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Dong C.,University of California at Los Angeles | Liu Y.,CAS South China Sea Institute of Oceanology | Liu Y.,University of Chinese Academy of Sciences | Lumpkin R.,National Oceanic and Atmospheric Administration | And 4 more authors.
Journal of Atmospheric and Oceanic Technology

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 closing curve 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. © 2011 American Meteorological Society. Source

Liu Y.,CAS South China Sea Institute of Oceanology | Liu Y.,University of Chinese Academy of Sciences | Dong C.,University of California at Los Angeles | Guan Y.,CAS South China Sea Institute of Oceanology | And 3 more authors.
Deep-Sea Research Part I: Oceanographic Research Papers

There are two zonal bands of eminently high eddy kinetic energy (EKE) in the North Pacific Ocean. The highest one is located in the Kuroshio Extension and the second one is in the subtropical area. This paper focuses on the latter. An eddy detection scheme based on velocity vector geometry is applied to the SSHA-derived geostrophic currents to identify and track eddies, and to generate an eddy dataset which includes spatial and temporal information on eddy generation, evolution and termination. Analysis of this dataset allowed the investigation of a broad range of eddy parameters. From 1993 to 2010, about six thousand eddies with lifetime longer than eight weeks were generated within the band. All eddies moved westward, and both cyclonic and anticyclonic eddies deflected northward south of 21°N and southward north of 21°N, respectively. Statistically, three different stages of an eddy's lifetime can be identified: the first one-fifth of its lifetime corresponds to the growing period; the successive three-fifths after that to its stable stage; the last one-fifth to its decaying period. Observed Argo vertical profiles collected within the detected eddy areas are used to investigate the eddy-induced vertical displacement of the thermocline and the halocline. Frontal intensity derived from the SST data is used to explain the mechanism modulating temporal and spatial eddy variations within the zonal band. © 2012 Elsevier Ltd. Source

Dong C.,State Key Laboratory of Satellite Oceanic Environment and Dynamics | Dong C.,University of California at Los Angeles | Dong C.,Nanjing University of Information Science and Technology | McWilliams J.C.,University of California at Los Angeles | And 2 more authors.
Nature Communications

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 Earth's 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. © 2014 Macmillan Publishers Limited. All rights reserved. Source

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