Relative Dispersion of Surface Drifters in the Benguela Upwelling Region

Abstract

AbstractWe examine the relative dispersion of surface drifters deployed in groups of triplets at the boundaries of a filament in the upwelling region off Namibia for both the entire ensemble and the two main subgroups. For the drifters in the group released at the northern boundary of the filament, close to the upwelling front, we find that the mean-square pair separation 〈s2(t)〉 shows the characteristic distinct dispersion regimes [nonlocal, local (Richardson), and diffusive] of an ocean surface mixed layer. We confirm the different dispersion regimes by a rescaled presentation of the moments 〈sn(t)〉 and thereby also explain the anomalous slow decay of the kurtosis in the transient regime. For the drifter group released at the southern boundary, 〈s2(t)〉 remains constant for a short period, followed by a steep “Richardson like” increase and an asymptotic diffusive increase. In contrast to the northern release, the corresponding moments reveal a narrow distribution of pair separations for all regimes. The analysis of finite-size Lyapunov exponents (FSLEs) reveals consistent results when applied to the two releases separately. When applied to the entire drifter ensemble, the two measures yield inconsistent results. We relate the breakdown of consistency to the impact of the different dynamics on the respective averages: whereas, because of separation in scale, 〈s2(t)〉 is dominated by the northern release, the decay of the FSLEs for small distances reflects the drifter dynamics within the filament.