Turbulent motions in the ocean are classified in the following 2 basic classes according to their nature, spatio-temporal scale, direction of mixing (isopycnal/diapycnal) and intensity:
- Mesoscale turbulence:
Mesoscale (or synoptic) eddies are created by mixed instability (i.e. by either baroclinic instability which feeds on the potential energy of the tilted density field or barotropic instability which feeds on the mean field kinetic energy) and are believed to mix tracers along surfaces of constant density called isopycnals. Mesoscale eddies have a typical length scale of the order of the internal Rossby Radius (ranging from 10-100km) and cannot be resolved by the coarse horizontal resolution of climate ocean models. Hence, their effect is parameterized by use of a rotated tensor which mixes along isopycnal surface. It is woeth noting that isopycnals are quasi-horizontal except in frontal regions and convection zones.
- Small-scale turbulence:
Small scale turbulence encompasses processes such as isotropic shear turbulence and wave breaking which range from 1mm to 1m. These turbulent motions cannot be resolved explicitly because of the coarse vertical resolution of models and the hydrostatic assumption. Hence, their effect is parameterized by use of a mixing along the cross-isopycnal direction which in the bulk of the ocean can be approximated by the vertical direction.
GOTM focuses on the small-scale turbulent motions. The design of vertical mixing schemes is of paramount importance in ocean modelling because it largely determines model's performances for the following reasons:
- Small-scale turbulence controls to a large extent the vertical exchanges in estuaries (erosion/sedimentation of suspended matter, ...).
- The vertical visosity largely controls the intensity of the oceanic overturning circulation via the dynamical Munk balance (in the ocean interior, the momentum equation simplifies to a balance between vertical mixing and advection).
- Small-scale turbulence strongly controls the exchange of mass at the air-sea interface. The choice of the mixing scheme has therefore a key importance in the evolution of the ocean SST and heat content in coupled climate models.
- Turbulence in the mixed layer influences biological processes, via for example the regeneration of nutrients used for primary production.