Predictability Of Spatial Distribution Of Summer Extreme High Temperature Days In Eastern China

Summer extreme high-temperature events over eastern China have increased significantly,which has severer impact on human health,ecological environment,social and economic development in recent years.However,to what extent the anomaly pattern of the extreme high temperature days(EHDs)over eastern China is predictable remains largely unexplored.To address this issue,we analyzes the characteristics of major EOF(Empirical Orthogonal Function,EOF)modes of summer extreme high-temperature days in southern China(SC,south of 33°N,east of 105°E)/ northern China(NC,north of 33°N,east of 105°E),reveals the related physical mechanism,establishes a set of physical-based empirical models,predicts the major modes of extreme high temperature days in summer in southern China/northern China and estimates the predictability of its spatial distribution by employing predictable mode analysis method.The conclusions are summarized as follows:1)There are significant seasonal differences in extreme high temperature days in eastern China.The EHDs in southern China mainly occur in July to August.The first EOF mode explains 48.6% of total variance of summer(July-August)EHDs over southern China,which exhibits a homogeneous pattern related to a local low-level high-pressure anomaly when EHDs increased over SC.The explained variance of the second mode of EHDs is 17.5%,which is a meridional dipole pattern associated with the Pacific-Japan pattern over East Asia/Western Pacific sector and Pacific Meridional Mode-like SSTA over North Pacific.Via tracking the boundary anomalies forcing of EHDs over SC,physically meaningful predictors are selected to establish a set of physics-based empirical(P-E)models.The results show that the first two leading principal components(PCs)can be predicted reasonably well by the P-E models.Thus,they can be regarded as the predictable modes.Assuming that these two modes can be predicted perfectly,66.1% of the total variance for the distribution of summer EHDs over SC are potentially predictable.Using the first two predicted PCs and corresponding observed spatial pattern of EOF modes,the prediction of anomalies pattern of EHDs over SC can be obtained.The independent forecast skill(2010–2019)of the domain-averaged temporal correlation coefficient reaches 0.37,while the PCC skill in the whole period(1980-2019)is 0.24.2)The EHDs in northern China are mainly concentrated in June-July.The explained variance of first mode EHDs in northern China in summer(June-July)is 49.4%,which is a uniform pattern.The increased EHDs over northern China are modulated by the warm SST in the northwest Pacific Ocean and its related high pressure anomaly.The second mode is meridional dipole mode and its explained variance is 14.4%.This mode is affected by the upper-level northeast-southwest dipole pressure anomalies.Based on the understanding of the physical mechanism of the simultaneous correlation,we selected two physical meaningful predictors from the lead-lag correlation maps between the principal component(PC)and the preceding lower boundary forcing anomalies to establish the P-E model for each mode,respectively.The hindcast and independent forecast achieved skillful prediction skills.Therefore,the first two modes are considered as predictable.Assuming that these two modes can be predicted perfectly,63.8% of the total variability of the spatial distribution of summer EHDs over northern China is potentially predictable.By further reconstructing the predicted first two principal components and the corresponding observed EOF modes,the forecasted results show that the areal averaged TCC skill is 0.26 during the independent period(2010-2019),and the temporal averaged PCC skill is 0.25 during the whole period(1980-2019).