Journal of the Oceanographical Society of Japan \rol. 30, pp. 282 to 288, 1974 How does the Ocean Response to the Withdrawal of Water from a Watergate Located at the Eastern Boundary on a Beta-plane ?* Toshio YAMAGATA** Abstract : The formation o[ the 'spun up' region by the withdrawal of water from a watergate located in the eastern boundary in a homogeneous ocean on a beta-plane at low Rossby number is presented. The 'spun up' region penetrates only westward from the watergate because of the special character of the generated Rossby waves. The growth rate and the final longitudinal length of the 'spun up' region in the dissipative system is much affected by the watergate scale in the north-south direction. The relation with the experiment of LONG (1952) and the geophysical application are presented. 1. Introduction It is well known that the growth of Taylor columns in a uniformly rotating homogeneous fluid at low Rossby number is analogous to the formation of the blocked region by an obstacle of two-dimensional motion in a stably stratified fluid at small internal Froude number. The growth of these phenomena is interpreted in terms of the propagation of inertial waves or internal waves generated by the obstacle (BRETHERTON, 1967). In this analysis, I treat a similar but a little different phenomenon caused by the propagation of Rossby waves. The motif is based on a following simple question. How does the ocean response to the withdrawal of water from a watergate located at the eastern boundary on a beta-plane ? The beta-plane imposes a con- straint on a motion of a vortex element and, thus, has a beta-restoring force field. Rossby waves are due to the above restoring force and has a special character about propagation; if the Coriolis parameter is positive and the wave length in the east-west direction is infi- nitely large, its group velocity is westward. * Received August 14, 1974 ** Ocean Research Institute, University of Tokyo, Minamidai, Nakano-ku, Tokyo, 164 Japan Owing to this character, it is shown that the induced flow field, which is called 'spun up' region hereafter, penetrates only in the western side of the watergate. The effect of the free surface cannot be neglected when the wave length in the north- south direction is larger than the barotropic radius of deformation. The free surface re- duces the beta-restoring effect by the horizontal divergence and suppresses the growth of the 'spun up' region. If a two-layered system is considered, another characteristic length scale called baroclinic radius of deformation becomes important. The horizontal divergence of the interface cannot be neglected when the wave length in the north-south direction is larger than the baroclinic radius of deformation. The 'spun up' region due to the internal mode is expected to be greatly suppressed in this case. But the interests in this paper are focused in the homogeneous fluids since the essential dy- namics in the layered model is a straight exten- tion of that in the homogeneous model. In the absence of diffusivity, the 'spun up' region penetrates infinitely in the western side of the watergate. On the other hand, a steady finite 'spun up' region is formed by including the Ekman layer dissipation. The longitudinal length of the 'spun up' region can be deter- (32)