Crossroads of the mesoscale circulation

Abstract

Quantifying the mechanisms of tracer dispersion in the ocean remains a central question in oceanography, for problems ranging from nutrient delivery to phytoplankton, to the early detection of contaminants. Until now, most of the analysis has been based on Lagrangian concepts of transport, often focusing on the identification of features that minimize fluid exchange among regions, or more recently, on network tools which focus instead on connectivity and transport pathways. Neither of these approaches, however, allows us to rank the geographical sites of major water passage, and at the same time, to select them so that they monitor waters coming from separate parts of the ocean. These are instead key criteria when deploying an observing network. Here we address this issue by estimating at any point the extent of the ocean surface which transits through it in a given time window. With such information we are able to rank the sites with major fluxes that intercept waters originating from different regions. We show that this allows us to optimize an observing network, where a set of sampling sites can be chosen for monitoring the largest flux of water dispersing out of a given region. When the analysis is performed backward in time, this method allows us to identify the major sources which feed a target region. The method is first applied to a minimalistic model of a mesoscale eddy field, and then to realistic satellite-derived ocean currents in the Kerguelen area. In this region we identify the optimal location of fixed stations capable of intercepting the trajectories of 43 surface drifters, along with statistics on the temporal persistence of the stations determined in this way. We then identify possible hotspots of micro-nutrient enrichment for the recurrent spring phytoplanktonic bloom occuring here. Promising applications to other fields, such as larval connectivity, marine spatial planning or contaminant detection, are then discussed. arXiv:1802.02743v4 [physics.ao-ph]

Explicit and implicit network connectivity: Analytical formulation and application to transport processes

“…Since water (air, respectively) parcels carry numerous particulate and dissolved substances, connectivity is tantamount to evaluating how any almost-passive tracer is transported and dispersed by the oceanic (atmospheric, respectively) circulation. As such, relevant applications of transport networks already include studying the spread of oceanic tracers [41,42], micro-plastics [43], biological propagules [44][45][46][47] and of atmospheric pollutants [48].…”

Lagrangian betweenness as a measure of bottlenecks in dynamical systems with oceanographic examples

“…From a more applied perspective, the effectiveness of betweenness in assessing systems sensitivity could also contribute to the design of optimized observing and monitoring systems on targeted areas of the ocean 57 , 58 . High-betweenness hotspots could indeed provide early-warning signals to anticipate when and where the marine environment would be affected by multiple stressors such as pollutants, pathogens, invasive species, or climatic-induced threats.…”

The streaming of plastic in the Mediterranean Sea

“…In the present work, instead of focusing on areas that are a final destination for plastic debris, we target regions through which large quantities of plastic debris pass (plastic crossroads); plastic debris converges to these crossroads but then passes on. We combine (i) the largest field dataset of plastic concentrations in the Mediterranean Sea to date from the Tara Expedition 16 (called here the in situ plastic concentrations); (ii) the implementation of a recent numerical model, TrackMPD, to simulate the plastic-debris paths at sea 17 ; and (iii) a particular Lagrangian diagnostic, the crossroadness, used to identify the plastic crossroads 18 . The Tara Expedition covered, for the first time, the whole Mediterranean basin (122 sampling stations, Fig.…”