Lagrangian geography of the deep gulf of mexico

Abstract

Using trajectories from acoustically tracked (RAFOS) floats in theGulf ofMexico, we construct a geography of its Lagrangian circulation within the 1500-2500-m layer. This is done by building aMarkov-chain representation of the Lagrangian dynamics. The geography is composed of weakly interacting provinces that constrain the connectivity at depth. Themain geography includes two provinces of near-equal areas separated by a roughlymeridional boundary. The residence time is about 4.5 (3.5) years in the western (eastern) province. The exchange between these provinces is effected through a slow cyclonic circulation, which is well constrained in the western basin by preservation of f/H, where f is the Coriolis parameter and H is depth. Secondary provinces of varied shapes covering smaller areas are identified with residence times ranging from about 0.4 to 1.2 years or so. Except for the main provinces, the deep Lagrangian geography does not resemble the surface Lagrangian geography recently inferred from satellite-tracked drifter trajectories. This implies disparate connectivity characteristics with potential implications for pollutant (e.g., oil) dispersal at the surface and at depth. Support for our results is provided by a Markov-chain analysis of satellitetracked profiling (Argo) floats, which, while forming a smaller dataset and having seemingly different waterfollowing characteristics than theRAFOS floats, replicate the main aspects of the Lagrangian geography.Our results find further validation in independent results from a chemical tracer release experiment.