Study On Mesoscale Eddy In Kuroshio Extension Region And Their Effect On Chlorophyll–a

The Kuroshio extension?KE?region is one of the regions with the most frequent mesoscale eddy activities.Combined with satellite remote sensing data and in-situ data,this paper makes a long-term and seasonal statistical analysis of the mesoscale eddies in the KE region from 1993 to 2018 by using the Vector Geometry?VG?method based on the sea surface flow field.The life history process and three-dimensional structural characteristics of a typical strong mesoscale cold eddy in March 2019 are reproduced,and the seasonal characteristics and physical mechanism of the influence of mesoscale eddy on the surface chlorophyll concentration distribution are preliminarily explored in this paper.According to the statistical results,a total of 1759 eddies with a lifetime more than4 weeks were detected in the target area,including 962 anticyclones eddies and 797cyclones.The average lifetime of all eddies is 7.8 weeks,the number of eddies decreases exponentially with the increase of lifetime,and 82%of the eddies have a lifetime less than 10 weeks,only 1%of them have a lifetime more than 30 weeks.In the long lifetime?more than 30 weeks?eddy,the number of anticyclone is more than twice that of cyclone.By the sum of the times of each detection time,There are 14,382 eddies,including 8275 anticyclones and 6007 cyclones.The statistical analysis of the detected eddies shows that the probability distribution of the eddies radius obeys the structure similar to the normal distribution.Most of the eddies have a radius of 40-80 km,and a few have a radius of more than 180 km.The polar distribution of eddies shows that mesoscale eddies is predominantly anticyclonic in north of the KE and cyclonic in south of it.The average eddy kinetic energy,eddy number and radius in the target area have significant seasonal characteristics.Generally speaking,they show the characteristics of"strong in summer,weak in spring and winter".The number and radius of anticyclone in each season are more than those of cyclone.Thereafter,the three-dimensional structural characteristics of the typical mesoscale cold eddy with the longest lifetime in the first half of 2019 are studied by using the in-situ data of"Song Hang"ship near the Kuroshio Extension flow axis in March 2019 by Shanghai Ocean University.Its average radius is 58.37 km,the average energy intensity?EI?is 4.7×10^-1 cm²/?s² km²?,and the average moving speed is 8.13 cm/s;Vertical temperature-salinity structure of this eddy exhibits a strong upwelling effect with an influence depth exceeding 900 m;the cold core is located in the water layer between 250m and 350 m,where the maximal temperature anomaly is up to-6.2 ^oC.Moreover,results of biological survey in the same time show that the zooplankton biomass was the most abundant at the eastern boundary of this eddy.The mesoscale eddy is one of the most important factors influencing the distribution of chlorophyll–a?Chl-a?in the Kuroshio extension of the Northwest Pacific.Based on statistical analysis of the weekly remote-sensing ocean surface Chl-a concentration during the period of 2012-2018 from NOAA and the data of geostrophic current from COPERNICUS,we found that the influence of mesoscale eddy on the surface Chl-a concentration shows significant seasonal characteristics.About 17%of the cyclonic eddies in Spring have high Chl-a concentration in the center and propagate westward.However,the cyclonic eddies in other seasons have little effect on the Chl-a concentration.The anticyclonic eddies generate high Chl-a ring in both spring and autumn,with little effects on Chl-a concentration in summer and winter.The seasonal characteristics of Chl-a in mesoscale eddies may relate to seasonal nutrient-limitation,light-limitation or the change of pycnocline,indicating that wind-and flow-induced vertical nutrient transport cannot be the only factor to study.Chl-a concentration can represent phytoplankton biomass in some extent,which is important for the ecosystem.Thus,this seasonal feature of mesoscale eddy effect should be considered in the studies for fisheries.