Relative dispersion in the South Western Mediterranean as derived from satellite-tracked surface drifting buoys

Abstract. Relative dispersion (D²) in the South Western Mediterranean is analyzed using surface drifter pairs deployed during the period from 1986 to 2016. The results show the existence of four well-known regimes. The first regime, characterized by an exponential increment of the relative dispersion (Lundgren or exponential regime), corresponds to the chaotic advection at small scales and small separation distances, lasts for a few days. In the second regime, extending from 1.5 to roughly 7 days, for scales between 25 and 57 km and 1–3 km of initial distance, D² increases as time cubed (Richardson regime). The third regime occurs for initial distances of 5–10 km and times of 1.5–13 days; D² increases quadratically with time (Ballistic regime). The forth regime corresponds to time scales larger than 34 days for initial distances of 1–3 km and to 23 days for 35–40 km with a linear increase in time of D² (Rayleigh or diffusive regime). The relative diffusivity and characteristic dispersion time exhibit three different phases based on the initial pair separations and corresponding with Lundgren, Richardson and Rayleigh regimes, respectively. In the first phase (enstrophy cascade range) the diffusivity is ~ D² for distances smaller than 15 km and initial separation distances between 5 km and 10 km, and also for distances smaller than 40 km for initial separation distances between 35 km and 40 km; characteristic dispersion time is constant. In the second phase (inverse energy cascade), the diffusivity and characteristic dispersion time increase with growing distances following the 4/3 and 2/3 power laws, respectively, for scale ranging between 3 and 15 km and for initial distances smaller than 3 km. The third phase occurs for distance larger than 55 km, all pair velocities are uncorrelated and both relative diffusivity and characteristic dispersion time are approximately constants.