A fourth-order hyperdiffusion with a coefficient r = 2.5 x 10~4 is incorporated in the model for computational purposes. The frictional effect is represented by a linear drag. It may be viewed as a linearized form of a bulk aerodynamic formula for representing a surface stress. A reasonable value of the drag coefficient is a = 0.03, corresponding to a damping time of about 5 days (Whitaker and Sardeshmurkh, 1998). When we take into account the influence of continents and oceans, we use a = 0.015 in the oceanic sectors and a = 0.045 in the land sectors.
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