Passive tracer advection in the equatorial Pacific region: statistics, correlations, and a model of fractional Brownian motion

Abstract

Abstract. Evaluating passive tracer advection is a common tool to study flow structures, particularly Lagrangian trajectories ranging from molecular scales up to the atmosphere and oceans. Here we report on numerical experiments in the region of equatorial Pacific (20° S–20° SN), where 6600 tracer parcels are advected from a regular initial configuration during periods of one year, 25 years altogether. We demonstrate that the strength of the advection exhibits a surprisingly large year by year variability. Furthermore an analysis of cross-correlations between advection strength and El-Niño and Southern Oscillation Indices (SOI) reveal a significant anti-correlation between advection intensity and ONI (Oceanic Niño Index) and a weaker positive correlation with SOI, both with a time lag of about 3 months (the two indices are strongly anti-correlated near real-time). The statistical properties of advection (first passage time, and mean squared displacement) suggest that the westward moving tracers can be mapped into a simple 1D stochastic process, namely fractional Brownian motion. We fit the model parameters and show by numerical simulations of the fractional Brownian motion model that it is able to well reproduce the observed statistical properties of the tracers' trajectories.