Variations Of Cross-Shelf Volume Transports And Mesoscale Eddy Effects On Chlorophyll In The Northern South China Sea

Mesoscale processes are prevail in the South China Sea(SCS),such as mesoscale eddy,cross-shelf volume transport,jet,front,coastal upwelling and so on.This thesis mainly investigates variations of cross-shelf volume transports and mesoscale eddy effects on chlorophyll in the northern South China Sea(NSCS).This study has important implications on the dynamics in coastal regions where observational data is limited and helps us gain a deeper understanding of the role of mesoscale eddies in marine ecology.Seasonal cycle and interannual variability in cross-shelf volume transports in the NSCS are investigated using satellite altimeter data from March 1993 to September 2016.To begin with,we use along-track sea level anomalies to calculate along-shelf velocity anomalies,which are in good agreement with acoustic Doppler current profilers(ADCP)observations at an adjacent location.Along-shelf volume transport anomalies in the NSCS could integrate from along-shelf velocity anomalies from the coastline to 200 m isobaths.The results indicate northeastward transports from April to August and southwestward transports from October to February,which consistent with previous studies in this region.According to convergence and divergence in the target control volumes,cross-shelf volume transports are estimated as the differences between two neighboring along-shelf volume transport anomalies,with the assumption that long-term mean along-shelf volume transports at each cross-sections are identical.The results show offshore transports in June and onshore transports in October.The comparison between altimeter-derived and ADCP-estimated cross-shelf volume transports is encouraging,especially when the region has relatively low mesoscale activities and low freshwater input.We further reconstruct cross-shelf volume transports in the NSCS through multiple linear regression,which reveals that seasonal harmonics is the primary force in driving cross-shelf volume transports in the NSCS,while wind and El Nino have secondary effects on controlling cross-shelf volume transports in different regions.And Pearl River discharge and Kuroshio intrusion have important effects in cross-shelf volume transport off Pearl River Estuary.This study also investigates mesoscale eddy effects on chlorophyll in the NSCS during their lifecycles based on eddy dataset and chlorophyll data from January 2003 to December 2016.Firstly,chlorophyll data are pre-processed to cover the SCS and the signal relates to mesoscale eddies are isolated.Then,the variations of chlorophyll are composited in mesoscale eddies generated in the two areas of NSCS(the Luzon Strait(LS)and the area off northwest Luzon Island(NWLI))during their lifecycles.Our results show that chlorophyll in cyclones generated in LS is very high,then it drops quickly as cyclones propagate westward and stays stable until vanishing with increasing values,and chlorophyll in eddies generated in the NWLI ascends stably and keeps high until their disappearance with dropping.While anticyclone effects on chlorophyll are weaker than cyclones,with chlorophyll in anticyclones generated in the LS decreases quickly,then keeps steady and bounds back stably when they disappear.Furthermore,different eddy mechanisms(eddy advection,eddy trapping,eddy pumping and eddy-induced Ekman pumping)on chlorophyll in the NSCS are quantified during their lifecycles.Different spatial distributions and temporal evolutions of chlorophyll due to different mechanisms are compared with empirical orthogonal function(EOF)results for chlorophyll in eddies during their lifecycles.Our consequences reveal that EOF mode 1(with variance of 43.80%)of chlorophyll in cyclones generated in the NWLI is a positive monopole,which is affected by eddy pumping that causes chlorophyll rises,and this strengthening effect reaches its strongest stage at half of the lifecycles.The sum of variances of mode 2 and mode 3 of chlorophyll in cyclones generated in the LS is 34.06%.The former is introduced by eddy advection with periphery extremes,while the latter is caused by eddy advection and linear Ekman pumping.The spatial distribution of mode 1 of chlorophyll in anticyclones generated in the LS is negative,and the time series first increases and then decreases but always keep positive.This mode is introduced by eddy trapping and nonlinear Ekman pumping,after the generation of the anticyclones,these two factors undergo a trend of increasing and then gradually weakening,thereby inhibiting the same change of chlorophyll.Mode 2 is influenced by eddy pumping.