| The Western Subarctic Front?WSAF?in Kuroshio-Oyashio Confluence Region?KOCR?affects water generation,material transport and three-dimensional temperature,salinity and current field of fine structure.Based on satellite remote sensing data,HYCOM reanalysis data and oceanographic surveying data,the intensity and location of the Western Subarctic Front?WSAF?,seasonal variation,thermohaline finestructure and mixing characteristics in frontal zone are analyzed,the specific works are as follows:1.Analyse the seasonal variation of Western Subarctic FrontThe average position of the subarctic front?SAF?is mainly located in 143°-172°E,38°-44°N,there are East and West frontal zones in SAF in different seasons,and the eastern front intensity is weaker than the western front.WSAF is mainly located in 146°-152°E,37°-45°N,the intensity of winter WSAF are highest in different seasons,the strongest intensity of winter is 0.069?/km;the intensity of summer WSAF are lowest in different seasons,the strongest intensity of summer is 0.043?/km,and the maximums are stable located in 149°E in all seasons;when average mixing rate in KOCR is high,the water properties close to the Kuroshio Extension,and WSAF intensity is weak,the position is north;when average mixing rate in KOCR is low,Oyashio water are increased in KOCR,the Kuroshio Extension characteristic is not significant,and WSAF intensity is strong,the position is south,this result shows that WSAF intensity in different seasons is closely related to water properties of KOCR.2.Reveal three dimensional fine structure of WSAF in spring 2016?1?WSAF is located in 143°-153°E,38°-42°N,the front shows a V distribution,and the maximum intensity is 0.08?/km,the temperature is largest?17??at 147°E,37°N,the geostrophic velocity of the south side of the frontal zone is strong,the maximum is 0.8m/s?149°E,39°N?,the geostrophic velocity of the north side of the frontal zone is weak.?2?Based on HYCOM reanalysis data,the distribution characteristics of WSAF in june 1-2 are analyzed,during this period,WSAF is mainly located in 143°-154°E,38°-42.5°N,the front shows a V distribution,and the maximum intensity in sea surface is located in 143°-146°E,41°-42.5°N,the maximum is 0.15?/km;at 100m,the front intensity is significantly enhanced,the maximum is higher than 0.15?/km,the entire distribution of WSAF is inclined to the south,the southernmost reaches 37.5°N;at500m,WSAF disappears.?3?The results of three main Sections of WSAF situ fine observation shows that,with increasing depth,the temperature front narrows from width to width in Section S01,and inclines to the south,the front is widest?30km?at 30m,WSAF disappears at 400m,and the region with the highest front intensity is mainly located in 200m shallow;the average width of temperature front is about 20km in Section S02,the narrowest width of front is about 18km at sea surface,the temperature front widens from narrow,and inclines to the south with increasing depth,WSAF disappears at 400m,and the region with the highest front intensity is located in the 100-280m water south of 38.3°N;the temperature front of Section S03?147°E,38.1°-38.97°N?shows a"double front?north and south?"structure over 250m,the average width of the south temperature front is about 20km,the narrowest width of front is about 16km at sea surface,the front widens from narrow,and inclines to the south with increasing depth,WSAF disappears at 560m,front intensity enhance with increasing depth,the maximum intensity is located in 450m;the front north of 38.4°N is mainly located in shallow 250m,the front widens from narrow with increasing depth,the narrowest part?9km?is located in 50m,the widest part?21km?is located in 150m,the region with maximum intensity is at 38.7°N,30m.Moreover,the location of salinity front is the same as the temperature front in all Sections,this feature is familiar with the Roden's study about SAF.During the whole observation period,there are Oyashio water,Kuroshio extension water and North Pacific Intermediate Water?NPIW?in WSAF,the cold and fresh Oyashio is on the north side of the front,and the warm and salty Kuroshio extension is on the south side of the frontal zone,fresh NPIW is located in water between 26.5 and 26.9??,simultaneously,Oyashio surface water is characterized by high temperature and low salinity,which is not present in the HYCOM reanalysis data;Kuroshio extension on the south side of WSAF has a strong current,the maximum is 1.9m/s,and Oyashio on the north side is weak,with an average velocity of about 0.4m/s;The current direction in Sections S01 and S02 are northeastward,it is different from HYCOM data and sea surface geostrophic current data,that the current in Sections S03-S06 are ageostrophic with northwest direction,affectd by a anticyclone eddy.3?Explore thermohaline finestructure of WSAF in spring 2016In the spring of 2016,thermohaline intrusions,thermohaline staircase and double-diffusion characteristics of WSAF show that:?1?Thermohaline intrusions is ubiquitous in WSAF,mainly in the region shallower 200m and deeper than 350m?NPIW?,and intrusions above 200m level are strong?±0.1-±0.2 Psu?,the vertical scale is 10-40m,the horizontal scale is about 10-15km;intrusions in NPIW are weak?±0.05-±0.1 Psu?,the vertical scale is about 40-70m,the horizontal scale is 20-30km.?2?Thermohaline staircase is distributed sporadically in WSAF shallower than 200m and deeper than 350m?NPIW?.The vertical scale of thermohaline staircase above 200m level is about 5m,with high temperature and high salinity feature above the staircase and low temperature and low salinity feature under the staircase;the vertical scale of thermohaline staircase in NPIW is about 5-20m,the shape of the staircase is clear,with low temperature and low salinity feature above the staircase and high temperature and high salinity feature under the staircase.?3?Turner angle in region shallower than 26.5??is higher than 45°,which is beneficial to the development of salt fingers?SF?type double-diffusion;turner angle of water deeper than 26.5??is generally lower than-45°,which is beneficial to the development of diffusive-layer?DL?type double-diffusion,therefore,thermohaline staircase is mainly SF type staircase in water shallower than 200m,thermohaline staircase is mainly DL type staircase in NPIW.Compared with the double-diffusive significant sea area such as the Arctic Ocean,the spatial continuity of thermohaline staircase structure in WSAF is poor,the shape of staircase is not obvious,and the vertical scale is small.?4?Thermohaline intrusions in the whole WSAF sea area is mainly caused by DL type double-diffusion,in addtion,upper ocean is also caused by SF type double-diffusion,as well as inertial instability,possible near-inertial internal waves?NIWs?may affect the intrusions in the front shallower than 200m near 147°E in the north of 38.5°N.4?Quantificationally analyse mixing characteristics of WSAF in spring 2016Through the calculation of turbulent eddy diffusivity?K??,thermal diffusivity?K??and other parameters,the mixing characteristics of WSAF are analyzed,the results show that:?1?the mean value of K?is about 10-4.2m2/s,and that of K?is about 10-5m2/s;therefore,the diapycnal mixing of WSAF is generally the turbulent mixing;the strong turbulent mixing region(K?>10-3.5m2/s)is mainly distributed in water layers of26.1-26.7??along the front.Furthermore,the K?in the water layers of 25-26.1??are10-4.5m2/s,higher than K?(10-5.5m2/s),and turner angle is generally higher than 45°,thus the diapycnal mixing of this region is mainly caused by SF type double-diffusion;There are some DL type thermohaline staircase in partial water layer deeper than 26.5??,in this water layer,the diapycnal mixing is mainly caused by DL type double-diffusion.The mean value of K?in the south,middle and north of WSAF are 10-4.2?10-4.1?10-4m2/s,respectively,the mean value of K?in the south,middle and north of WSAF are10-5?10-5.1?10-5.2m2/s,indicate that the turbulent mixing is stronger in the north,and weaker in the south,double-diffusion mixing is stronger in the siuth,and weaker in the north.?2?Symmetrical instability?SI?occurs mainly in the water layer near the front of the 25.5-26.7??in WSAF,and SI is the main cause of turbulent mixing enhancement in the frontal zone;there are also many scattered water layers in the state of shear instability?KI?in water deeper than 100m,147°E,38.15°-38.5°N and between350-500m,near 147°E,38.65°N,indicate that enhancement of turbulent mixing is mainly caused by KI;meanwhile,KI in the region shallower than 200m,near 147°E,38.5°N and deeper than 200m,147°E,38.4°-38.5°N may be related to NIWs.Except for the analysis of the influence of atmospheric forcing on turbulent mixing,the enhanced mechanism of turbulence in WSAF is similar to that of D'Asaro on the Kuroshio front. |
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