Tracking areas with increased likelihood of surface particle aggregation in the Gulf of Finland: A first look at persistent Lagrangian Coherent Structures (LCS)

TL;DR

This study investigates surface particle aggregation in the Gulf of Finland by comparing Lagrangian Coherent Structures and Finite-Time Compressibility to identify regions prone to patch formation, revealing seasonal and dynamic mixing patterns.

Contribution

It introduces a novel combined approach using LCS and FTC to identify potential pollution patch formation areas in the Gulf of Finland.

Findings

Backward FTLE shows seasonal variation in mixing strength.

Upwelling and downwelling regions have higher mixing intensity.

Combined FTC and LCS methods can predict surface patch formation.

Abstract

We explore the possibility to identify areas of intense patch formation from floating items due to systematic convergence of surface velocity fields by means of a visual comparison of Lagrangian Coherent Structures (LCS) and estimates of areas prone to patch formation using the concept of Finite-Time Compressibility (FTC, a generalisation of the notion of time series of divergence). The LCSs are evaluated using the Finite Time Lyapunov Exponent (FTLE) method. The test area is the Gulf of Finland (GoF) in the Baltic Sea. A basin-wide spatial average of backward FTLE is calculated for the GoF for the first time. This measure of the mixing strength displays a clear seasonal pattern. The evaluated backward FTLE features are linked with potential patch formation regions with high FTC levels. It is shown that areas hosting frequent upwelling or downwelling have consistently stronger than average mixing intensity. The combination of both methods, FTC and LCS, has the potential of being a powerful tool to identify the formation of patches of pollution at the sea surface.