A Study On The Air-sea Interaction Process Affecting The Interdecadal Variability Of The Western North Pacific Summer Monsoon In The 1990s

As the subsystem of the Asian summer monsoon,the western North Pacific summer monsoon(WNPSM)is one important part of the climate system.Considering the impact of monsoon rainfall on agriculture,economy and society,it is of great significance to study the change of the WNPSM and its related air-sea interaction processes.Focused on the interdecadal change of the WNPSM around the 1990 s,we investigated the interdecadal change of the WNPSM in the spatial structure of the wind field,the main period of variability,the relationship with the Australian monsoon system,and the impact on the genesis productivity of tropical cloud clusters in this thesis.In this study,we especially highlight that the tropical Atlantic sea surface temperature(SST)anomaly and related air-sea interaction process may play an important role in the interdecadal change of the WNPSM around the 1990 s.In terms of the spatial structure of the wind field,the results of this study suggest that the springtime SST anomaly in the tropical North Atlantic could enhance the correlation between the WNPSM and the equatorial zonal wind over the western Pacific through the cross ocean basin process after the 1990 s.The regime shift of the tropical oceanic SST configurations are found to be responsible for the interdecadal shift in the correlation between the WNPSM and the equatorial zonal wind.After the 1990 s,the joint modulation of the spring tropical North Atlantic SST anomaly,the faster decaying speed of El Ni(?)o-Southern Oscillation(ENSO)in the previous winter,and summertime SST anomaly in the equatorial central Pacific and maritime continent can strengthen the relationship between the WNPSM and the equatorial zonal wind.The relationship between the WNPSM and the equatorial zonal wind over the western Pacific can affect the feedback of the WNPSM to ENSO in the following winter.After the 1990 s,the closer WNPSM-equatorial zonal wind connection could support the stronger feedback of the WNPSM to ENSO.For 26 CMIP5 models,most of the models could reproduce the correlation between the WNPSM and the equatorial zonal wind relatively well.It is found that the model with good simulation effect on the correlation between the WNPSM and the equatorial zonal wind also has relatively good simulation ability on the correlation between the WNPSM and the ENSO in the subsequent winter.These results implicate that when considering improving the simulation of the relationship between the WNPSM and the ENSO in the following winter,it is necessary to pay special attentions to the simulation ability on the correlation between the WNPSM and the equatorial zonal wind in the same period in the model.Further,we investigated the correlation between the WNPSM and the equatorial zonal wind under the future global warming scenario.The correlation between the WNPSM and the equatorial zonal wind may not change significantly in the model results under the future warming scenario,compared with those in the present climate period.In terms of the dominant periodicity of variability,it is found that the interaction process between the tropical Atlantic and Pacific Ocean could enhance the quasi-biennial variation of the WNPSM after the 1990 s.There is an interdecadal variation of the WNPSM from the quasi four-year cycle before the 1990 s to the quasi two-year cycle after the 1990 s.By comparing with other global monsoon systems,we found that the decadal change of the dominant periodicity around the 1990 s can only be observed in the WNPSM.Other monsoons in the world do not show similar decadal variation.The enhancement of the quasi-biennial variability of the WNPSM after the 1990 s could be explained by the theory referred to as the tropical Atlantic "capacitor" effect.Significant quasi-biennial variability of the WNPSM could be induced by the alternation of the "charging"(ENSO affecting SST anomalies in the tropical North Atlantic through the "atmospheric bridge" mechanism)and "discharging"(SST anomalies in the tropical North Atlantic feeding back to ENSO through the subtropical teleconnection mechanism)process through the tropical Pacific-Atlantic interactions.After the 1990 s,according to the joint influence of the Atlantic decadal oscillation(AMO)(from the negative phase before 1990 s to the positive phase after 1990s)and the global warming trend,the enhanced influences on the Pacific Ocean from the tropical Atlantic(the "discharging" process)could enhance the Atlantic "capacitor" effect,thus promoting the enhancement of the quasi-biennial variation of the WNPSM.In terms of the linkage with other monsoon systems,the study reveals that the influence of the tropical North Atlantic SST anomaly in spring could enhance the relationship between the WNPSM and the Australian monsoon after the 1990 s.The recent shift of the negative correlation between the WNPSM and the Australian summer monsoon may be related to the interdecadal variation in the spatial SST configuration in the tropical oceans.After the 1990 s,the modulation of the SST anomaly in the boreal springtime tropical North Atlantic and the subsequent evolution of ENSO are responsible for the significant enhancement in the negative correlation between the WNPSM and the Australian summer monsoon.The correlation results between the WNPSM and the Australian summer monsoon simulated by various CMIP5 models are quite different,and the proportion of models whose simulation results are close to the observations is relatively small,indicating that the simulation ability of CMIP5 models on the relationship between the WNPSM and Australian summer monsoon needs to be further improved.In terms of the influence on other weather systems,we noted that the intensification of the WNPSM in May could contribute to the increasing trend of the genesis productivity of tropical cloud clusters.We found that the genesis productivity of tropical cloud clusters over the western North Pacific(WNP)have obvious seasonality.May was the only month in which an increasing trend was observed.The intensification of the WNPSM may be closely related to the increasing trend of tropical cloud cluster genesis productivity in May.By comparing May(the month with an increasing trend)and October(the month with the strongest decreasing trend),we propose that the spatial configuration of SST anomalies in different tropical oceans could induce circulation anomalies in the lower atmospheric,which could explain for the differences in the trends of tropical cloud cluster genesis productivity in different months.In May,stronger SST warming in the tropical western Pacific could prompt increased anomalous westerlies associated with anomalous cyclonic circulation,accompanied by the strengthening of the WNPSM and the weakening of the WNP subtropical high.Such changes in background atmospheric circulations could favor the enhancement of atmospheric eddy kinetic energy and barotropic energy conversions,resulting in recent intensified genesis productivity of WNP tropical cloud clusters in May.The results in this thesis will help to understand the interdecadal variability of the WNPSM and its mechanisms around the 1990 s more systematically,by further revealing the air-sea interaction process affecting the 1990 s interdecadal variability of the WNPSM.These results in this study could further increase the understanding of the impact of the interactions among three tropical oceans on the climate variability during recent decades,which are helpful for improving the ability to predict climate variability and mitigate climate disasters.