I will present in this seminar two sets of laboratory experiments devoted to the study of rotating stratified flows. The first one concerns the coalescence of two lenticular anticyclones in a linearly stratified rotating fluid. This type of events,classically met in the oceans has also been observed in the Jovian atmosphere. Our results show that the merging critical distance between the vortices depends drastically on their Rossby radius of deformation. This is in complete agreement with previous numerical modelling of vortex coalescence. We have also observed that mergers involve threedimensional processes as the vortices intertwine together possibly because of the presence of an elliptic instability that tilts the vortex cores. They are also accompanied by the emission of vorticity filaments and internal gravity waves radiation although we cannot prove that in our experiments these waves are solely due to the merging process.

In the second set of experiments, I will present a new instability mechanism that affects shear flows when these are stably stratified in density along the vertical direction, i.e. orthogonal to the horizontal shear. Stratified shear flows are ubiquitous in nature and we may think to water flows in submarine canyons, to winds in deep valleys, to currents along sea shores or to laminar flows in canals where density stratification can be due to temperature or salinity gradients. Our study follows recent investigations on instabilities in stratified rotating or non rotating shear flows: the stratorotational instability or the stratified boundary layer instability where it was shown that these instabilities belong to a class of instabilities caused by the resonant interaction of Doppler shifted internal waves. Our laboratory experiments for Plane Couette and Plane Poiseuille flows show in both cases the appearance of braided wave patterns when the Reynolds and Froude numbers are above a threshold. The non linear saturation of the instability leads to a meandering in the horizontal plane arranged in layers stacked along the vertical direction.