Identification and census statistics of multicore eddies based on sea surface height data in global oceans
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Abstract: This study produced a statistical analysis of multicore eddy structures based on 23 years' altimetry data in global oceans. Multicore structures were identified using a threshold-free closed-contour algorithm of sea surface height, which was improved for this study in respect of certain technical details. Meanwhile a more accurate definition of eddy boundary was used to estimate eddy scale. Generally, multicore structures, which have two or more closed eddies of the same polarity within their boundaries, represent an important transitional stage in their lives during which the component eddies might experience splitting or merging. In comparison with global eddies, the lifetimes and propagation distances of multicore eddies were found to be much smaller because of their inherent structural instability. However, at the same latitude, the spatial scale of multicore eddies was found larger than that of single-core eddies, i.e., the eddy area could be at least twice as large. Multicore eddies were found to exhibit some features similar to global eddies. For example, multicore eddies tend to occur in the Antarctic Circumpolar Current, some western boundary currents, and mid-latitude regions around 25°N/S, the majority (70%) of eddies propagate westward while only 30% propagate eastward, and large-amplitude eddies are restricted mainly to reasonably confined regions of highly unstable currents.
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Key words:
- satellite altimetry /
- mesoscale eddy /
- multicore structure /
- eddy identification /
- eddy characteristic
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Figure 1. Eddy rotational velocities. a. Distribution of mean rotational velocities U on eddy contours with 1-cm interval in Gulf Stream region from SLA field on March 27, 2015. Arrows represent the surface geostrophic velocity components calculated from SLA, gray curves represent eddy boundaries determined by the UEC method, black lines represent eddy boundaries based on the maximum rotational speed method, and red points represent eddy cores that are the local extremum. The lines with color are the eddy contours with 1-cm internal that are normalized into perfect circles (the number of contours equals the eddy amplitude); the color means the value of mean rotational velocities U (cm/s). b. Variations of mean rotational velocity on eddy contours with distance to the core on March 27, 2015 globally. Each line represents an entire eddy structure with 1-cm contour interval. The numbers of the different type eddies are labeled in the panel.
Figure 2. Example of fitting a multicore eddy using an anisotropic two-dimensional Gaussian kernel in the SLA field. At the bottom, the black line represents the multicore eddy boundary, black asterisks represent the eddy centers, dots with color represent the SLA gridded points within the eddy interior (the colors reflect the value of the SLA), and lines with color represent the SLA contours with 2-cm intervals. The upper two independent Gauss surfaces G1(x, y) and G2(x, y) are fitted using the SLA gridded points, and the middle composite surface is the superposition of G1(x, y) and G2(x, y) with a basal SLA B (here B = –0.3 m). The fitting eddy scales σ’ are shown in the black circles at the top, and L is the distance between the composite eddy centers.
Figure 3. Eddies and multicore structures detected based on SLA map of October 3, 2009. Blue and red points represent cyclonic and anticyclonic eddies, respectively; blue and red lines represent the multicore eddy boundaries. There are 124 multicore cyclones and 125 multicore anticyclones in global oceans. The green block area in the western Pacific was selected to study the evolution of multicore eddies that was shown in Fig. 4.
Figure 4. SLA maps of eddy evolution from September 5 to October 19, 2009. Color shading represents the value of the SLA field; arrows represent the surface geostrophic velocity components calculated from the SLA; blue and red lines represent boundaries of cyclonic and anticyclonic eddies, respectively; blue and red dots represent cyclonic and anticyclonic eddy cores, respectively; and black lines represent multicore eddies. The multicore eddy structure Am persisted for 16 d from September 20 to October 5 before merging into a single-core eddy, and the multicore eddy structure Bm persisted for 13 d during October 2–14 before splitting into two eddies.
Figure 5. Upper-tail cumulative histograms of lifetimes (a) and propagation distances (b) of cyclonic and anticyclonic multicore eddies over a 23-year period (January 1993 to December 2015). The lower thick and thin dashed lines represent purely cyclonic and anticyclonic multicore eddies, respectively, for which no single-core eddy was matched through hybrid tracking.
Figure 6. Trajectories of cyclonic (blue lines) and anticyclonic (red lines) multicore eddies (abbreviate as “meddy-trajs”) over the 23-year period (January 1993 to December 2015) for all eddies (a), eddies with net eastward displacement (b), eddies with lifetime≥15 d (c), and eddies with lifetime≥ 30 d (d). The trajectories of purely multicore eddies for all (e) and lifetime≥ 30 d (f) are also given. The numbers of multicore eddies of each polarity are labeled at the top of each panel.
Figure 7. Maps of the number (upper), average amplitude (middle), and average scale/radius (lower) of multicore eddies for each 1° × 1° region of the World Ocean over the 23-year period (January 1993 to December 2015). Right-hand panels show meridional profiles of the average for multicore eddies (thick lines) and global eddies (thin lines) in 1° latitude bins (equatorial region with fewer eddies is not shown).
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