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 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.

我们探索了通过拉格朗日相干结构（LCS）的视觉比较，并通过有限时间的概念来估计由于表面速度场的系统收敛而从漂浮物中形成强烈斑块的区域的可能性，并估计了易于形成斑块的区域可压缩性（FTC，发散时间序列的概念的概括）。使用有限时间李雅普诺夫指数（FTLE）方法评估LCS。测试区域是波罗的海的芬兰湾。第一次为GoF计算了全盆地范围内向后FTLE的空间平均值。混合强度的这种度量显示出明显的季节性模式。评估的后向FTLE特征与具有高FTC水平的潜在补片形成区域相关联。结果表明，经常发生上涌或下涌的区域始终比平均混合强度强。FTC和LCS两种方法的结合具有成为识别海面污染斑块的有力工具的潜力。