A Study Of Pinch-off Rings In The Kuroshio Extension Region

The Kuroshio Extension region is the key region of the mid-latitude air-sea interaction,and is also one of the most active regions of ocean eddies.The study of rings shed from the KE jet plays an important role in analyzing and understanding the transport of mass and energy in the ocean and the sea-air interaction.However,previous studies have not distinguished the rings shed from the KE jet,“meanders” and other mesoscale eddies in the region very well.A comprehensive description of the spatial distribution characteristics,motion properties,seasonal,interannual and decadal-like variations of rings shed from the Kuroshio Extension(KE)jet using AVISO satellite altimeter observations from 1993-2015.A total of 276 anticyclonic rings and 242 cyclonic rings shed from the KE jet in 23 years using the manual eddy detection method based on WA(Winding-Angle)method were identified.Pinched-off rings mostly distributed in the region west of the Shatsky Rise.According to the spatial distribution of the ring formations in the KE region,there were high-incidence areas of cyclonic rings.One is located at the upstream region between 144°E–146°E around the steady meander of the KE jet,the other is located at 156°E west of the Shatsky Ridge.There were high-incidence areas of anticyclonic rings,one located at downstream region west of the Shatsky Ridge and the other located at 148°E.These pinched-off rings in both the upstream and downstream regions generally propagated westward and the average moving speed of anticyclonic(cyclonic)ring is 3.52 cm/s and 3.89 cm/s respectively,but about 88% of the rings were reabsorbed by the jet.The lifetime of the ring is 50 d,the average radius of the cyclonic and anticyclonic ring is 92.4 km and 103.9 km respectively,and their average amplitudes is 35 cm and 32 cm,respectively.The number of rings showed substantial interannual to decadal-like variabilities.In the upstream and downstream KE regions,decadal-like and interannual variabilities was dominant,respectively.In the upstream region,these fluctuations of the ring formations were negatively correlated with the strength of the KE jet.In terms of seasonal variation,the most rings formed in summer,and the least in winter.On this basis,a comprehensive description of 3D structures of rings shed from the KE based on the objective analysis method were povided,using Argo temperature/salinity float profiles from the Ifremer,the JAMSTEC cruise and buoy data and climatological temperature/salinity data from NOAA.The heat transport of rings was also estimated.The cyclonic(anticyclonic)rings present consistent negative(positive)anomaly of potential temperature,while the depth of their maximum anomaly center gradually becomes shallower(deeper)as the potential temperature anomaly decreases from the west to the east.The composite cold ring between 140°E and 150°E has a double-core vertical structure due to the existence of mode water with low potential vorticity.The composite cyclonic(anticyclonic)ring has a relatively large negative(positive)salinity anomaly above 600 m,and has a relatively small opposite salinity anomaly below 600 m.The causes for temperature and salinity anomaly of the composite rings in each area of the KE region are not the same,which may be related to the intensity of background current and the temperature and salinity fields in each region.The average depth of rings' influences on temperature and salinity can be deeper than 1 000 m.The calculation of heat transport caused by rings based on the trajectory of rings showed that the meridional heat transport value of the rings is about 0.03 PW,which accounts for one third of the meridional heat transport of the eddy at the latitude of KE jet and makes a significant contribution to the meridional heat transport of the North Pacific.Finally,the distribution characteristics of the western Pacific subtropical mode water(NPSTMW)and the influence of the rings on it were studied.The results show that the average mixing layer depth and NPSTMW thickness in the anticyclonic eddies are thicker than those in the cyclonic eddies in KE regions,which indirectly proves that mode water is more easily generated in the anticyclonic eddies.The distribution of the intensity of low potential vorticity water in the composite rings shows that NPSTMW in the anticyclonic ring is significantly stronger than that in the cyclonic ring,and the high potential vorticity water mass is transported southward to the east of the anticyclonic ring,while the low potential vorticity water mass is confined to the center of it.The trapped depth in the anticyclonic ring is as high as 950 m and that in the cyclonic ring is 800 m.