It would be desirable to have a dynamical theory of how ocean current patterns vary with time in response to variation in the patterns of mass, heat and momentum transfer at the surface, but severe difficulties, particularly the uncertain effects of vertical mixing, nonlinear interactions and bottom topography, oppose the development of such a theory, while its evaluation through comparison with observation is impeded by the insufficiency of both input and output data. The characteristic wavenumber components associated with different parts of the input frequency spectrum at different latitudes can, however, be expected to play a particularly important role in determining response. Several of the means by which this may happen are discussed, with some particular reference to the dynamic response of the Indian Ocean to onset of the Southwest Monsoon.