 dimension(:,:,:)
             Tendency of t, q  up to this point
             (The final value of dt_t and dt_q are output by 
              gcm_vert_diff_up)
             Units are [xi]/s where [xi] = [T], [q]


    input/output:

         tau_u, tau_v -- real, dimension(:,:)
             Surface stresses (positive upwards) 
             shape must conform to first two dimensions of input fields

             Units:  Kg/(m s**2) = (Kg m/s)/(m**2 s) 
                               = (Kg/(m**3))((m/s)**2) =