Mesoscale Eddies In The Kuroshio Extension Region From Satellite And In-situ Data

The Kuroshio Extension(KE)region(25oN-45 oN and 130oE-180oE)is one of the most energetic regions of the global ocean.The KE shed off vigorous mesoscale eddies toward north and south,interacting with the fronts,and other eddies originating from the subarctic and subtropical Pacific Ocean.All these eddies contribute to the meridional heat and salt transports,impacting the Kuroshio path variability,regional hydrography,water mass modification,marine biology,fisheries,and even storm tracks.A systematic study of mesoscale eddies in the KE is not only meaningful for the understanding of the material and energy balance in the northwest Pacific Ocean,but also beneficial to researches on mesoscale phenomena in other western boundary currents.Based on satellite remote sensing data and Argo floats,we investigate and characterize mesoscale eddies in the KE,including their property statistics,propagations,three-dimensional structures,heat and salt transports,as well as one specific eddy type,Anticyclonic Mode Water Eddy(ACME),south of Japan.The main achievements of this dissertation are listed as follows:(1)Eddies are identified based on 22-year satellite sea level anomaly(SLA)data using two different eddy detection methods(FAG and NEN);both methods are evaluated against each other.We verified the FAG method can detect more long-lived eddies,favorable to the automatic eddy tracking;and made one eddy dataset near the KE using the FAG method.(2)Mean eddy composites of SLA,sea surface temperature(SST)and salinity(SSS)are calculated.A westward shift is found for the mean eddy SST and SSS patterns relative to the mean SLA patterns.This asymmetry is larger for SST than SSS composites,larger for anticyclonic eddies(AEs)than cyclonic eddies(CEs),and larger south than north of the KE.(3)The vertical structures of eddies' temperature and salinity fields are analyzed.Eddies predominantly have subsurface(near-surface)intensified temperature and salinity anomalies south(north)of the KE.(4)To obtain the spatial distributions of heat and salt transports by eddy movements in a Lagrangian framework,a new method based on eddy trajectories and eddy structures is proposed,lessening the problem of low sampling number of Argo floats.The magnitude of eddy-induced meridional heat(freshwater volume)transport is several times of 0.01 PW(103 m3/s).The eddy heat flux divergence results in an oceanic heat loss south and heat gain north of the KE,thereby indicating northward eddy heat transports through the KE.(5)Based on altimetry and Argo float datasets,we found cold temperature anomalies in the upper 100 m for AEs near the Shikoku Recirculation Gyre(SRG),which is unusual for regular warm AEs.We demonstrate that AEs in this region are indeed dominantly by ACME instead of surface-intensified AEs by presenting the vertical structures of ACME,analyzing the SSTA values inside the cores of ACME,and studying the relationship between SSTAs and eddy amplitudes.Eddy merger and wintertime convection are supposed to contribute to the generation of ACMEs.During cold seasons,deeper mixed layer depths are found inside ACMEs compared with CEs or regions outside of eddies,which may be favorable for the generation of the North Pacific Subtropical Mode Water.The combined impacts of local topography and hydrography conditions(the SRG)help to attract AEs,making the AE merger more frequent,and generation of ACMEs more efficient.