Impacts Of ENSO And Pacific Meridional Mode On The Mesoscale Eddy And SST In The South China Sea

El Ni?o-Southern Oscillation(ENSO)and Pacific Meridional Mode(PMM)are the leading mode of climate variability in the tropical and subtropical Pacific Oceans,respectively.While the influence of ENSO on the South China Sea(SCS)climate has been extensively studied,the influence of the PMM on the SCS climate has not received much attention.In this dissertation,we perform statistical analyses with observational datasets to investigate the impacts of the PMM on SCS climate,the relative importance of the PMM and ENSO impacts on SCS climate,and interdecadal variations of these impacts during the past seven decades.In the study,both the PMM in the northern subtropical Pacific(i.e.,the northern PMM;nPMM)and the southern subtropical Pacific(i.e.,the southern PMM;sPMM)are considered for investigations.In the first part of this study,we focus on one important element of the SCS climate-the mesoscale eddies.We find that the correlation between ENSO and the activity of mesoscale oceanic eddies in the SCS changed around 2004.The mesoscale eddy number determined from satellite altimetry observations using the geometry of velocity vector method was significantly and negatively correlated with the Nino3.4 index before 2004,but the correlation weakened and became insignificant afterward.Further analyses reveal that the relationship between ENSO and eddy is controlled by two major wind stress forcing mechanisms:one directly related to ENSO and the other indirectly related to ENSO through its subtropical precursors-the nPMM and sPMM.Both mechanisms induce wind stress curl variations over the SCS that link ENSO to SCS eddy activities.While the direct ENSO mechanism always induces a negative correlation between ENSO and eddy through the Walker circulation,the indirect mechanism is dominated by the nPMM resulting in a negative correlation between ENSO and eddy before 2004 but dominated by the sPMM after 2004,resulting in a positive correlation between ENSO and eddy.As a result,the direct and indirect mechanisms enhance each other to produce a significant positive correlation between ENSO and eddy relation before 2004 but cancel out,resulting in a weak correlation between ENSO and eddy afterward.The relative strengths of the northern and southern PMMs are the key to determining the relation between ENSO and eddy and may be related to a phase change of the Interdecadal Pacific Oscillation(IPO).In the second part of this study,we focus on another key element of the SCS climate-the interannual variations of sea surface temperature(SSTs).We examine the impacts of ENSO and PMMs on the SCS SST variability on interannual time scales.The leading variability mode,which is characterized by basin-wide warming or cooling in the SCS,exhibits a double-peak feature in its intensity.The basin-wide variability is the strongest during both the boreal winter and summer.While this basin-wide variability mode was linked in previous studies to the ENSO and East Asian winter monsoon forcing,we find that this mode is directly driven by the two PMMs.The nPMM drives the SCS SST mode during boreal winter via a wave train mechanism,while the sPMM drives the SCS SST mode during boreal summer via a Gill-type response mechanism.The mechanisms enable the negative nPMM to produce an anomalous anticyclonic circulation to the east of the SCS that counteracts the climatological winds to warm up the SCS basin during winter.Conversely,the positive sPMM produces anomalous anticyclonic circulation to the north of the SCS that counteracts the summer climatological winds to give rise to an SCS basin warming.The double-peak feature of the SCS basin mode is a result of its seasonallyvarying dependence on the two PMMs.In the last part of the study,we examine the inter-decadal variations in the ENSO and PMMs impact on the SST anomaly in the SCS.We find that,while the sPMM’s impact on the summer SCS basin mode remains the same during the past seven decades(1950-2018),the impacts of nPMMs and ENSO on the winter SCS basin mode are not stationary and exhibit an inter-decadal variation.During boreal summer(June-August)in 1962-1979 and boreal autumn to winter(October-February)in 19862004,the basin-wide SST anomaly in the SCS can be connected to nPMM through a wave train mechanism.Meanwhile,the synchronized correlation between ENSO and SCS’s SST anomaly shows an opposite change during 1962-1979 and 1986-2004.The boreal winter SST anomalies in the SCS significantly and positively correlated with the Nino3.4 index during 1962-1979,but the correlation weakened and became insignificant during 1986-2004.We find the inter-decadal variation in the relationship between the SCS basin mode and the nPMM and ENSO to be caused by the phase shift of the IPO around the last 1970s.During the epoch of the negative IPO phase(i.e.,1962-1979),the nPMM activity was overly weak and enabled the ENSO to play a more important role in inducing the SCS basin mode of SST variability.During the epoch of the positive IPO phase(i.e.,1986-2004),the nPMM activity was overly strong and dominated over the ENSO in inducing the SCS basin mode of SST variability.We noticed that the southern hemispheric center of the IPO did not have a large amplitude change from the negative to positive phases of the IPO.The sPMM activity did not experience a strong inter-decadal change from the 1962-1979 epoch to the 1986-2004 epoch.As a result,the impact of the sPMM on the SCS basin mode did not show an apparent inter-decadal variation.All these findings reveal a trans-basin and inter-hemispheric influences mechanism of the Pacific Ocean on the SCS climate variability and help us better understand the response of SCS climate to the tropical and subtropical forcing in the Pacific Ocean.Findings reported in this dissertation have the potential to improve the projections of future climate variability in the SCS.