Submesoscale Eddies in the Taiwan Strait Observed by High-Frequency Radars: Detection Algorithms and Eddy Properties

Abstract

This study compared the efficiencies of two widely used automatic eddy detection algorithms—that is, the winding-angle (WA) method and the vector geometry (VG) method—and investigated the submesoscale eddy properties using surface current observations derived from high-frequency radars (HFRs) in the Taiwan Strait. The results showed that the WA method using the streamline and the VG method based on the streamfunction field have almost the same capacity for identifying eddies, but the former is more competent than the latter in capturing the eddy size. The two algorithms simultaneously identified 1080 submesoscale eddies, with the centers and boundaries determined only by the WA method, and they were further used to investigate the eddy properties. In general, no significant difference was observed between the cyclonic and anticyclonic eddies in terms of radius, life span, and kinematics, as well as the evolution during their life cycles. The typical radius of the eddy in this region was 3–18 km. And a strong correlation was observed between the life span and the radius. The spatial distribution of the eddies indicated that topography played a significant role in the generation of the eddies. And the trajectories of the eddies suggested that all the eddies in this area mostly tended to move southeastward. Statistically, three different stages of the eddy’s life span could be identified by the significant growth and decay of the radius and the mean kinetic energy. This study shows the great capability of HFRs in oceanography research and applications, especially for observing the submesocale dynamics.

Spectral Descriptions of Submesoscale Surface Circulation in a Coastal Region

“…Although technological advances have resulted in increases in the density of sampling rates over time and space and the development of the Lagrangian sampling strategy, which involves following and crossing the target phenomena, these observations have mainly focused on specific submesoscale events and phenomena and are available for a limited number of realizations (e.g., Baschek & Molemaker, ; Callies et al., ; D'Asaro et al., ). In contrast, operational and concurrent Eulerian observations using remote sensing instruments can thoroughly investigate submesoscale processes via statistical analyses using available data such as high‐frequency radar (HFR)‐derived surface currents (e.g., Kim et al., ; Kirincich, ; Lai et al., ) and geostationary ocean color imagery (GOCI)‐derived surface concentration maps of chlorophyll, total suspended solids, and colored dissolved organic matter (e.g., Choi et al., ), which are available at hourly and kilometer‐scale resolutions and are abundant relative to classic in situ measurements.…”

Multi-Sensor Observations of Submesoscale Eddies in Coastal Regions

“…This method is suitable for measuring the velocity field. It has been successfully applied to different study regions, such as the leeward side of Hawaii [42], the subtropical zonal band of the North Pacific Ocean [43], the Taiwan Strait [18], and the Kuroshio Extension Region [44]. We invite the reader to refer to Nencioli et al [33] for more details.…”

“…However, in recent years, high-precision observations have mainly focused on some special submesoscale events and phenomena, due to increased sampling rates in time and space [6,11]. Remote-sensing observations can also be used to investigate SPs, such as surface currents derived by high-frequency radar [2,[17][18][19], geostationary ocean color imagery (GOCI)-derived chlorophyll surface concentration maps, and colored dissolved organic matter [20], which are available at hourly and kilometer-scale resolutions. HFRs can provide information on ocean surface currents over a large, horizontal distance with high spatial-temporal resolution, which can reach up to 200 m in space (e.g., SeaSonde [21]) and 20 min in time.…”

Ocean Surface Current Observation with a Dual Monopole-Cross-Loop Antenna Array

“…Many studies of HFR surface current measurements have validated the capacity of the direction-finding HFR to remote sensing the ocean surface currents through comparisons with in situ current measurements, such as [8][9][10][11][12]. These studies demonstrated that there is a 7-20 cm/s differences between the current measurements derived from the directionfinding HFRs and those from the in situ instrument.…”