A Lagrangian Study Of The Agulhas System

As an important link connecting the Indian Ocean and Atlantic,Agulhas leakage transports warm and salty water from the Indian Ocean into the Atlantic,composing an important source of the upper arm of the Atlantic Meridional Overturning Circulation.Therefore,changes of Agulhas leakage play a vital role in global ocean circulation and global climate change.Recent studies have shown that Agulhas leakage was increasing in the past few decades,but there are discrepancies on the factors which influence Agulhas leakage.Exploring the relationship among Agulhas system will help to understand the physical mechanism.A Lagrangian particle tracking method is used here to estimate volume transport of Agulhas leakage and Agulhas return current,in the 1/12°HYCOM current field.At the same time,the position of Agulhas retroflection is identified using upstream mean sea surface height,and then relationships among different Agulhas system’s components are investigated on interannual and seasonal scales respectively.Considering the time it takes for Agulhas current parcels to leave as Agulhas leakage or return current,this study sums the fluid particles as Agulhas leakage or Agulhas return current at the time of the release,and then analyze the relationship of each summation with the Agulhas current volume transport.In addition,this thesis takes advantage of Lagrangian particle tracking tool to track the Agulhas current backward,and explored the main sources of Agulhas current and their virtical distribution.After a comprehensive analysis of the interactions among Agulhas system components,the followings have been found:the larger the Agulhas current,the smaller the proportion of Agulhas leakage;"viscous regime" dominates when position of Agulhas retroflection shifts eastward extremely,and "turbulent regime" dominates when position of Agulhas retroflection shifts westward extremely;The more westward the monthly mean Agulhas retroflection is,the greater the Agulhas Leakage Flow and the smaller the Agulhas Return Current Flow.More comprehensive understanding of the interaction among Agulhas system was got base on these conclusions,which is helpful to comprehend the response of Agulhas system in the background of climate change at different time scales.