The mixing and spreading phases of MEDOC. I

The processes which produce bottom water in the Mediterranean are studied. Observations show three phases. The first, called ‘preconditioning’, is not studied. The second ‘violent mixing’ phase occurs when the cold Mistral begins. Cooling at the surface leads to intense vertical mixing in a narrow chimney-like region. When the wind stops, the ‘sinking and spreading’ phase begins. Interleaving of water masses occurs; horizontal eddying on scales of 15 km is observed; and 600 m columns of water can be lifted up to 500 m.A two-dimensional model is used to explain the mixing phase. Non-penetrative vertical convection explains the observations well. Advection of water from outside the column is small, and slows down the descent of the outer parts of the column as observed. The column does not break up even when it reaches the bottom, or if the strong winds cease.The most efficient mechanism for the spreading phase is baroclinic instability. In conditions of vertical overturning there is a large amount of potential energy available to drive the instability. For an eddy viscosity of 50 m² s⁻¹, after 10 days sinking the growth time is 3–4 days and the eddies are mainly concentrated at the surface with a major axis length of about 15 km. These tend to stabilize the top few hundred meters of the column. Later, finite amplitude effects will produce slower perturbations in the rest of the column.