Rossby normal modes in basins with barriers

The Rossby waver linear, normal modes for a barotropic fluid in a basin on the beta plane are calculated in the presence of a thin barrier, which near ly divides the basin in two. Narrow gaps allow one subbasin to communicate to the other. In the case of a meridional barrier it is shown that the mode s split into two categories: either full-basin modest which have strong exp ression in both subbasins, or subbasin modes, which are limited to one or t he other of the subbasins. The full-basin modes have natural frequencies ve ry close to the eigenfrequencies of the basin in the absence of a barrier w hile the subbasin modes oscillate at the eigenfrequencies of the subbasins as if they were isolated. In the former case, the oscillation is accompanie d by strong flow through the gaps in the barrier while, in the latter case, there is essentially no flow through the barrier. If the barrier is opened by adding additional gaps, some of the modes turn from subbasin modes to lull-basin modes. If the barrier is zonally oriented rather than meridionally oriented, the c lear distinction between the two mode types disappears, although it is stil l possible to associate each of the modes with a subbasin mode restricted p redominantly to one or the other of the subbasins. The analytic theory, based on a narrow-gap assumption, is checked and exten ded by considering a linearized numerical model for the forced response of the basin. Of particular significance is the ability of localized forcing i n one subbasin to excite oscillations of the full basin in spite of narrow gaps that restrict the communication between adjacent subbasins.