Ocean Dynamics manuscript No.

(will be inserted by the editor) Optimal boundary conditions at the staircase-shaped coastlines. Eugene Kazantsev the date of receipt and acceptance should be inserted later Abstract A 4D-Var data assimilation technique is ap-plied to the rectangular-box configuration of the NEMO in order to identify the optimal parametrization of boun-dary conditions at lateral boundaries. The case of the staircase-shaped coastlines is studied by rotating the model grid around the center of the box. It is shown that, in some cases, the formulation of the boundary conditions at the exact boundary leads to appearance of exponentially growing modes while optimal bound-ary conditions allow to correct the errors induced by the staircase-like appriximation of the coastline