Changes and Dependencies of Compound Hot–Dry Extremes in China During 1961–2022

Concurrent hot and dry days (CHDDs) can significantly impact agricultural production, the social economy, and natural ecosystems. However, most previous studies used fixed percentile thresholds to determine the occurrence of CHDDs, without considering the intra-annual threshold changes and comparing the differences from different thresholds. This study investigated the patterns, differences, and changes of CHDDs in China based on daily maximum temperature and precipitation observations. The results indicated that CHDDs were most likely to occur in southeastern North China (NC), the northwestern parts of the middle and lower reaches of the Yangtze River (YR), and southeastern Southwest China (SWC), and the intensity of CHDDs was more severe in South China (SC), most of the YR, central and southern NC, and some parts of southeastern SWC. During 1961–2022, the frequency of CHDDs showed a significant increasing trend at nearly 90% of the stations, and the intensity of CHDDs presented a significant strengthening trend at nearly 30% of the stations in China. Different percentile thresholds had effects on the patterns and trends of CHDDs. With the heightening of the threshold, the increasing trend of the frequency slowed down, but the strengthening trend of the intensity further accelerated, and the range with a strengthening trend expanded. The changes of hot days were consistent with those of CHDDs during 1961–97 and 1998–2022, but the changes of dry days did not correspond to those of CHDDs. Due to the increase in hot days after 1998, the increasing rate of CHDDs was accelerated. We conclude that the comparisons of multiple thresholds and the latest daily observations can yield new insights into compound extremes. Significance Statement Under the background of global warming, the frequency and intensity of temperature and precipitation extremes and their impacts are increasing in some regions of the world. Many studies have analyzed the changes of individual extreme events such as temperature or precipitation, but there are few studies on the simultaneous occurrence of temperature and precipitation extremes (e.g., concurrent hot and dry days or events). The impacts of concurrent hot and dry days (CHDDs) on human society and natural ecosystems are often more severe and destructive than those of individual days. Based on daily maximum temperature and precipitation data from 2225 meteorological stations in China during 1961–2022, the spatial patterns, trends, and interannual variations of CHDDs under four different thresholds, as well as the spatiotemporal characteristics and dynamics of the influence or contribution of hot days and dry days to the occurrence of CHDDs in the two periods of 1961–97 and 1998–2022 were analyzed. The results have indicative significance and practical values for agricultural and forestry production, disaster prevention and management, sustainable economic development, and human health.

Dependence of compound hot and dry extremes on individual ones across China during 1961–2014

Based on daily maximum and minimum temperature data from 437 weather stations over China, this study examined the spatiotemporal change of temperature extremes in China from 1960 to 2011. Results showed a general downward trends in the occurrence of cold days (TX10) and nights (TN10) (base period 1961–1990), but upward tendency on the occurrence of warm days (TX90) and nights (TN90), the temperatures of coldest day (TXn), coldest night (TNn), warmest day (TXx), and warmest night (TNx) in China and most climate regions. At the national scale, TX10 and TN10 have significantly decreased by −1.89 and −4.39 days/decade, and TX90 and TN90 have significantly increased by 2.49 and 4.72 days/decade from 1960 to 2011. The national average trends for TXn, TNn, TXx, and TNx were 0.28, 0.54, 0.17, and 0.27 °C/decade, respectively. The temporal changes of extremes indices showed that changes in cold (warm) relative indices may be primarily related to that of corresponding winter (summer) Tmax and Tmin, respectively. Regionally, the magnitudes of changes in extreme indices decreased from the north to south of China. However, we found significant increase of warm extremes, especially warm days and nights in Southeast China. For most climate regions, the trend magnitudes in warm days/nights were larger than that in cold days/nights, but the trend in coldest temperature was much higher than that in warmest temperature. The trend magnitudes in minimum temperature indices were larger than those based on daily maximum temperature, explaining the faster increase of Tmin than Tmax in China.

With global warming, the probability of summer compound hot and dry extreme (CHDE) days, which are higher risk compared with single-factor extreme events, increases in some regions. However, there have been few studies on the winter precursor signals of such events. In this study, we found that summer CHDEs have generally increased in the last 20 years, with the increases in the middle and lower reaches of the Yangtze River region and Southwest China being more than double those in other regions of China. The dominant mode of summer CHDEs in China is characterized by more hot–dry days in the Yangtze–Huaihe River Basin (YHRB). Importantly, we found that there is an obvious cross-seasonal relationship between the first mode of winter snow cover in the Northern Hemisphere (NH) and summer CHDEs in China. When the mode of winter snow cover in the NH is in a positive phase with a negative-phase Arctic Oscillation (AO), i.e., more snow cover in Europe, Northeast China, and the northern United States, and less snow cover in central Asia and the midlatitudes in winter, more CHDEs in China in the following summer. Compared with the signals from the AO, these signals from winter snow can be better stored and transmitted into summer through the snow, soil and ocean, inducing a northward shift of the upper-level westerly jet and strengthening of South Asia high. Through the strong dynamic forcing of negative vorticity advection with the change of westerly jet, the subsidence movement in the western Pacific subtropical high (WPSH) region is strengthened, resulting in the stable maintenance of the WPSH in the YHRB. Under the synergy of a remote mid- and high-latitude wave train in summer, which also relates closely to winter snow cover, more CHDEs ultimately occur in the YHRB of China.

Observed trends in extreme temperature events over northern part of the Korean Peninsula during 1960–2019 and a comparative overview

Droughts and hot extremes may lead to tremendous impacts on the ecosystem and different sectors of the society. A variety of studies have been conducted on the variability of the individual drought or hot extreme in China. However, the evaluation of compound droughts and hot extremes, which may induce even larger impacts than the individual drought or hot extreme, is still lacking. The aim of this study is to investigate changes in the frequency and spatial extent of compound droughts and hot extremes during summer in China using monthly precipitation and daily temperature data from 1953 to 2012. Results show that a high frequency of compound droughts and hot extremes mostly occur in the regions stretching from northeast to southwest of China. There is an overall increase in the frequency of co-occurrence of droughts and hot extremes across most parts of China with distinct regional patterns. In addition, an increasing trend in the areas covered by compound extremes has been observed, especially after the 1990s. At regional scales, the increase of the frequency and spatial extent of compound extremes has been shown to be most profound in North China (NC), South China (SC), and Southwest China (SWC), while the decrease of compound extremes was found in Central China (CC). These results show the variability of compound droughts and hot extremes and could provide useful insights into the mitigation efforts of extreme events in China.

The China Meteorological Administration (CMA) recently produced a CMA Global Atmospheric Interim Reanalysis (CRAI) dataset for the years 2007–2016. A comprehensive evaluation of the ability of CRAI to capture the spatiotemporal variability of observed precipitation, in terms of both mean states and extreme indicators over China, is performed. Comparisons are made with other current reanalysis datasets, namely, the ECMWF interim reanalysis (ERAI), Japanese 55-yr reanalysis (JRA55), NCEP Climate Forecast System Reanalysis (CFSR), and NASA Modern-Era Retrospective analysis for Research and Applications version 2 (MERRA2), as well as NCEP Climate Prediction Center (CPC) observations. The results show that, for daily variations of rainfall during warm seasons in eastern China, CRAI and CFSR overestimate the precipitation of the main rain belt, while the overestimation is confined to the area south of 25°N in JRA55 but north of 24°N in MERRA2; whereas ERAI tends to underestimate the precipitation in most regions of eastern China. Two extreme metrics, the total amount of precipitation on days where daily precipitation exceeds the 95th percentile (R95pTOT) and the number of consecutive dry days (CDDs) in one month, are examined to assess the performance of reanalysis datasets. In terms of extreme events, CRAI, ERAI, and JRA55 tend to underestimate the R95pTOT in most of eastern China, whereas more frequent extreme rainfall can be found in most regions of China in both CFSR and MERRA2; and all of the reanalyses underestimate the CDDs. Among the reanalysis products, CRAI and JRA55 show better agreement with the observed R95pTOT than the other datasets, with fewer biases, higher correlation coefficients, and much more similar linear trend patterns, while ERAI stands out in better capturing the amount and temporal variations of the observed CDDs.

Based on daily maximum and minimum surface air temperature and precipitation records at 303 meteorological stations in China, the spatial and temporal distributions of indices of climate extremes are analyzed during 1961–2003. Twelve indices of extreme temperature and six of extreme precipitation are studied. Temperature extremes have high correlations with the annual mean temperature, which shows a significant warming of 0.27°C/decade, indicating that changes in temperature extremes reflect the consistent warming. Stations in northeastern, northern, northwestern China have larger trend magnitudes, which are accordance with the more rapid mean warming in these regions. Countrywide, the mean trends for cold days and cold nights have decreased by −0.47 and −2.06 days/decade respectively, and warm days and warm nights have increased by 0.62 and 1.75 days/decade, respectively. Over the same period, the number of frost days shows a statistically significant decreasing trend of −3.37 days/decade. The length of the growing season and the number of summer days exhibit significant increasing trends at rates of 3.04 and 1.18 days/decade, respectively. The diurnal temperature range has decreased by −0.18°C/decade. Both the annual extreme lowest and highest temperatures exhibit significant warming trends, the former warming faster than the latter. For precipitation indices, regional annual total precipitation shows an increasing trend and most other precipitation indices are strongly correlated with annual total precipitation. Average wet day precipitation, maximum 1-day and 5-day precipitation, and heavy precipitation days show increasing trends, but only the last is statistically significant. A decreasing trend is found for consecutive dry days. For all precipitation indices, stations in the Yangtze River basin, southeastern and northwestern China have the largest positive trend magnitudes, while stations in the Yellow River basin and in northern China have the largest negative magnitudes. This is inconsistent with changes of water vapor flux calculated from NCEP/NCAR reanalysis. Large scale atmospheric circulation changes derived from NCEP/NCAR reanalysis grids show that a strengthening anticyclonic circulation, increasing geopotential height and rapid warming over the Eurasian continent have contributed to the changes in climate extremes in China.

Summer hot and dry extremes (defined as high air temperature and low atmospheric humidity) in monsoon (climatologically high-humidity) region, may cause severe disasters, such as flash droughts. However, it remains unclear whether hot (dry) extremes are amplified on dry (hot) days to warming temperature. Here, taking eastern monsoon China (EMC) as a typical monsoon region, we find a fastest positive (negative) response of air temperature (atmospheric humidity) on driest (hottest) days to per unit warming, indicating amplified warming (drying) of hot (dry) extremes on dry (hot) days (i.e. coupling hotter and drier extremes) especially in southern EMC.  The southern EMC is also a hotspot where the coupling of hot and dry extremes has become significantly stronger during the past six decades. The increasing hot-dry extremes in southern EMC is associated with anomalies in large-scale environmental conditions, such as reduced total cloud cover, abnormal anticyclone in upper atmosphere, intense descending motion, and strong moisture divergence over this region. Land-atmosphere feedbacks play another important role in enhancing the hot-dry coupling via increasing land surface dryness (described as decreasing evaporation fraction). The decreasing evaporation fraction is associated with drying surface soil moisture which is controlled by decreases in pre-summer 1-m soil moisture and summer-mean precipitation. Given hot extremes (atmospheric humidity) are (is) projected to increase (decrease) in the future, it is very likely to witness more hot-dry days in monsoon regions and associated disasters, which should be mitigated by adopting adaptive measures. 

ABSTRACTTrends in indices based on daily temperature and precipitation are examined for two periods: 1948–2016 for all stations in Canada and 1900–2016 for stations in the south of Canada. These indices, a number of which reflect extreme events, are considered to be impact relevant. The results show changes consistent with warming, with larger trends associated with cold temperatures. The number of summer days (when daily maximum temperature >25°C) has increased at most locations south of 65°N, and the number of hot days (daily maximum temperature >30°C) and hot nights (daily minimum temperature >22°C) have increased at a few stations in the most southerly regions. Very warm temperatures in both summer and winter (represented by the 95th percentile of their daily maximum and minimum temperatures, respectively) have increased across the country, with stronger trends in winter. Warming is more pronounced for cold temperatures. The frost-free season has become longer with fewer frost days, consecutive frost days, and ice days. Very cold temperatures in both winter and summer (represented by the 5th percentile of their daily maximum and minimum temperatures, respectively) have increased substantially across the country, again with stronger trends in the winter. Changes in other temperature indices are consistent with warming. The growing season is now longer, and the number of growing degree-days has increased. The number of heating degree-days has decreased across the country, while the number of cooling degree-days has increased at many stations south of 55°N. The frequency of annual and spring freeze–thaw days shows an increase in the interior provinces and a decrease in the remainder of the country. Changes in precipitation indices are less spatially coherent. An increase in the number of days with rainfall and heavy rainfall is found at several locations in the south. A decrease in the number of days with snowfall and heavy snowfall is observed in the western provinces, while an increase is found in the north. There is no evidence of significant changes in the annual highest 1-day rainfall and 1-day snowfall. The maximum number of consecutive dry days has decreased, mainly in the south.

At thousands of stations worldwide, the mean daily surface air temperature is estimated as a mean of the daily maximum (T max) and minimum (T min) temperatures. We use the NOAA Surface Radiation Budget Network (SURFRAD) of seven US stations with surface air temperature recorded each minute to assess the accuracy of the mean daily temperature estimate as an average of the daily maximum and minimum temperatures and to investigate how the accuracy of the estimate increases with an increasing number of daily temperature observations. We find the average difference between the estimate based on an average of the maximum and minimum temperatures and the average of 1440 1-min daily observations to be − 0.05 ± 1.56 °C, based on analyses of a sample of 238 days of temperature observations. Considering determination of the daily mean temperature based on 3, 4, 6, 12, or 24 daily temperature observations, we find that 2, 4, or 6 daily observations do not reduce significantly the uncertainty of the daily mean temperature. The bias reduction in a statistically significant manner (95% confidence level) occurs only with 12 or 24 daily observations. The daily mean temperature determination based on 24 hourly observations reduces the sample daily temperature uncertainty to − 0.01 ± 0.20 °C. Estimating the parameters of population of all SURFRAD observations, the 95% confidence intervals based on 24 hourly measurements is from − 0.025 to 0.004 °C, compared to a confidence interval from − 0.15 to 0.05 °C based on the mean of T max and T min.

Summertime hot extremes in China are categorized into three distinct types, i.e., independent hot days, independent hot nights, and compound events, based on differing configurations between daily maximum and minimum temperature. Linear trends for multiple indictors of these subtypes and traditionally defined hot days/nights exhibited remarkable differences in significance, magnitude, and even sign, especially for events involving daytime extremes. Thus, some significant changes masked in conventional analyses are successfully uncovered. Particularly, the dominance of independent hot days has decayed significantly, accompanied by a rapid boom of compound events and/or independent hot nights in different regions. These nighttime‐accentuated hot extremes have exhibited significant increases in duration, intensity, and spatial extent, with much stronger trends detected in severest events.

Cold and warm temperature extremes predominantly occurring in winter gained much more attention than mean temperatures. On the basis of daily maximum and minimum surface air temperature records at 303 meteorological stations in China, the spatial and temporal distributions of five indices for winter (DJF: December, subsequent January and February) temperature extremes are analysed during 1961–2003. For the majority of stations, the frequency of cold days/nights decreases by − 1.33/− 2.98 and warm days/nights increases by 0.92/2.35 d/decade, respectively. Cold days/nights are significantly negatively correlated with the Arctic Oscillation (AO) index, while warm days/nights are positively correlated with the AO. The diurnal temperature range (DTR) has a declining trend with rate of − 0.25 °C/decade and positive correlation with the AO index. Compared with other regions in China, stations in the northern China have larger trend magnitudes and stronger correlations with the AO index, and the AO can explain more than 50% of winter temperature extreme change in China. Compared with the annual basis, the winter temperature extremes have larger trend magnitudes, which reflect the rapid warming. During strongly positive AO index years, enhanced contrast tropospheric temperature (defined as the average of air temperature vertically integrated between 200 hPa and 1000 hPa based on the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis) between the north of China and the southern China weakens the East Asian winter monsoon which in turn reduces cold outbreaks in the northern and eastern China. The composites of large‐scale atmospheric circulation are consistent with the asymmetrical changes of the geopotential height, zonal and meridional winds at high and mid latitudes at troposphere. Meanwhile, the linkage between the AO and solar activity also modulates the winter temperature extremes, while the mechanism needs to be investigated. Copyright © 2012 Royal Meteorological Society

This study evaluates the ability of 23 climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating extreme climate events over China. The multimodel ensemble (MME) performs better than most individual models in reproducing the climatological mean distribution of all extreme indices. The MME can reproduce well the climatological mean distributions of five extreme climate indices over China, including annual total precipitation (PTOT), maximum consecutive 5‐day precipitation (RX5), simple daily intensity (SDII), maximum daily maximum temperature (TXX), and minimum daily minimum temperature (TNN), with Taylor skill scores exceeding 0.7. SDII and TXX are the most skilful precipitation and temperature extreme indices simulated by the MME, respectively. The MME has relatively lower skill in simulating the climatological mean distribution of warm days (TX90P) and cold nights (TN10P) over China. Future projections of these extreme climate indices by the end of the 21st century are explored with the MME under the SSP1‐2.6, SSP2‐4.5, and SSP5‐8.5 scenarios. The PTOT and RX5 in northwestern China are all projected to increase by more than 30% under SSP5‐8.5. R20 is projected to increase by 4–5 days over southeastern China under SSP5‐8.5. There are fewer (more) consecutive dry days over north China (south China), with a change of 5 days under SSP5‐8.5. The extreme temperature indices, including TX90P, TXX, and TNN, all increase with time and higher SSP scenarios. The three indices increase by 40–55%, 4–6°C, and 4–7°C under SSP5‐8.5 over east China, respectively. The TN10P decreases by more than 6% over east China. The changes in these extreme indices under SSP1‐2.6 and SSP2‐4.5 are similar to those under SSP5‐8.5 but with a smaller magnitude. Large uncertainties still exist in the future projections, especially under the high SSP scenarios.

HKUST Electronic Theses Characterization of compound hot and dry extremes in China based on statistical and dynamical downscaled climate projections by Zhu Ziwei thesis 2024 1 online resource (ix, 59 pages) : color illustrations, color maps The increasing severity and…Read more ›