Changming Dong

A Vector Geometry–Based Eddy Detection Algorithm and Its Application to a High-Resolution Numerical Model Product and High-Frequency Radar Surface Velocities in the Southern California Bight

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.

Global heat and salt transports by eddy movement

Oceanic mesoscale eddies contribute important horizontal heat and salt transports on a global scale. Here we show that eddy transports are mainly due to individual eddy movements. Theoretical and observational analyses indicate that cyclonic and anticyclonic eddies move westwards, and they also move polewards and equatorwards, respectively, owing to the β of Earths rotation. Temperature and salinity (T/S) anomalies inside individual eddies tend to move with eddies because of advective trapping of interior water parcels, so eddy movement causes heat and salt transports. Satellite altimeter sea surface height anomaly data are used to track individual eddies, and vertical profiles from co-located Argo floats are used to calculate T/S anomalies. The estimated meridional heat transport by eddy movement is similar in magnitude and spatial structure to previously published eddy covariance estimates from models, and the eddy heat and salt transports both are a sizeable fraction of their respective total transports.

Island Wakes in Deep Water

Abstract Density stratification and planetary rotation distinguish three-dimensional island wakes significantly from a classical fluid dynamical flow around an obstacle. A numerical model is used to study the formation and evolution of flow around an idealized island in deep water (i.e., with vertical island sides and surface-intensified stratification and upstream flow), focusing on wake instability, coherent vortex formation, and mesoscale and submesoscale eddy activity. In a baseline experiment with strong vorticity generation at the island, three types of instability are evident: centrifugal, barotropic, and baroclinic. Sensitivities are shown to three nondimensional parameters: the Reynolds number (Re), Rossby number (Ro), and Burger number (Bu). The dependence on Re is similar to the classical wake in its transition to turbulence, but in contrast the island wake contains coherent eddies no matter how large the Re value. When Re is large enough, the shear layer at the island is so narrow that the ver...

SST–Wind Interaction in Coastal Upwelling: Oceanic Simulation with Empirical Coupling

Observations,primarilyfromsatellites,haveshownastatisticalrelationshipbetweenthesurfacewindstress and underlying sea surface temperature (SST) on intermediate space and time scales, in many regions inclusive of eastern boundary upwelling current systems. In this paper, this empirical SST‐wind stress relationship is utilized to provide a simple representation of mesoscale air‐sea coupling for an oceanic model forced by surface winds, namely, the Regional Oceanic Modeling System (ROMS). This model formulation is applied to an idealized upwelling problem with prevailing equatorward winds to determine the coupling consequencesonflow,SST,stratification,andwindevolutions.Theinitiallyuniformwindfieldadjuststhrough coupling to a cross-shore profile with weaker nearshore winds, similar to realistic ones. The modified wind stress weakens the nearshore upwelling circulation and increases SST in the coastal zone. The SST-induced wind stress curl strengthens offshore upwelling through Ekman suction. The total curl-driven upwelling exceeds the coastal upwelling. The SST-induced changes in the nearshore wind stress field also strengthen and broaden the poleward undercurrent. The coupling also shows significant impact on the developing mesoscale eddies by damaging cyclonic eddies more than anticyclonic eddies, which leads to dominance by the latter. Dynamically, this is a consequence of cyclones with stronger SST gradients that induce stronger wind perturbations inthis particularupwellingproblemandthatarethereforegenerally moresusceptibletodisruption than anticyclones at finite Rossby number. The net effect is a weakening of eddy kinetic energy.

An Automated Approach to Detect Oceanic Eddies From Satellite Remotely Sensed Sea Surface Temperature Data

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.

Three‐dimensional oceanic eddy analysis in the Southern California Bight from a numerical product

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 eddys 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.

A Model Study of Internal Tides in Coastal Frontal Zone

Abstract Internal tides near a midlatitude shelf–slope front are studied using an idealized numerical model, with emphasis on their structure, energetics, and mixing effects. It is found that the properties of internal tides are highly dependent on frontal configuration and tidal frequency. At a winter front, energetic internal tides are generated and arrested in the frontal zone; the cross-shelf flow tends to be surface (bottom) intensified by a large internal circulation cell at the diurnal (semidiurnal) frequency. At a summer front, the diurnal internal tide is still trapped, but a semidiurnal internal tide propagates out of the frontal zone in the offshore direction while arrested at the inshore boundary. The presence of the shelf–slope front enhances the generation of internal tides, and it also causes an amplification of the semidiurnal internal tide by trapping its energy in the frontal zone. This amplification is most prominent at the offshore boundary of the winter front and the inshore boundary ...

A Scheme to Identify Loops from Trajectories of Oceanic Surface Drifters: An Application in the Kuroshio Extension Region

When a drifter is trapped in an eddy, it makes either a cycloidal or a looping trajectory. The former case takes place when the translating speed is larger than the eddy spinning speed. When the background mean velocity is removed, drifter trajectories make loops. Thus, eddies can be detected from a drifter trajectory by identifying looping segments. In this paper, an automated scheme is developed to identify looping segments from Lagrangian trajectories,based on a geometric definition of a loop, that is, a closingcurve with its starting point overlapped by its ending point. The scheme is to find the first returning point, if it exists, along a trajectory of a surface drifter with a few other criteria. To further increase the chance that detected loops are eddies, it is considered that a loop identifies an eddy only when the loop’s spinning period is longer than the local inertial period and shorter than the seasonal scale, and that at least two consecutive loops with the same polarity that stay sufficiently close are found. Five parameters that characterize an eddy are estimated by the scheme: location (eddy center), time (starting and ending time), period, polarity, and intensity. As an example, the scheme is applied to surface drifters in the Kuroshio Extension region. Results indicate that numbers of eddies are symmetrically distributed for cyclonic and anticyclonic eddies, mean eddy sizes are 40‐ 50 km, and eddy abundance is the highest along the Kuroshio path with more cyclonic eddies along its southern flank.

The pattern and variability of winter Kuroshio intrusion northeast of Taiwan

The variations of the Kuroshio path and velocity northeast of Taiwan are analyzed based on along-track satellite altimeter data as well as high-resolution model experiments. Observations reveal that in winter the Kuroshio intrusion into the East China Sea (ECS) at this location is manifested by a secondary maximum current core (SMCC) shoreward of the Kuroshios main path. The SMCC varies significantly on interannual time scale, and its variability is strikingly out of phase with that of the Kuroshio entering the ECS, meaning that the stronger the Kuroshio, the weaker the SMCC, and vice versa. Model experiments corroborate the observational results and, more importantly, indicate that the Kuroshio intrusion here follows two primary routes, a large anticyclonic loop that separates from the Kuroshio at the northern end of Taiwan and moves forward to form the SMCC, and a straight northward path onto the shelf when the Kuroshio turns sharply eastward along the continental slope of the ECS. The intrusion is controlled by both local forcing and remote effect, with its pattern and variability depending mostly on the local heat flux and the inertia of the Kuroshio Current.

Tidally Induced Cross-Frontal Mean Circulation: Analytical Study*

An analytical model is developed to study the tidally induced mean circulation in the frontal zone. Four distinct forcing mechanisms are identified, which result in the generation of the counterclockwise Bernoulli cell, the clockwise Ekman cell, the clockwise frontal cell, and the Stokes drift (facing in the direction with the shallow water to the left). The decomposition of the cross-frontal circulation provides a dynamical framework for interpreting and understanding its complex structure. To illustrate the underlying physics, three model configurations are considered pertaining to a homogenous ocean and winter and summer fronts. For a homogeneous ocean, the circulation is dominated by three cells; for the winter front, the offshore Bernoulli cell is strengthened; and for the summer front, two counterrotating cells are found in the vertical direction, associated with the two branches of the front. The dependence of the cell structure on the Ekman, Burger, and other dimensionless numbers is examined.