The Strengthened Relationship Among The Western North Pacific Summer Monsoon,ENSO And Tropical Cyclone Genesis In Recent Decades

The Western North Pacific summer monsoon is an important subcomponent of the Asian monsoon system.It has important scientific and realistic significance to study the variation process and the physical mechanism of the Western North Pacific summer monsoon.We concentrated on the strengthened relationship between the Western North Pacific summer monsoon and the El Ni?o-Southern Oscillation(ENSO)/Typhoon genesis number in recent decades,and analyzed the physical process and physical origins of these observed interdecadal variations in the research of this thesis.The results of the study revealed that the enhanced influence of the tropical Atlantic sea surface temperature(SST)is an important reason why the correlation between the Western North Pacific summer monsoon and ENSO/Typhoon has increased in recent decades.The correlation between the Western North Pacific summer monsoon and the summer ENSO index has increased significantly in recent decades.The correlation between summertime Ni?o3.4 index and Western North Pacific summer monsoon index has strikingly enhanced since the early 1990 s,with nonsignificant correlation before the early 1990 s but significant correlation afterward.The interdecadal variation of the relationship between the two indexes may be related to the interdecadal variation of ENSO-related SST anomalies.During the post-1990 s period(the pre-1990 s period),highly noticeable tropical Atlantic(Indian)Ocean SST anomalies tend to co-occur with the summertime Ni?o3.4 SST anomalies.The concurrent tropical Atlantic(Indian)Ocean SST anomalies could constructively reinforce(destructively mitigate)the Western North Pacific monsoon circulation anomalies induced by the summertime Ni?o3.4 SST,thus boosting(muting)the correlation between summertime Ni?o3.4 SST and Western North Pacific monsoon.In addition,the faster decaying rate of preceding-winter El Ni?o after the 1990 s,which may have been mainly induced by the influences from the spring tropical North Atlantic SST anomalies,could also have contributed to the enhanced correlation between the summertime Ni?o3.4 index and Western North Pacific monsoon.These results suggest that the enhanced influences from the tropical Atlantic SST may have triggered the intensified correlation between summertime ENSO and Western North Pacific monsoon since the early 1990 s.The relationship between the Western North Pacific summer monsoon and summer ENSO simulated in the models was analyzed using the CMIP5 multi-model data.During the current climate period,most models could simulate the positive correlation between the Western North Pacific summer monsoon and the summer ENSO.In the results of future global warming scenarios,the positive correlation between the Western North Pacific summer monsoon and summer ENSO still existed,and there was no significant differences compared with the current climate period.The results demonstrated that the average ENSO decay rate simulated by the model may be the primary factor for the difference in the correlation between the Western North Pacific summer monsoon and the summer ENSO index simulated by each model.The increased biennial relationship between the Western North Pacific summer monsoon and ENSO has also occurred in recent decades.The enhancement of the monsoon—ENSO biennial relationship was primarily due to the strikingly increased correlation coefficient between the Western North Pacific summer monsoon index and the Ni?o3.4 index in the following winter,indicating that the Western North Pacific summer monsoon may be able to exert a more important impact on the subsequent ENSO evolution after the 1990 s.The results revealed that the anomalous wind field associated with the Western North Pacific summer monsoon significantly expanded toward the equatorial region after the early 1990 s.After the early 1990 s,the westerly anomalies over the equatorial region,which accompanied by the strong Western North Pacific summer monsoon,could influence the subsequent evolution of ENSO by triggering the eastward oceanic Kelvin wave,and further enhanced the biennial relationship between the Western North Pacific summer monsoon and ENSO in recent decades.The increased influence of the tropical Atlantic SST was considered to be the major cause for the wind field anomaly associated with the Western North Pacific summer monsoon to significantly expand towards the equatorial region after the 1990 s.In addition,we conducted a comparative study for the Western North Pacific and Indian summer monsoons in terms of their biennial relationship with ENSO.The results showed that there was an out-of-phase see-saw biennial relationship between the Western North Pacific/Indian summer monsoon and ENSO on the interdecadal time scale.During the recent decades,the stronger ENSO—Atlantic coupling could enhance the biennial relationship between the Western North Pacific summer monsoon and ENSO,but weakens the biennial relationship between the Indian summer monsoon and ENSO.The increased influence of the tropical Atlantic ocean temperature may trigger the observed see-saw of the out-of-phase variation between the two monsoons.The relationship between the intensity of the Western North Pacific summer monsoon and the typhoons genesis number has also increased in recent decades.This may be related to the interdecadal variation of the distribution characteristics of the atmospheric wind field and the SST anomalies associated with the Western North Pacific summer monsoon.Before the early 1990 s,accompanied by anomalous westerly north of 5°N associated with the strong Western North Pacific summer monsoon,the negative shear vorticity was generated in the southern region,which suppressed the local Tropical cyclone(TC)genesis;After the early 1990 s,the negative vorticity of the wind field disappeared and eliminated the inhibitory effect on TC genesis of the southeast region in the previous period,due to the fact that the anomalous westerly associated with the strong Western North Pacific summer monsoon moved southward significantly.At the same time,the warm SST anomalies in the equatorial central Pacific accompanied by the strong Western North Pacific summer monsoon could also increase the TC genesis number on the southeast region of Western North Pacific.The combined effect from the southward shift of anomalous equatorial westerly and the warm central Pacific SST anomalies were the major factors to enhance the correlation between the summer monsoon and the TC genesis number since the early 1990 s,which may also be closely associated with the stronger influence of the tropical Atlantic SST on the Pacific climate in recent decades.After the 1990 s,the impact of tropical Atlantic SST on the Pacific climate has significantly increased,which was considered to be an important driving factor for the enhancement of the relationship between the Western North Pacific summer monsoon and the ENSO/typhoons genesis number during recent decades.The Atlantic background SST has shown a notable increase under the combined influence of the phase transition of the Atlantic Multi-decadal Oscillation(AMO)and the global warming trend since the early 1990 s.This warmer Atlantic SST may support the stronger impact of tropical Atlantic SST on the Pacific climate.These findings demonstrated that we need to pay more attentions to the effect of the tropical Atlantic SST when we study the climate variability of the tropical Pacific(including the climatic characteristics of the Western North Pacific summer monsoon,ENSO and TC).