| The extreme high-temperature events seriously impact the livelihood,productivity,and economy in North China.In this thesis,hierarchical clustering was used to classify 47 North China heatwaves from 1961-2019 according to the spatial intensity distribution,and the possible factors contributing to the differences were analyzed.Besides,the impacts from Atlantic multidecadal oscillation(AMO)and Pacific decadal oscillation(PDO)on the 1994 interdecadal shift of heatwaves in North China and the associated physical mechanisms were evaluated.In addition,we also projected the future heatwaves for 2041-2060 under three major emission pathway scenarios(SSP1-2.6,SSP2-4.5,and SSP5-8.5)with the newly released CMIP6 simulations.In the end,a dynamical downscaling simulation for one selected CMIP6 model was performed by the CWRF and combined with population and GDP data,this thesis also has estimated the heat risk in North China under SSP5-8.5.The main results are as follows:(i)Two types of heatwaves in North China and their linkage with sea ice changes in the Barents Sea-Karst Sea are revealed.Hierarchical cluster analysis of 47 heatwave events in North China from 1961 to 2019 revealed that there could be two types according to the central spatial intensity,which was S-type centered in the southern part and N-type centered in the northern part.The number of S-type(36)was much higher than that of N-type(11).The results concluded that the Barents-Kara Sea(BK)sea ice changes could be the main reason for the difference between the two types of heatwaves.The reduction of sea ice would enhance cyclonic circulation over the BK-Siberian and in conjunction with the continental high,generating a cyclonic-anticyclonic-cyclonic(C-A-C)pattern from the BK to the North ChinaJianghuai Region,the C-A-C would eventually bring the southern part of North China under the control of cyclonic anomalous.This cyclonic anomalous would,on the one hand,lead to rainy days,increasing soil moisture,which was unfavorable to the occurrence and maintenance of heatwaves,and on the other hand,it would also move the continental high northward,thereby shifting the center of heatwaves northward,in which case an N-type heatwave event came out.When there was more sea ice in the BK,the upper cyclonic circulation could not get strong and eventually failed to excite the cyclonic circulation in the south of North China through the CA-C(actually the C-A-C would not show).This would make the whole region of North China completely under control by the continental high,then the S-type heatwave occurred.(ii)The impacts from AMO and PDO on the interdecadal shift of North China heatwaves around 1994 were pointed out,and the AMO was found to be dominant.Compared with the cold period(1972-1994),the heatwaves during the warm period(1995-2010)exhibited significant increases in terms of intensity,duration,and scope.The atmospheric circulation anomalies indicated that the strengthened continental high and the weakened upper westerly over North China could be the direct circulation factors.Moreover,the AMO and PDO demonstrated significant positive and negative correlations with the heatwaves,respectively.Both the observational analysis and numerical experimental results agreed that the AMO and PDO could influence North China through the anticyclone-cyclone-anticyclone(A-C-A)wave trains in the mid-latitudes and the Pacific-East Asia pattern(PEA),respectively.For the warm period,the AMO is in a positive phase(+AMO)while the PDO is in a negative phase(-PDO).The warmer sea surface temperatures in the North Atlantic and the North Pacific could cause anticyclonic circulation anomalies over the ocean basins.Through the A-C-A,the anticyclonic circulation over North Atlantic could enhance the anticyclonic circulation over North China,which would then strengthen the continental high and weaken the westerly jet,resulting in heatwaves.The anomalous anticyclonic circulation over the North Pacific would enhance the anticyclone over North China mainly through the PEA,and would also generate a counterclockwise vertical circulation anomaly from North China to the Sea of Japan.This system could make downward movements and weaken the upper westerly jet over North China.Whereas,for the cold period,-AMO and +PDO would produce a similar but reversed atmospheric mechanism,which would finally generate atmospheric circulations that were not favorable for temperature rise.The numerical experiments also demonstrate that the individual effects of AMO and PDO can both affect the temperature in North China,and AMO is dominant.(iii)The future changes of North China heatwaves in 2041-2060 relative to the current(1995-2014)were projected,and it was found that the high emission scenario would increase 1.5 times as much as the low and medium emission scenarios.Twenty-seven selected CMIP6 models were grouped into three ensembles based on the simulated performance for the North China heatwave during the present days,i.e.,better simulated,poorly simulated,and all models included.Based on this,the future changes in the duration and intensity of heatwaves for 2041-2060 under SSP1-2.6,SSP2-4.5,and SSP5-8.5 were projected.The projections showed that the future heatwaves in North China for the three ensembles were consistent: both the duration and intensity would increase,with a similar magnitude under SSP1-2.6 and SSP2-4.5,and the most significant increases under the high-emission pathway(SSP5-8.5).For the spatial distribution,the heatwave days would increase more in southern North China,and the areas with larger increases in intensity will be mainly in northwestern and southern parts.In addition,the increases in annual highest heatwave intensity would be greater than the annual average heatwave intensity,and the increase in annual longest heatwave days would be 40% of the annual heatwave days.(iv)Quantify the future heat risks to population and GDP from North China heatwaves.In this section,we used a regional climate model(CWRF)to dynamically downscale the simulations from MPI-ESM1-2-HR under SSP5-8.5 and increased the horizontal resolution to?30 km.The simulation performance of the heat stress index and the annual heatwave days(HWAD)in North China is enhanced.The south-central part of North China was the leading region for the heat risk days(RD,defined by heat stress and HWAD),and would exceed 5 RDs per year during 2041-2060.The projected heat risk(i.e.,population exposure and GDP exposure)displayed a similar spatial distribution of RD in North China,that is,the high-value areas were concentrated in the central-southern,with major cities as the heart locations.The high-value areas of population exposure and GDP exposure to the heat risk were also concentrated in southcentral North China and centered on big cities.The future population exposure of North China and its major cities for the mid-term was projected to be triple the present level.The RD would be a major part of the population exposure with a contribution of 80%.For the GDP,the exposure would reach almost 45-70 times the present level in North China.Among the GDP exposure,the joint contribution would be the main contributor(?60%),and the contribution from GDP would be 30% while the RD would be no more than 5%.For quantitative estimation of future heat risk in North China,more factors need to be considered and more experiments need to be conducted to allow for strategic planning for different climate risks. |
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