A Study On The Statistical Characteristic Of Mesoscale Eddies And The Inversion Method Of Their Temperature And Salinity Fields

Mesoscale eddies are universal in the global ocean.As an important physical process in the ocean,the statistical characteristics and three-dimensional structure of eddies are the focus of domestic and foreign scholars in recent years.Mesoscale eddies play a key role in energy,material transport,and other biochemical processes in ocean dynamics,which further affect the ocean circulation,horizontal and vertical distributions of temperature and salinity.Therefore,the study on mesoscale eddies have a high research and utilization value not only in the theoretical research such as ocean dynamics,but also in the practical applications such as military defense.Based on three independent mesoscale eddy datasets,we selected eddies in the global ocean from 1993 to 2018,then analysed the statistical characteristics of them and the comparisons with the basic features of mesoscale eddies in different regions.Futhermore,four probability density functions were used to fit the distribution histograms of the eddy's amplitude,radius,rotation speed and Rossby number.The results showed the lognormal distribution is the best choice to describe the distributions of eddy characteristics.On this basis,taking the northern of the South China Sea as an example,combined with the historical observations data of the temperature and salinity profiles and the sea surface temperature(SST)and sea surface height(SSH)data from satellite remote sensing,the three-dimensional ocean temperature and salinity fields were expanded,their reliability and availability were examined by observed data.Besides,the characteristics of mesoscale eddies below the sea surface were reflected by the structure and variation of the expanded three-dimensional temperature field.First,the histograms of eddy amplitude,radius,rotation speed and Rossby number in the global ocean with the fitting curves of four probability density functions are given.The results showed that the fitting error of the lognormal distribution is the smallest,which is the best choice to fit eddies in the global ocean and the fitting results is independent of the selection of datasets.Then,through the comparisons of the characteristics of eddies in different seas,we revealed the similar results.Although they were all well followed by the lognormal distribution,there were obvious differences in the fitting errors of different seas.Mesoscale eddies in eddy-active regions were further analysed.Unlike the large basins,the values of the studied eddy characteristic were significantly increased in these regions,and the difference between cyclonic and anticyclonic eddies were more obvious.These results reflect the significant regional characteristics,which may be related to the local background flow fields,stratification,and baroclinic instability.However,the lognormal distribution is still the best function for fitting the histograms of eddy characteristics in the eddy-active region,and the fitting errors in these region were significantly reduced.The results showed that in different sea regions,the fitting errors of the lognormal distribution on the characteristic of mesoscale eddies are related to the activities of eddy.Where eddies are more active,the fitting errors are smaller and the fitting effects are better.Futhermore,the spatial distribution,propagation characteristics,seasonal and interannual changes of eddies in the STCC region were analysed and the results are consistent with the previous studies.In order to better understand the characteristics of mesoscale eddy inside the ocean,we selected the historical observations of the temperature and salinity profiles from the northern of the South China Sea to the east of the Luzon Strait.Then an empirical regression model was constructed by the method of regression statistical analysis,which is used to expand the three-dimensional temperature field in this region with a time resolution of day and a spatial resolution of 0.25°×0.25° by the SST and SSH data from satellite remote sensing.Furthermore,the same method was used to establish an empirical regression model to expand the three-dimensional salinity field by temperature field in there.Comparing with the observed data for past 30 years from the northern of the South China Sea to the east of the Luzon Strait,the results showed that the monthly errors of the expanded temperature profiles are generally less than 1.0°C,which are slightly larger near the thermocline and the maximum is 1.55°C.The monthly errors of the expanded salinity profiles is generally less than 0.25 psu and reach about 0.7 psu near the sea surface in summer.The expanded temperature and salinity profiles at single stations were also similar to the actual observed results.Furthermore,the expanded temperature and salinity field clearly described the temperature vertical structure of a cross section,and accurately simulated the structure and variation of the three-dimensional temperature profile in an observed region under the influence of the Luzon Warm Eddy.