Low-frequency motions in the interiors of the subtropical gyres include distinct multiple, alternating quasi-zonal jet-like features (striations) which slowly, at a speed of about 0.3-0.5 km/day, propagate toward the equator. In their presence, the space-time distribution of mesoscale eddies is not completely random but organized into some sort of “storm tracks”, elongated along the jets and moving with them. Transient quasi-zonal jets and their associated “storm tracks” represent a marked mode of oceanic low-frequency variability and may play an important role in the ocean component of the Earth’ climate system.

The effect of the jets and eddy organization on the horizontal transport and mixing of tracers is investigated using Lagrangian particle trajectories. Numerical experiments with “virtual” particles are conducted in the eastern North Pacific and utilize altimetry-derived velocity fields. To separate the effects of the large-scale flow, quasi-zonal jets and eddies, three different experiments are conducted, using (1) total velocity fields (include large-scale and mesoscale), (2) high-pass filtered velocity fields to retain only mesoscale phenomena (include zonal jets and eddies) and (3) synthetic eddy velocity fields composed of contributions of perfectly isotropic Gaussian eddies, but distributed in space and time as the observed mesoscale eddies in the eddy dataset. The results demonstrate that, in all three cases, spreading rates in the zonal direction systematically exceed spreading rates in the meridional direction. The zonal anisotropy is mainly due to quasi-zonal jets. The secondary and generally weaker effect is due to eddy organization. This effect is not negligible, however. Despite the fact that the jets are weak relative to mesoscale eddies, their role in localizing eddy pathways can be important for mixing contributions. Quantitative analysis of the dispersion curves also reveals significant deviations from the diffusive regime even on scales longer than half a year. These deviations are particularly pronounced in the synthetic eddy-only case, where the dispersion curves show super-diffusive regime at longer time lags. This is qualitatively consistent with the eddy organization into persistent “storm tracks”, localized on the jets, which results in the long-time Lagrangian velocity correlations.