Features And Seasonal Variation Of Hydrographic Structure And Watermass Of Southwest Australian Coast

The eastern boundaries of the world have been a hot topic of oceanography and climatology.The eastern-boundary currents flow to the equator that provide abundant fishery resources for the surrounding waters,but the eastern boundary of the South Indian Ocean are short of nutritious extremely because the eastern-boundary current flow polarward,and unique physical environment makes the sea a high-energy mesoscale eddy area.The life cycle and spatial distribution of the local marine life are greatly affected by the east boundary flow and related water masses.In addition,the eastern boundary of the South Indian Ocean is the world's only eastern boundary that transport heat to the atmosphere,and the sea-air heat flux reaches the largest in the Southwest of Australia.Therefore,the physical marine environment along the Southwest Australian coast has great significance to the local ecological environment,the distribution of fishery resources and the impact of climate change.The Southwest waters of Australia are known as the unique eastern-boundary current – Leeuwin Current(LC).The origin,variation characteristics and the mesoscale phenomena of LC have been the focus of attention of scientists.At the same time,other characteristics of related the current are also interesting,so far there is no conclusion about it.In recent years,the distribution of fishery resources and the climate change caused by the current patterns along the Southwest Australian coast are the concern of many scholars.Therefore,it is great significance to study the hydrological environment and the distribution of water masses,it is helpful to understand the ocean process and solve the above problems.This paper provides a scientific basis for the systematic study of current,water masses and mesoscale eddies in the Subtropical Indian Ocean.This paper uses the CTD sections data that obtained by Institute for Marine and Antarctic Studies(IMAS)of the University of Tasmania in Australia,that implemented research programme Eastward flows,ocean mixing and air-sea interaction in the southeast Indian Ocean in July 2013.The hydrological characteristics and the water mass structure of the Southwest Australia coast were analyzed,and the judgment index of the water masses are redefined.In combination with the data of 32 ° S in November 2000 and November 2000,The Variation of Hydrographic Structure and Watermass were discussed.The main conclusions are as follows:First,the CTD section data of winter(July 2013)shows that he coastal surface temperature is higher than the open sea of the study area,the salinity is significantly lower than the offshore,the surface temperature of the meridional sections is obvious high in the North and low in the South.There are strong thermocline,halocline and pycnocline in the subsurface.The high / low salt zone and the high / low dissolved oxygen zone are shown in different sections,and the high salt zone is the most obvious in the meridional section,that is,the lowest value of salinity and dissolved oxygen are in middle layer water.The seawater density in the study area mainly depends on the temperature difference,which is less affected by salinity.The hydrological condition of the study area is affected by the mesoscale process,especially in the 105 ° E section,and there is obvious structure of the cold and warm eddies,and the lower cyclonic eddy is stronger than the upper anticyclonic eddy.In the spring and winter,the mesoscale phenomenon is more significant,while the autumn is less affected by the mesoscale process.In the winter,the geostrophic flow field in the study area is characterized by the distribution of the north-south flow in the 32 ° S section.The coastal waters are more complicated and have many strong flow zones,and the offshore waters are affected by the mesoscale process.SICC is present on both 105 ° E and 100 ° E sections,but EGC is not found.There is a cyclonic cold eddy on the 105 ° E section,and the presence of the cyclonic eddy changes the flow direction of seawater.The section was weakly influenced by the mesoscale process in autumn(May 2009),and the velocity of the area outside the shelf was weak.LC flow becomes smaller,the section did not observe CC in 2009.Compared with the winter(July 2013)geostrophic field,it can be seen that the mesoscale process may reduce the LC and CC,but will enhance the LUC.It is found that there is no significant difference in the general trend of temperature,salinity and density distribution from the hydrological characteristics of the Argo grid climate data and the observed features of the CTDs observed in different seasons.However,since the Argo climate state salt field is excessive smoothing,eddy and wave signals are greatly suppressed.In particular,the climatic data of the Argo grid in the spring(November 2000)are quite different from the hydrological characteristics observed by the CTD,which is reflected in the deepening of the depth of the mixed layer,the decrease of the thermocline and the presence of the high / low salinity,it may relate to the strong mesoscale process.Second,from the satellite sea surface height anomaly and the geostrophic flow field can be seen that the mesoscale eddy are very active in the winter,autumn and spring,and is more intense in the winter and spring;It is relatively calm,only in the near shore of the section.The superposition of mesoscale eddies appears only on the winter section.The temperature and salinity of the anti-cyclonic eddies in the offshore area are significantly higher than those in the eddies,and the salinity is lower than that outside of the eddies.In the spring,on the 109 ° E and 111 ° E sections,there are obvious anti-cyclonic eddy and cyclonic eddy.The temperature and salinity in the eddy also appear in the doublelayer structure,and the temperature inside the eddy,salinity above the depth of the observation are lower than the eddy.In the Argo grid data,only large-scale ocean phenomena(such as high / low salinity bands,thermocline and geostrophic fields)can be presented,and the mesoscale eddy is hardly hard present.Although the mesoscale eddy is active in the study area,there is no long-life or permanent mesoscale eddy.Thirdly,the surface and sub-surface water masses of the study area are affected by runoff and advection,and some of the water masses are change greatly than those of the water source.The middle and lower water masses are mixed with the surrounding water in the process of transporting to the north,the nature of the water mass changes.In addition to the several water masses,the majority of waters have been denatured,the water mass has the obvious characteristics of the southeast Indian Ocean,in addition to the southern Indian Ocean central water(SICW),other water masses are named subtropical southeast Indian Ocean surface water(STSEISW),subtropical southeast Indian Ocean subsurface water(STSEISSW),subtropical southeast Indian Ocean modal water(STSEIMW),subtropical southeast Indian Ocean water(STSEIIW),middle subtropical southeast Indian Ocean deep water(STSEIDW)and subtropical southeast Indian Ocean bottom water(STSEIBW),and the characteristic parameters are given.The characteristics of the water masses in the study area have obvious seasonal variation: the surface STSEISW and the sub-surface STSEISSW are more mixed in spring and autumn.In the spring,the temperature of STSEISW is lower than that of the other two years,and the dissolved oxygen of TSW is higher in the autumn.The maximum value of the salinity is in the sub-surface water STSEISSW,but the minimum value of the salinity of autumn is shallow,the winter is the second,and it's the deepest in spring,and the autumn is higher.At 10.0 ?,the dissolved oxygen content of STSEIMW is higher than that of the other two seasons in July(July 2013),and the dissolved oxygen content of the middle layer STSEIIW is higher than that of the other two seasons at 6.0 ?.