Sensitive dependence of trajectories on tracer seeding positions – coherent structures in German Bight backward drift simulations

Ulrich Callies

doi:10.5194/os-17-527-2021

Abstract

Abstract. Backward drift simulations can aid the interpretation of in situ monitoring data. In some cases, however, trajectories are very sensitive to even small changes in the tracer release position. A corresponding spread of backward simulations implies attraction in the forward passage of time and, hence, uncertainty about the probed water body's origin. This study examines surface drift simulations in the German Bight (North Sea). Lines across which drift behaviour changes non-smoothly are obtained as ridges in the fields of the finite-time Lyapunov exponent (FTLE), a parameter used in dynamical systems theory to identify Lagrangian coherent structures (LCSs). Results closely resemble those obtained considering two-particle relative dispersion. It is argued that simulated FTLE fields might be used in support of the interpretation of monitoring data, indicating when simulations of backward trajectories are unreliable because of their high sensitivity to tracer seeding positions.

Highlights

A comprehensive monitoring network is operated in the German Bight area (North Sea), including the Marine Environmental Monitoring Network in the North Sea (MARNET), the Coastal Observing System for the North and Arctic Seas (COSYNA) and other stations
Lines across which drift behaviour changes non-smoothly are obtained as ridges in the fields of the finite-time Lyapunov exponent (FTLE), a parameter used in dynamical systems theory to identify Lagrangian coherent structures (LCSs)
The analysis of backward surface tracer simulations in the German Bight region revealed the intermittent presence of linear structures (LCSs) across which the past history of water bodies substantially changes

Summary

Introduction

A comprehensive monitoring network is operated in the German Bight area (North Sea), including the Marine Environmental Monitoring Network in the North Sea (MARNET), the Coastal Observing System for the North and Arctic Seas (COSYNA) and other stations. Chen et al, 2016; Kim and Hwang, 2020) When it comes to the interpretation of specific data, a merely statistical description of spatial connectivity falls short of what can be achieved if hydrodynamic current fields from either models or remote sensing are available. Backward tracer trajectories seeded at monitoring stations provide insight into the background of water bodies that were probed (e.g. Supplement Video S1; d’Ovidio et al, 2015; Lucas et al, 2016; Teeling et al, 2012). They help distinguish between temporal and spatial variability, i.e. local changes and advection from somewhere else. The present study focuses on this latter aspect of coherent structures shaping the separation of simulated backward trajectories

Methods

Results

Discussion

Conclusion