
!>
      CALL mp_exchange4d (ng, tile, iTLM, 1,                            &
     &                    LBi, UBi, LBj, UBj, 1, N(ng), 1, NT(ng),      &
     &                    NghostPoints,                                 &
     &                    EWperiodic(ng), NSperiodic(ng),               &
     &                    tl_t(:,:,:,3,:))
#  endif
# endif

      RETURN
      END SUBROUTINE tl_pre_step3d_tile
#endif
      END MODULE tl_pre_step3d_mod
