Blob instability in rotating compositional convection

The onset and structure of compositional convection in a rotating system ar e investigated experimentally. A vertically oriented cylindrical annulus fi lled with NH4Cl-H2O solution is cooled from the bottom and can be rotated a bout its axis at rates ranging up to 10.5 rad s(-1), corresponding to Ekman numbers down to 7.5 x 10(-6). The Coriolis force has a strong effect on th e structure of plumes above the mush-liquid interface. Helical motion of th e conduit, which is weakly developed in the non-rotating case, is amplified by Coriolis forces that twist the plume conduits to lie nearly horizontall y. This results in secondary plumes (or blobs) that rise from the sub-horiz ontal primary plume conduits. This new instability could be an efficient me chanism for producing small-scale flow in the form of buoyant blobs that as cend through the polar regions of the outer core.