Possible Influence Of The Southern Hemiphere Annular Mode On The Climate Over The Tibetan Plateau And Its Surrounding Areas

As a unified whole,the Northern and Southern hemispheres interact with each other through the ocean-atmosphere coupling processes and the associated exchange of mass,momentum and heat.The Southern Hemisphere(SH)annular mode(SAM)is the principal mode of SH extratropical circulation variability.The climate effect of SAM may not be limited to SH,it can also stretch to the Northern Hemisphere.Using observation,reanalysis and model simulations,this dissertation investigates the possible relationship between the SAM and the climate change of Tibetan plateau and its surrounding areas,and reveals the corresponding physical mechanisms.The main results concluded as follows:(1)It is found that there is a significant positive correlation between the spring SAM and the interannual variability of western Tibetan Plateau snow cover(TPSC)and Indian summer monsoon rainfall(ISMR)in boreal summer.The Indian Ocean surface temperature anomaly(SSTA)plays an “ocean bridge” role and the tropical rainfall anomalies acts as an “atmosphere bridge” role in the process of May SAM impacting the following summer Asia climate variability.The results show that SAM signal can be restored in the Indian Ocean meridional tripole(IOT)SSTA mode through air-sea heat flux exchanging.Due to the thermal inertia of the ocean,the IOT SSTA can persist to the following summer.On one hand,the dipole SSTA in middle-low latitudes of IOT directly affects the ISMR by regulating the local vertical meridional circulation and low-level cross-equatorial flow.On the other hand,the IOT SSTA transmits the SAM signal to the tropics,causing a tropical dipole rainfall(TDR)anomaly in the tropical western Pacific and Maritime continents(MC).The results of wave ray diagnostic analysis indicate that the latent heat energy released by the rainfall dipole can be propogated to the western TP,which drives the variation of the TPSC.(2)Using 21-year sliding correlation,this paper finds that there is a significant interdecadal change in the relationship between spring SAM and Yangtze River valley summer rainfall(YRVSR).The in-phase correlations between SAM and YRVSR are statistically insignificant during the period of 1958–1987,whereas it becomes significant during 1988–2012.The interdecadal variation of the relationship between SAM and SST in the Indian Ocean may be the main reason for this interdecadal transition.Before 1987,the amplitude of SAM itself was weak,and the heat exchange between SAM and the ocean was not significant,so it is difficult to imprint the SAM signal into the Indian Ocean.Consquently,the SAM signal cannot persist to the ensuing season and cannot influence the YRVSR.After 1988,through the strong air-sea interaction,the spring SAM anomaly leads to a tripole SSTA mode in Indian Ocean,which is consistent with the IOT SSTA mode mentioned above.The IOT SSTA persists to the following summer and drives precipitation anomaly over MC.The results of the wave tracing show that the wave energies generated by the MC rainfall(MCR)can not only propagate to the western TP,but also propagate northward to the YRV along the great circle route,and then modulate the YRVSR.In addition,those models from phase 5 of the Coupled Model Intercomparison Project(CMIP5)which well captures the May SAM-YRVSR relation can reproduce the IOT SSTA pattern and MCR anomalies associated with SAM.This result further affirms that both the IOT SSTA and MCR are responsible for the strengthened relationship between the SAM and the YRVSR after 1988.(3)Our work shows that the autumn SAM exhibits a significant inverse relationship with the winter precipitation over East Asia.Based on the results of observation and simulation,it is found that the corresponding mechanism is different to the mechanism responsible for the potential influence of spring SAM on summer East climate.The South Atlantic–Pacific dipole(SAPD)SSTA is the key factor to link the autumn SAM and the East Asia winter precipitation.The SAPD prolong the SAM signature to the ensuing winter and then modulate the variability of the Inter-tropical Convergence Zone(ITCZ)in Pacific.Subsequently,a distinguished atmospheric teleconnection pattern is induced and prevails over the Northern Hemisphere midlatitude region as a response to the anomalous ITCZ.Large areas of high(or low)pressure anomalies are triggered over East Asia,which favors less(or more)precipitation over East Asia.