Turbulent dispersion properties from a model simulation of the western Mediterranean Sea

摘要

Using a high-resolution primitive equation model of the western MediterraneanSea, we analyzed the dispersion properties of a set of homogeneouslydistributed, passive particle pairs. These particles were initially separatedby different distances D₀ (D₀ = 5.55, 11.1 and 16.65 km), and wereseeded in the model at initial depths of 44 and 500 m.This realistic ocean model, which reproduces the main features of theregional circulation, puts into evidence the three well-known regimes ofrelative dispersion.The first regime due to the chaotic advection at small scales lasts only afew days (3 days at 44 m depth, a duration comparable with the integraltimescale), and the relative dispersion is then exponential. In the secondregime, extending from 3 to 20 days, the relative dispersion has a power lawtα where α tends to 3 as D₀ becomes small. In thethird regime, a linear growth of the relative dispersion is observed startingfrom the twentieth day. For the relative diffusivity, the D2 growth isfollowed by the Richardson regime D4/3. At large scales, where particlevelocities are decorrelated, the relative diffusivity is constant.At 500 m depth, the integral timescale increases (> 4 days)and the intermediate regime becomes narrower than that at 44 m depth due tothe weaker effect of vortices (this effect decreases with depth). Theturbulent properties become less intermittent and more homogeneous and theRichardson law takes place.