Spin up/down in linear density stratified fluid

Abstract

Vertical transport of momentum due to baroclinic instabilities is investigated in a rotating stratified fluid. The experiments were carried out in the rotating tank of the Coriolis/LEGI in Grenoble. The mean flow was generated by increasing or decreasing the rotation rate of the platform. The velocity fields were measured by PIV technique. The dimensionless parameter that are important in determining the observed response are the Burger and the Rossby numbers. Our experimental study reveals two very different behaviors. For 0(1) Burger numbers, the bulk of the flow remains axisymmetric with a very slow decay. We have checked that the decay is well described by the usual vertical diffusion law. The mismatch pertains to the presence of Ekman pumping/suction, but this mechanism is confined by stratification to a relatively shallow layer. For Burger numbers << 0(1), baroclinic instabilities are observed. The increased vertical momentum transport leads to a much faster spin up. Neither the diffusion nor the Ekman pumping/suction can be considered responsible for this enhanced transport of momentum and we interpret it as the effect of the baroclinic instabilities.