**Titre/Title :**

Instabilities in a rotating and stratified fluid

**Contact :**

Nicolas Mordant

**Résumé/Abstract :**

Geosismic observations have revealed the stacking of horizontal layers of water

with different densities in the ocean, particularly above and beneath lens-shaped

eddies. We present a simplified model together with an experimental setup to re-

produce and identify the mechanism responsible for this layering phenomenon: we

consider the stably stratified flow around a rotating, solid ellipsoid. Experimentally,

a flat oblate rotating ellipsoid reproduces faithfully the boundary condition of an

oceanic eddy, whereas the case of a rotating sphere provides an analytically tractable

base flow, suitable for a numerical linear analysis. Two instabilities are witnessed

experimentally and numerically. The first instability is the classical, inviscid, strato-

inertial instability that tends to develop at the equator of the ellipsoid independently

of the value of the Schmidt number. The second instability is localised in the vicin-

ity of the poles and appears only if the Schmidt number differs from one. Hence,

this instability is reminiscent of the double-diffusive McIntyre instability, a valuable

candidate to explain layering in oceanic eddies.