Automated eddy detection methods are fundamental tools to analyze eddy activity from the large datasets derived from satellite measurements and numerical model simulations. Existing methods are either based on the distribution of physical parameters usually computed from velocity derivatives or on the geometry of velocity streamlines around minima or maxima of sea level anomaly. A new algorithm was developed based exclusively on the geometry of the velocity vectors. Four constraints characterizing the spatial distribution of the velocity vectors around eddy centers were derived from the general features associated with velocity fields in the presence of eddies. The grid points in the domain for which these four constraints are satisfied are detected as eddy centers. Eddy sizes are computed from closed contours of the streamfunction field, and eddy tracks are retrieved by comparing the distribution of eddy centers at successive time steps. The results were validated against manually derived eddy fields. Two parameters in the algorithm can be modified by the users to optimize its performance. The algorithm is applied to both a high-resolution model product and highfrequency radar surface velocity fields in the Southern California Bight.
Mesoscale eddies may play a critical role in ocean biogeochemistry by increasing nutrient supply, primary production, and efficiency of the biological pump, that is, the ratio of carbon export to primary production in otherwise nutrient-deficient waters. We examined a diatom bloom within a cold-core cyclonic eddy off Hawaii. Eddy primary production, community biomass, and size composition were markedly enhanced but had little effect on the carbon export ratio. Instead, the system functioned as a selective silica pump. Strong trophic coupling and inefficient organic export may be general characteristics of community perturbation responses in the warm waters of the Pacific Ocean.
[1] With eight islands, complex coastlines and bottom topography, strong wind curls, and frequent upwelling fronts, the Southern California Bight (SCB) is an area with strong eddy activity. By applying an automated eddy detection scheme to a 12 year high-resolution numerical product of the oceanic circulation in the SCB, a three-dimensional eddy data set is developed. It includes information for each eddy's location, polarity, intensity, size, boundary, and moving path at nine vertical levels. Through a series of statistical analyses applied to the eddy data set, three-dimensional statistical characteristics of mesoscale and submeoscale eddy variations in the SCB are elucidated; these shed light on how eddies are generated, evolve, and terminate. A significant percentage of eddies is found to be generated around islands and headlands along the coastline, which indicates that islands in the SCB play a vital role in eddy generation. Three types of eddies, based on shape, are identified from the numerical product: bowl, lens, and cone. A dynamic analysis shows that some submesoscale eddies with finite local Rossby numbers tend to be ageostrophic balanced while mesoscale eddies are in geostrophic balance. The present research results are useful for the interpretation of data sets obtained during the interdisciplinary Santa Barbara Channel Radiance in a Dynamic Ocean (RaDyo) field experiment conducted on September 3-25, 2008.
Cyclonic (anticyclonic) oceanic eddies drive local upwelling (downwelling), leaving footprints in the sea surface temperature (SST) field as local extremes. Satellite-measured SST images can therefore be used to obtain information of the characteristics of oceanic eddies. Remotely sensed measurements represent very large data sets, both spatially and temporally. Manual eddy detection and analysis are thus practically impossible. In this letter, an automated scheme for eddy detection from remote sensing SST data is presented. The method is based on the analysis of velocity fields derived from SST measurements (thermal-wind velocity field). Using the geometric features of the velocity field, we can identify positions of eddy centers and derive eddy size, intensity, path, and lifetime. The scheme is applied to a realistic remotely sensed SST data set in a strong eddy activity region: Kuroshio Extension region
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