LABORATORY STUDIES OF EQUATORIALLY TRAPPED WAVES USING FERROFLUID

We introduce a new technique to model spherical geophysical fluid dyna mics in the terrestrial laboratory. The local vertical projection of p lanetary vorticity,f, varies with latitude on a rotating spherical pla net and allows an important class of waves in large-scale atmospheric and oceanic flows. These Rossby waves have been extensively studied in the laboratory for middle and polar latitudes. At the equator f chang es sign where gravity is perpendicular to the planetary rotation. This geometry has made laboratory studies of geophysical fluid dynamics ne ar the equator very limited. We use ferrofluid and static magnetic fie lds to generate nearly spherical geopotentials in a rotating laborator y experiment. This system is the laboratory analogue of those large-sc ale atmospheric and oceanic flows whose horizontal motions are governe d by the Laplace tidal equations. As the rotation rate in such a syste m increases, waves are trapped to latitudes near the equator and the d ynamics can be formulated on the equatorial beta-plane. This transitio n from planetary modes to equatorially trapped modes as the rotation r ate increases is observed in the experiments. The equatorial beta-plan e solutions of non-dispersive Kelvin waves propagating eastward and no ndispersive Rossby waves propagating westward at low frequency are obs erved in the limit of rotation fast compared to gravity wave speed.