While coupled to LMDZ, use the variable cell_area instead of calculating the grid-box area using resolution(:,1)*resolution(:,2).
Use the variable area in routing.nc instead of recacluating each time the grid-box area.
Note: Results change in the routing only because the order of calculation is changed in the line : gridarea(:) = contfrac(:)*resolution(:,1)*resolution(:,2) now
gridarea(:) = contfrac(:)*area_g(:)