Material Transport in Oceanic Gyres. Part I: Phenomenology

Abstract

Material spreading and mixing by oceanic mesoscale eddies are analyzed in an idealized, numerical model of the wind-driven, midlatitude oceanic circulation. The main focus is on the regime with large Reynolds number, Re, and vigorous mesoscale eddies, although brief comparisons are made with less turbulent solutions at smaller Re. The analyses are based on ensembles of Lagrangian particle trajectories. The authors find that tracer transport by mesoscale eddies differs in many ways from the commonly used model of homogeneous, isotropic eddy diffusion. The single-particle dispersion, which describes the spreading process, is generally anisotropic and inhomogeneous and in most places it is not diffusive (i.e., not linear in time) during intermediate-time intervals after tracer release. In most of the basin and especially in the deep layers, subdiffusive single-particle dispersion occurs due to long-time trapping of material by coherent structures such as vortices near the strong currents and planetary waves in the eastern part of the gyres. Superdiffusive dispersion behavior is found in the western part of the subtropical gyre and near the boundaries in fluctuating jetlike flows. Sub- and superdiffusion are associated with a strong first negative and second positive lobe, respectively, in the Lagrangian velocity autocorrelation function. The two-particle dispersion, which describes the mixing process, is characterized by initial exponential growth, and its exponent has strong geographical inhomogeneities, with faster rates in the upper western gyres. The finite-time Lyapunov exponents and recurrence times?other descriptors of the mixing?indicate a similar geographical partitioning, but are better alternatives to the two-particle dispersion on short and long time intervals, respectively. Over large time intervals, due to inhomogeneity of the transport properties, material spreads and mixes much faster within the subtropical than the subpolar gyre. The eastward jet extension of the weaker subpolar western boundary current behaves as a strong barrier to the intergyre flux, but the eastward jet of the stronger subtropical gyre behaves as a weaker transport barrier. It is shown that net Lagrangian fluxes across the eastward jets are less intensive but more efficient in the deep ocean. The permeability of the barrier and the main pathways across it are measured with distributions of crossing particles, with finite-time Lyapunov exponents, and, at small Re, with the geometry of turnstyle lobes.