  We propose to use the ensemble Kalman smoother (EnKS) as linear least squares
solver in the Gauss-Newton method for the large nonlinear least squares in
incremental 4DVAR. The ensemble approach is naturally parallel over the
ensemble members and no tangent or adjoint operators are needed. Further,
adding a regularization term results in replacing the Gauss-Newton method,
which may diverge, by^M the Levenberg-Marquardt method, which is known to be
convergent. The regularization is implemented efficiently as an additional
observation in the EnKS.
