Multi-scale Oceanic Variations Over The Kuroshio Extension Region And Their Influences On East Asian Climate

In this thesis,multiple atmospheric and oceanic observational and reanalysis datasets and various statistical methods are utilized to investigate the characteristics of multi-scale oceanic low-frequency variations over the Kuroshio Extension(KE)region.Then,the atmospheric responses to these oceanic variations are discovered and their relationships with the synoptic-scale transient eddy feedback tied to storm tracks are also analyzed.Furthermore,the impacts of KE multi-scale variations on the East Asian climate in different seasons are examined.At last,the interdecadal change in the relationship between the summer KE sea surface temperature(SST)anomalies and the East Asian summer monsoon(EASM)is discussed.The main conclusions are as follows:(1)The low-frequency variation of SST anomalies averaged over the KE region is not stable with respect to the time domain,which shows significant ~6-yr oscillation during 1930–1950 and significant ~10-yr oscillation after the late 1970 s.These dual-period KE SST variations over the two different time spans are forced by the oceanic Rossby waves formed in the central subtropical and North Pacific,which are excited by the intensity change and meridional shift of Aleutian Low(AL),respectively.(2)The KE front(KEF)intensity shows significant ~6-and ~10-yr oscillations during 1996–2010 and 1984–2008,respectively.The KEF position shows significant oscillations with periodicities of ~6 yr and longer than 9 yr during 1983–2000 and after 1981,respectively.The low-frequency variation of KEF position is directly modulated by the westward-propagating oceanic Rossby waves,which leads that of KEF strength by 21 months.(3)Based on the SST data,this thesis develops a new index to depict the characteristics of KE mesoscale eddy activity.This index is used to reveal the low-frequency variation of KE mesoscale eddy activity and its relationship with the westward-propagating oceanic Rossby waves over the recent three decades.It is found that the KE mesoscale eddy activity shows significant ~6-and ~10-yr oscillations during 1998–2015 and 1992–2006,respectively.This low-frequency variation is closely related to the KE jet strength and pathlength changes,but it is independent of oceanic Rossby waves.(4)The atmospheric responses to the KE multi-scale variations basically present a equivalent barotropic structure with strong seasonal variation,which resemble the North Pacific Oscillation/western Pacific(NPO/WP)pattern over the western North Pacific–East Asia section,the East Pacific/North Pacific Oscillation(EP/NP)over the Pacific–North America section and the North Atlantic Oscillation(NAO)over the North Atlantic and its adjacent areas.The synoptic-scale transient eddy feedback tied to storm tracks primarily contributes to the formation of EP/NP(NPO/WP)-type atmospheric responses to the KE SST variation(mesoscale eddy activity)and KEF meridional shift.(5)The impacts of KE multi-scale variations on the East Asian climate have obvious seasonal differences.These oceanic variations generally affect the winter and spring surface air temperature(SAT),and the summer and autumn rainfall over East Asia mostly.In particular,both the winter KEF meridional shift and spring KE mesoscale eddy activity can result in the wide-ranging SAT anomalies simultaneously.However,the scope of KE oceanic forcings at different scales on the summer and autumn rainfall has little differences.(6)The summer KE SST and the EASM show a significant positive and negative relationship before and after the late 1980 s,respectively.This interdecadal change in the KE–EASM relationship may be attributed to the enhancement of summer PDO,which is most probably induced by the stronger NPO-like atmospheric forcing,especially its southern pole(SP),in the preceding spring after the late 1980 s.The spring NPO-like SP forces the KE SST and PDO more directly during the following summer than the full NPO-like dipole and thus can be a precursor for the following EASM.