code: def barrier_layer_thickness(SA, CT):
      
      import gsw
  
      sigma_theta = gsw.sigma0(SA, CT)
      mask = mixed_layer_depth(CT)
      mld = np.where(mask)[0][-1]
      sig_surface = sigma_theta[0]
      sig_bottom_mld = gsw.sigma0(SA[0], CT[mld])
      d_sig_t = sig_surface - sig_bottom_mld
      d_sig = sigma_theta - sig_bottom_mld
      mask = d_sig < d_sig_t  # Barrier layer.
      return Series(mask, index=SA.index, name="BLT")
docs: Compute the thickness of water separating the mixed surface layer from the
      thermocline.  A more precise definition would be the difference between
      mixed layer depth (MLD) calculated from temperature minus the mixed layer
      depth calculated using density.