Statistical analysis of long-lived mesoscale eddies in the Lofoten basin from satellite altimetry

摘要

The paper presents a statistical analysis of long-lived detected mesoscale eddies in the Lofoten Basin (LB). An automated eddy identification and tracking method is applied to the altimeter data during the time-period 1993-2017 to detect and track anticyclonic (ACEs) and cyclonic eddies (CEs) in the LB. Our analysis found that only one percent of the eddies detected are long-lived (eddy-life > 35 days). Even though only 1%, the detected 330 long-lived mesoscale eddies (CEs, 120; ACEs, 210) account for >11,550 daily individual eddy-observations. The lifetime, occurrence, generation sites, size, intensity, and drift of these long-lived eddies are quantified. The average drift speed of long-lived eddies is found to show a pronounced seasonal variation with a maximum from October to March. Long-lived eddies in the LB are further divided into four groups based on their region of generation and dissipation. Long-lived eddies generated and dissipated outside the region of the Lofoten Vortex (group 2 eddies) are found to be the predominant type (CEs, 73.3%; ACEs, 69.5%). The eddies generated and dissipated in the region of the permanent Lofoten Vortex form the second dominant type (CEs, 14.3%; ACEs, 26.2%). Based on their lifetime, properties of the two predominant groups of eddies are examined in detail. The difference found in the temporal variability of the eddy characteristics of the two groups reflects their different genesis. The analysis revealed that the mesoscale eddies of group 2 have a longer life than eddies of group 1, and ACEs are more long-lived in comparison to CEs. The analysis also found three main areas of eddy generation in the frontal zone of the NwASC from where mesoscale eddies propagate to the north-west, forming three main corridors of trajectories. The study further provides evidence of long-lived cyclonic CEs surrounding the large quasi-permanent Lofoten Vortex (LV) and forming a shield around it. Small CEs located in two areas with centers at 69.5° N, 4° E, and 70° N, 2.5° E survive nearby the LV resisting the vortex-vortex interaction with the large and strong LV.