Causes And Projections Of Regional Changes In Summer Precipitation In China Based On Self-organizing Map Circulation Classification

In China,the mechanism of changes in regional summer precipitation during the past few decades and the projections are the key issues in policy-making for addressing climate change.This research establishes the connection between the observed daily precipitation and the key circulation patterns reflecting the evolution of summer monsoon by Self-organizing map(SOM)approach.First,the characteristics and influence factors of rainband movement in China are explored.Then based on an improved idea in decomposing the precipitation trend,the changes of summer precipitation over China are decomposed into trends induced by circulation pattern frequency,precipitation probability and intensity respectively.The causes of these change components are investigated from the perspectives of external forcings and decadal signals.On the basis of the attribution results during historical period,attribution-constrained projections of precipitation distribution over China induced by frequency of circulation pattern are further obtained.Furthermore,through the multi-model and multi-predictor statistical downscaling scheme,the refined projection of summer precipitation over the middle and lower reaches of Yangtze River is realized,and the possible physical causes of future precipitation changes are also revealed from the perspective of circulation patterns.The main conclusions are as follows:(1)Using the SOM approach,the relationships between key circulation patterns affecting summer rainfall and observed precipitation distribution in China from 1961-2019 are established.The circulation and precipitation distribution between adjacent patterns are found with evolutionary characteristics.During the process of the rainband moving from south China to northeast China,the circulation is manifested as continuous expansion of the positive anomaly range of geopotential height,the northeastward retreat of the western Pacific subtropical high,and the reverse change of the trough and ridge structure.This evolution of circulation patterns and corresponding precipitation can reflect the movement process and the main pathway of rain belt.In addition,it is found that the northward moving velocity of the rainband has interdecadal fluctuations.When the North Pacific Interdecadal Oscillation(PDO)is in negative(positive)phase,the probability of the rain belt transferring from Yangtze–Huaihe to Huanghuai river basin increases(decreases),and the duration of the rainband in north China increases(decreases)significantly,finally the northward of the rainband accelerates(decelerates).(2)Based on the improved idea in decomposition of precipitation trend,the summer precipitation trends in China are decomposed into three components induced by the changes in circulation pattern frequency,precipitation probability and precipitation intensity.The spatial distribution of precipitation changes caused by circulation pattern frequency shows a northern flood-southern drought pattern.Combining the attribution by optimal fingerprint method,the external forcing leads to the reduction of the pattern with southward precipitation,and the anthropogenic influence is detectable in the increases of circulation pattern with northeastward western Pacific subtropical high and northward rainband.Besides,decadal signals would also affect a few patterns'frequencies and corresponding rainfall changes.In the historical stage,the changes of precipitation probability and intensity together dominate the complex spatial distribution(southern flood-northern drought)of summer precipitation in China,especially the former.The pattern of precipitation probability in eastern China may be related to the suppression of weak precipitation in the north and the increase of strong precipitation in the south.This phenomenon may be affected by aerosols.The trend of precipitation intensity(7.2%K^-1)is relatively uniform and highly consistent with the variation of moisture content,and its regional averaged trend is almost same with the increasing rate of water vapor caused by thermodynamic factor,indicating the direct effect of global warming.(3)The attribution in the historical period revels that GCMs underestimate the responses of circulation patterns'frequencies to external forcing.By correcting this simulation bias,the attribution-constraint projection of circulation pattern is realized.The results show that the increasing(decreasing)trend of pattern with northeastward(southwestward)western Pacific subtropical high and northward(southeastward)precipitation is 2.4 times(5 times)in 2061-2080 than that of the historical period.Under the influence of circulation pattern frequency,the precipitation increases as a whole after attribution constraints compared with that before the constraint,especially in northern and southwestern China.After constraints,the precipitation at most stations is projected to increase significantly in the 21^st century.Only the rainfall in the southeastern coastal regions and parts of the southwestern area may decrease.Considering the uniform enhancement of moisture content under warming,precipitation may experience a general increase with the larger augment in the north than south in the future.(4)The SOM statistical downscaling scheme is further established to realize the projection of summer precipitation at the station scale.Taking the middle and lower reaches of Yangtze River as an example,the scheme shows good behaviors in terms of probability distribution of daily rainfall and spatial distribution of rainfall indices.In the future,daily rainfall at most sites(63%)is projected to shift towards larger values.For the early 21^st century(2016–2035),precipitation in the central basin rises,yet decreases occur over the middle reaches of the Yangtze River as well as a part of southeast area.For the late 21^st century(2081–2100),the precipitation indices experience an overall increase except for a few southeast stations.The total precipitation in the lower reaches of the Yangtze River and in its south area increase from7%at 1.5°C global warming to 11%at 2°C,while the intensity enhancement is more significant in southern and western sites.All SOM nodes are regrouped into four distinct regional regimes which show remarkable stability for future climate.The overall intensification of precipitation in the future is linked to the increase of a wet regime bringing longitudinally closer the south Asia high and the western Pacific subtropical high,as well as the reduction of a dry pattern making the two atmospheric centers of action move away from each other.