Fix bug in multi-region SplitExplicitFreeSurface (#4947)

* fix bug

* remove one line

Author: simone-silvestri

src/MultiRegion/multi_region_split_explicit_free_surface.jl

@@ -28,11 +28,10 @@ function materialize_free_surface(free_surface::SplitExplicitFreeSurface, veloci
     @apply_regionally u_bcs = barotropic_velocity_boundary_conditions(u_baroclinic)
     @apply_regionally v_bcs = barotropic_velocity_boundary_conditions(v_baroclinic)
 
-    U = Field{Center, Center, Nothing}(extended_grid, boundary_conditions = u_bcs)
-    V = Field{Center, Center, Nothing}(extended_grid, boundary_conditions = v_bcs)
-
-    U̅ = Field{Center, Center, Nothing}(extended_grid, boundary_conditions = u_bcs)
-    V̅ = Field{Center, Center, Nothing}(extended_grid, boundary_conditions = v_bcs)
+    U = Field{Face, Center, Nothing}(extended_grid, boundary_conditions = u_bcs)
+    V = Field{Center, Face, Nothing}(extended_grid, boundary_conditions = v_bcs)
+    U̅ = Field{Face, Center, Nothing}(extended_grid, boundary_conditions = u_bcs)
+    V̅ = Field{Center, Face, Nothing}(extended_grid, boundary_conditions = v_bcs)
 
     filtered_state = (η = η̅, U = U̅, V = V̅)
     barotropic_velocities = (U = U, V = V)
@@ -71,4 +70,3 @@ iterate_split_explicit!(free_surface, grid::MultiRegionGrids, GUⁿ, GVⁿ, Δτ
 @inline augmented_kernel_offsets(grid, ::XPartition) = (- halo_size(grid)[1] + 1, 0)
 @inline augmented_kernel_offsets(grid, ::YPartition) = (0, - halo_size(grid)[2] + 1)
 @inline augmented_kernel_offsets(grid, ::CubedSpherePartition) = (- halo_size(grid)[2] + 1, - halo_size(grid)[2] + 1)
-