Different Influences Of Mesoscale Oceanic Eddies On The North Pacific Subsurface Low Potential Vorticity Water Mass Between Winter And Summer

The generation and dissipation of North Pacific subsurface low potential vorticity water mass(SLPVW)affect various ocean-atmosphere dynamic and thermodynamic processes on a wide range of time scales.Mesoscale oceanic eddies play an important role in the variation of SLPVW.The study of the different influences of mesoscale oceanic eddies on the SLPVW between winter and summer is helpful to understand the seasonal variation of the ocean-atmosphere dynamic and thermal processes in North Pacific.NCEP/CFSR(National Centers for Environmental Prediction/Climate Forecast System Reanalysis)reanalysis coupled dataset during 1979-2010 has been analyzed to investigate the relation between mesoscale oceanic eddies and the distributions of SLPVW in winter and summer.The effects of strong and weak eddies in winter on the turbulent heat flux(THF),the upper ocean stratification and the distribution of SLPVW are revealed.Moreover,it is found that the cyclone(anticyclone)influences the potential vorticity(PV)through the extension(compression)of the subsurface ocean stratification in summer.Whether there is seasonal difference in the effect of mesoscale eddies on SLPVW,and whether there is a significant correlation between mesoscale eddies and SLPVW? In order to solve these questions,we calculate the correlation coefficient between the sea surface height(SSH)and the volume of SLPVW from the year 1979 to 2010 separately in winter and summer.It is found that the correlation coefficient between the SSH and the volume of SLPVW in winter is positive in the north of 30°N and negative in the south of 30°N,which reveals that more SLPVW tends to exist in the anticyclone in the north of 30°N,and less SLPVW tends to exist in the anticyclone in the south of 30°N.The distribution of correlation in summer is mostly negative,which means less SLPVW tends to exist in the anticyclone.It is found that the eddy vorticity strength is the main cause of the north-south opposite distribution of the correlation coefficient in winter.The ratio of the strong eddy in negative correlation value region is significantly larger than that in the positive correlation value region.The abnormal THF of strong eddy is much stronger than usual.Therefore,it has a great influence on mixed layer depth(MLD).The large abnormal THF(more heat loss than usual)of the strong anticyclone can induce deep convection which is beneficial to generate thicker mixed layer(ML).Although the deeper MLD correspond to more low PV water generation,the low PV water that actually exists the subsurface ocean below the MLD(namely SLPVW)is decreased due to the deeper MLD.On the contrary,affecting by the negative THF anomaly,the MLD in the strong cyclone is much shallower,and there is more low PV water under the MLD(namely SLPVW),although less low PV water generated in ML.The abnormal THF of the composite weak eddy is quite different with the composite strong eddy.In the atmosphere above composite weak anticyclone,temperature anomaly at 2m above sea surface is much larger than SSTA.Weak anticyclone may be the decay stage of the preexisting strong anticyclone.The distance of the motion of the eddies is far less than the radius of the eddies.In the early stage,strong anticyclone has been heating the atmosphere,and the large heat loss weakens anticyclone itself.Then it becomes the weak anticyclone in the decay period.At this time,the local temperature difference between the ocean and atmosphere is much smaller than before,and so does the local wind speed,therefore the abnormal THF is negative in the composite weak anticyclone.The positive temperature anomaly decreases with the deepening of depth and the negative THF anomaly in the weak anticyclones,result in shallower MLD and more SLPVW.On the contrary,the negative temperature anomaly of cyclone decreases with the deepening of depth,which is conducive to weaken the ocean stratification,and leads to deeper MLD in the weak cyclone where the THF anomaly is not obvious,finally resulting in SLPVW decreasing in the weak cyclone.In winter,mesoscale oceanic eddies also have obvious impacts on the subduction rate.The time-averaged total subduction rate of the North Pacific from the year 1979 to 2010 is 5.15 Sv.While,the time-averaged eddy subduction rate is 1.88 Sv,which accounts for 36.4% of the total subduction rate.The subduction rate varies greatly from year to year,suggesting the role of the interannual variation of eddies.The total subduction rate is positively correlated with the number of anticyclones,and the difference of the number of anticyclones and cyclones.In summer,SLPVW usually exists in the depth range of 100-250 m,while the mean MLD is 18.3 m,which is much shallower than winter,and shallower than the SLPVW depth range.Moreover,the summer ML hardly changes,and the seasonal thermocline disconnects the exchange of SLPVW and the upper ocean,so there is no SLPVW newly generated by subduction.The three-dimensional distribution suggests that the influence of the summer mesoscale oceanic eddies on the PV can reach the depth of 400 m,covering the depth range of the climatological mean SLPVW in summer.At this time,the effect of mesoscale oceanic eddies on SLPVW is mainly reflected in the dissipation or retention process.In summer,SSH is remarkably negatively correlated with SLPVW volume,indicating that SLPVW is less in anticyclones.In order to verify this point,we similarly compose eddies in summer.The composition results show that the SLPVW volume is abnormally decreased in the center of the composite anticyclone,and abnormally increased in the center of the composite cyclone.The three-dimensional structure of the composite eddy shows that positive(negative)divergence anomaly decreases with depth in the composite anticyclone(cyclone),which leads to compressing(stretching)of the distance between adjacent isopycnic surfaces.Therefore,the PV value increases(decreases)anomaly in the composite anticyclone(cyclone),which accelerates(hinders)the dissipation of SLPVW.Different from thermodynamic factors impacting the SLPVW generation process in winter,eddy leads to the variation of the subsurface divergence field,changes the ocean stratification,and affects the dissipation of the SLPVW in summer.