Spatiotemporal features and vertical structures of four types of mesoscale eddies in the Kuroshio Extension region
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Abstract: Except for conventional mesoscale eddies, there are also abundant warm cyclonic eddies (WCEs) and cold anticyclonic eddies (CAEs) in the global ocean. Based on the global mesoscale eddy trajectory atlas product, satellite altimetric and remote sensing datasets, and three-dimensional temperature/salinity dataset, spatiotemporal features of WCEs and CAEs are compared with traditional cold cyclonic eddies and warm anticyclonic eddies in the Kuroshio Extension (KE; 28°−43°N, 140°−170°E) region. Characteristics of abnormal eddies like radius, amplitude, eddy kinetic energy, and proportion in all eddies behave in significant asymmetry on the north and south sides of the KE jet. Unlike eddies in the general sense, temporal feature analysis reveals that it is more favorable to the formation and maintenance of WCEs and CAEs in summer and autumn, while winter is the opposite. The spatiotemporal variation of abnormal eddies is likely because the marine environment varying with time and space. Statistically, proportion of abnormal eddies increases rapidly in decaying stage during the whole eddy lifespan, resulting in smaller average radius, amplitude, sea surface temperature anomaly and sea surface height anomaly compared to normal ones. The three-dimensional composite structures for four types of eddies expose that the difference between abnormal and conventional eddies is not just limited to the sea surface, but also exists within the water below the sea surface. Vertical structures also indicate that the anomalous temperature signal is confined in the water from the sea surface to layers at about 30 m in the KE region.
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Figure 1. Distribution of average eddy kinetic energy (EKE, shading) and absolute dynamic topography (ADT, contours) in the Kuroshio Extension jet and its vicinity (a), correlation coefficient of mesoscale sea surface height anomaly (SSHA) and sea surface temperature anomaly (SSTA) (the dotted region means passing the significance test of 0.01) (b), climatological monthly regional-average EKE (c), and annual cycle of spatial correlation coefficient of mesoscale SSHA and SSTA (d). The shadings in c and d indicate the standard error.
Figure 2. Scatter plots of sea surface height (SSH) anomaly and sea surface temperature (SST) anomaly averaged inside the eddy core corresponding to CE (a) and AE (b). The color of scatter plots denotes the proportion
$ {\text{γ}} $ b of abnormal points within one radius from the eddy center.
Figure 3. The evolution process of the average T'in (blue curves) and proportion
$ {\text{γ}}$ b (red curves) inside the eddy core during the lifetime of a CE (a) and an AE (b). The orange shadings mark the abnormal eddy duration periods according to the definition conditions in this study.
Figure 4. The distribution of zonal average radius (a), amplitude (b), eddy kinetic energy (EKE, c) and EKE intensity (EI, d) for each type of eddies in 0.5°-width latitudinal bands. WCE, warm cyclonic eddy; CAE, cold anticyclonic eddy; CCE, cold cyclonic eddy; WAE, warm anticyclonic eddy.
Figure 5. Seasonal variation of the occurrence frequency of four types of eddies. a. Proportion of WCEs in all cyclonic eddies in each 0.5° latitudinal bands in four seasons; b. the same with a, but for CAEs in all anticyclonic eddies; c. average occurrence numbers for four types of eddies in each month; d. average proportion of abnormal eddies in all eddies in each month. WCE, warm cyclonic eddy; CAE, cold anticyclonic eddy; CCE, cold cyclonic eddy; WAE, warm anticyclonic eddy.
Figure 6. The proportion of abnormal eddies (a) and normal eddies (b) in total eddies with corresponding polarity. WCE, warm cyclonic eddy; CAE, cold anticyclonic eddy; CCE, cold cyclonic eddy; WAE, warm anticyclonic eddy.
Figure 7. Average potential temperature and salinity characteristics in study area for four seasons. The black contour lines represent potential density, the thin colored lines represent the region north of 35°N, while the thick ones represent the region south of it.
Figure 8. Composite sea surface temperature (SST) anomaly (color shading) and sea surface height (SSH) anomaly (contours, unit: cm) of WCEs (a), CCEs (b), CAEs (c) and WAEs (d). IAE, index of abnormal eddy.
Figure 9. Vertical profiles of the temperature anomaly averaged within the radius (a) and maximum Vg anomaly on each level of the composite eddy structures (b). WCE, warm cyclonic eddy; CAE, cold anticyclonic eddy; CCE, cold cyclonic eddy; WAE, warm anticyclonic eddy.
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