Multi-timescale Variations And Forecasts Of Summer Extreme High-temperature And Surface Air Temperature In Southern China

Global warming and the increase of extreme weather and climate events are the main features of the climate change during past decades,and the extreme heat is the main weather and climate disaster in Southern China,which poses important threats to human health,industrial and agricultural production and ecological environment.This paper focuses on the local variation characteristics of extreme high-temperature days(EHDs)and mean surface air temperature(SAT)in summer in Southern China,and explores the long-term,interannual and intra-seasonal variation characteristics of EHDs and SAT by station observation data,multiple sets of reanalysis data,dynamical model forecast data and various meteorological analysis methods.The corresponding external forcing signals and circulation characteristics are used to gain insight into the mechanisms of temperature variability and to improve temperature forecasts to some extent.The main conclusions obtained in this paper are as follows.(1)Midsummer extreme high-temperature types in Southern China and their corresponding large-scale circulation and sea surface temperature anomaliesThe results show that the annual EHDs in Southern China has a significant growth trend.Clustering analysis is used to classify the midsummer extreme high-temperature in Southern China into three types: Jianghuai type,South China type and Central China type.The Jianghuai type extreme high-temperature is centered on the Jianghuai area,with a wide range and high frequency.When the Jianghuai type extreme hightemperature occurs,the anomalous anticyclone is at high to low tropospheres,meanwhile the Western Pacific subtropical high(WPSH)is strong to the west,and its ridgeline is slightly northerly,so the Jianghuai type is a high temperature type under control of WPSH.This type of extreme high-temperature is mainly related to the attenuation of the central type El Ni?o from the pre-winter to the summer and the warmer sea surface temperature(SST)in equatorial Atlantic in spring.The central area of South China type is in the south of the Yangtze River to the South China.The typical circulation systems are the southerly position of East Asian subtropical jet,the stronger,westward WPSH and its ridgeline southerly.Meanwhile the South China type is directly under control of the WPSH,accompanied by the weakening of the southwest monsoon,and it has obvious characteristics of dry heat.This type of extreme high-temperature is closely related to the attenuation of the eastern type El Ni?o and its associated Indian Ocean capacitor effect.The Central China type extreme high-temperature is mainly located in Hubei province and Hunan province.The corresponding circulation is that the WPSH is slightly weaker and eastward,and it’s similar to the North Atlantic-Eurasia remote correlation pattern in the middle and high latitudes.It is the extreme high-temperature under control of continental high pressure,which makes the water vapor condition better than the other two types.The North Atlantic-Eurasian teleconnection is a possible signal source of the Central China type extreme hightemperature.(2)The main intra-seasonal modes of summer air temperature anomalies in Southern China and their corresponding predictable signalsThe intra-seasonal distributions and evolutions of SAT anomalies in Southern China are characterized by two main modes.The first mode is the anomalous warmth in the Yangtze River basin during July-August,which is mainly influenced by the anomalous high pressure in midsummer over Northern China-Southern Japan.The stable atmospheric conditions cause the increase of solar radiation and decrease of precipitation at surface,so the SAT is higher.The North Atlantic Oscillation(NAO)is its main signal and influences the temperature in Southern China through the anomalous southeast-oriented wave train over the Eurasian continent.The second mode is characterized by anomalous warmth in the southern region in July and September,with anomalous anticyclonic in the northwest and local zonal wind anomalous causing the intra-seasonal variation of the SAT anomalies.The developing La Ni?a during the same period is the main external forcing signal.The forecast model constructed based on the two main signal factors can better predict the interannual variability and intra-seasonal distribution characteristics of SAT anomalies in Southern China,and successfully predicted the main features of the heat wave in 2022.(3)The abrupt rise of midsummer extreme high-temperature days and surface air temperature over Southern China near early 2000 s and it’s influences to climate forecastsThe midsummer EHDs experienced an abrupt rise near early 2000 s during 1979-2021 instead of sustained growth or decadal changes,while there were no remarkable trends before and after the mutation.The interannual variability of EHDs changed little during two periods,but the correlation with El Ni?o-Southern Oscillation(ENSO)had changed from strong to weak,mainly due to the weakened influence of ENSO on the East Asian summer circulation after the mutation point.The long-term and interannual variations of SAT and EHDs are consistent.The current mainstream climate dynamical systems are not capable of capturing such abrupt rise in SAT,but rather exhibited a sustained growth trend,which adulterated with some spurious skills of the forecasts.The weakened relationship with ENSO reduced the skill of the models with high consistency with observations,while the rest were instead unaffected or have higher skills in the latter period.(4)Predictable patterns of midsummer surface air temperature over Eastern China and their corresponding signal sources in the ECMWF subseasonal forecastsECMWF has the highest forecast skill among all subseasonal models for the summer SAT over Eastern China,but decays significantly after more than 10 d of the lead time.The leading first predictable mode(MSN-EOF1)mainly presents a dipole mode over Eastern China.Both the ENSO and tropical Indian Ocean SST anomalies are the main predictable sources of the MSN-EOF1.A positive MSN-EOF1 is accompanied by the El Ni?o decaying,which can cause the strong and westward WPSH,and anomalous southwesterlies are over the north of Eastern China.The second predictable mode is the warmer characteristic of Yangtze-Yellow River basin,and the sea ice anomalies over the Barents Sea in the previous winter are the main predictable sources.Accompanied by the anomalous wave train propagating from Northwestern Russia to Northeast Asia.The third predictable mode is mainly the temperature trend item extracted from ECMWF model.The reconstructed predictions with the leading MSN-EOF components show higher skills than the model raw predictions,therefore this method can be used to filter the model noise and reduce the model random error.