Compound hot and dry events can pose significant threats to agricultural production, water-resource security, and socioeconomic development. Using multisource reanalysis datasets in combination with a three-dimensional connected component (CC3D) algorithm and the daily standardized precipitation index (SPI), this paper identified and investigated a large-scale compound hot and dry event that occurred over the Yangtze River Basin during the summer-autumn of 2024, with emphasis on its spatiotemporal characteristics and driving mechanisms. The event persisted for 65 days and affected nearly the entire basin, with a heat intensity of 2.1 °C and a drought intensity of -0.64. It exhibited a clear westward migration and progressive intensification, evolving from the middle-lower Yangtze Plain toward the Sichuan Basin. The period from 21 August to 20 September marked the peak stage, during which heat and drought anomalies were substantially stronger than those in midsummer, the positive anomaly of daily maximum temperature reached the highest level on record for the same period, and the negative daily precipitation anomaly was the strongest since 1998, forming a rare “autumn tiger” episode. Mechanistically, the event was jointly driven by the stage-wise alternation and superposition of mid-high-latitude Rossby wave trains and tropical convection. In the early stage, an East Asian anticyclone triggered by a “+-+” wave-train pattern dominated the circulation, inducing persistent subsidence and enhanced solar radiation. During the middle stage, enhanced tropical convection maintained and intensified the hot-dry conditions through a local meridional circulation in concert with high-latitude dynamical forcing. In the late stage, the process became primarily controlled by tropical convection, which continuously sustained the extreme heat and drought anomalies.
Conventional standardized precipitation index (SPI) datasets generally suffer from low temporal resolution, which limits their capability to support fine-scale monitoring of drought evolution. To address this issue, this study constructs a daily SPI dataset for China based on ERA5 reanalysis precipitation data (SPID-ERA5). Taking the middle and upper reaches of the Yellow River as the study area, two typical drought years, 1997 and 2001, are selected, and the applicability of SPID-ERA5 is evaluated using station-based monthly SPI and the daily meteorological composite index (MCI). The results show that SPID-ERA5 can clearly depict the onset, development, expansion, and weakening processes of drought, and can reasonably reflect the spatial extent and intensity variations of drought, while the daily MCI places more emphasis on the cumulative effects of moisture deficits and shows a relatively lagged response to drought evolution. The sign consistency rate and correlation coefficients between daily MCI and SPID-ERA5 are generally high, with broadly consistent variation trends, and MCI fully considers the contribution of antecedent precipitation, exhibiting evident cumulative characteristics. SPID-ERA5 can continuously capture fluctuations in drought intensity and process transitions, whereas station-based monthly SPI is constrained by temporal resolution and is insufficient to represent short-term drought variations. The high spatiotemporal resolution SPID-ERA5 provides a new approach to compensate for the deficiencies of traditional drought datasets and has promising application value in drought monitoring, risk assessment, and early warning services.
In the spring of 2019, Yunnan experienced a severe seasonal drought characterized by its extensive impact area and high intensity, which significantly affected agricultural production and water resources. By utilizing precipitation data from 125 meteorological stations in Yunnan, reanalysis data from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR), and high-resolution sea surface temperature data from the National Oceanic and Atmospheric Administration (NOAA), and employing statistical analysis methods such as correlation and synthesis analysis, this study analyzed the causes of the spring drought in Yunnan from the perspective of tropical atmospheric intraseasonal oscillation (Madden-Julian Oscillation, MJO) and abnormal sea surface temperature changes. The results are as follows: (1) The spring precipitation in Yunnan region is significantly modulated by the phase of MJO. When MJO is in the third and fourth phases, it usually leads to an increase in precipitation. However, during the critical period of precipitation from April and May 2019, MJO was mainly active in the Western Hemisphere and the African region, corresponding to its 8th and 1st phases, resulting in weak convection over the Indian Ocean-West Pacific region and blocking the northward transport of moisture by high-pressure systems and resulting in unfavorable moisture convergence conditions over Yunnan. (2) From March to May in 2019, there was a persistent sea surface temperature positive anomaly in the central-eastern Pacific, which weakened the tropical zonal circulation, and the convective activities over southeast Asia and South China Sea were suppressed, further weakening the conditions for water vapor transport to Yunnan. Most areas of Yunnan were in a moisture divergence zone, which was unfavorable for water vapor accumulation and resulted in less precipitation. (3) The positive anomaly of sea surface temperature in the central-eastern Pacific caused a weakening of the Walker circulation, and coupled with the circulation anomalies induced by the active MJO over the Western Hemisphere, both the two factors worked together and caused the strong and westward extension of the low-latitude subtropical high pressure system, which blocked the transport of water vapor from the Bay of Bengal to Yunnan, resulting in significant decrease of precipitation there in spring of 2019.
The alternation between drought and flood is more destructive than a single drought or flood disaster. Studying its evolution patterns and prediction methods is of great significance for enhancing disaster prevention and mitigation capabilities. Based on monthly precipitation data from 70 national meteorological stations in Hubei Province from 1960 to 2024, and the fifth-generation reanalysis data from the European Centre for Medium-Range Weather Forecasts, this study analyzes the temporal evolution characteristics of the Long-cycle Drought-Flood Abrupt Alternation Index (LDFAI) in typical years of summer drought-flood alternation (drought-to-flood and flood-to-drought) in Hubei Province, and explores the corresponding atmospheric circulation and water vapor transport anomalies. The results are as follows: 1) From 1960 to 2024, a total of 21 summer drought-flood alternation events occurred in Hubei Province, including 10 drought-to-flood events and 11 flood-to-drought events. The LDFAI showed a weak upward trend at a rate of 0.03 per decade, while its intensity decreased at a rate of 0.07 per decade, but both didn’t pass the significance test. 2) The LDFAI exhibits distinct decadal variation characteristics: the 1960s was a period with frequent drought-flood alternation, a total of 7 events occurrence (4 drought-to-flood and 3 flood-to-drought); in the 1970s, there was frequent flood-to-drought events, reaching 4 times; after the 1980s, the frequency of drought-flood alternation events tended to be balanced. The intensity of LDFAI experienced an abrupt change in 1973, and significantly decreased after 1980. 3) High-incidence areas for drought-to-flood events were concentrated in central and western Hubei, with the southern part of Shennongjia and the northwestern part of Yichang being high-risk regions. High-incidence areas for flood-to-drought events were widespread in central, western, and eastern Hubei, with Enshi being a high-risk region for flood-to-drought events. 4) During the drought period of drought-to-flood years, it was controlled by a southerly Western Pacific Subtropical High (referred to as “the subtropical high”) and mid-to-high latitude zonal circulation in Hubei Province, coupled with subsidence motion and water vapor divergence, leading to less precipitation. During the flood period, a mid-high latitude meridional circulation developed, and the ascending movement and water vapor convergence on the northwest side of the subtropical high increased, leading to more precipitation. The circulation and water vapor anomalies in flood-to-drought years were opposite to these.
Investigating the spatiotemporal characteristics of drought in western Guangdong and their correlation mechanisms with circulation indices is of great significance for regional disaster early warning and agricultural meteorological services. Meteorological data from 13 stations in western Guangdong during 1973-2022 and atmospheric circulation indices were used to calculate the multi-scale standardized precipitation evapotranspiration index (SPEI). Trend analysis, correlation analysis, and cross wavelet transform were applied to analyze the spatiotemporal drought characteristics and their correlations with circulation indices in western Guangdong over the past 50 years. The results showed that the annual, spring, and summer SPEI exhibited a drying trend in western Guangdong. The climate tendency rate reached -0.24 per decade in Huazhou and Yangchun, which were the centers of spring drying, with annual-scale drying dominated by precipitation decrease. Drought intensity in western Guangdong was mainly light to moderate. Although droughts occurred frequently in autumn and winter, their long-term trends were not significant. The correlation coefficient analysis indicated that spring drought was significantly correlated with the northward shift of the subtropical high ridge line over the South China Sea and the positive phase of the Western Pacific (WP) index in the preceding winter, while autumn drought was associated with the northward shift of the subtropical high ridge line six months earlier, a contracted Northern Hemisphere polar vortex nine months earlier, and the synergistic suppression of precipitation by positive phases of the Pacific Decadal Oscillation (PDO) index and Pacific-North American pattern (PNA) index. Cross-wavelet transform analysis indicated that the interdecadal variability of drought in western Guangdong was dominated by a 5-10 a periodicity associated with the PDO and subtropical high, which set the drought background, superimposed by inter-annual fluctuations driven by a 3-5 a periodicity of the Southern Oscillation Index (SOI) and the Northern Hemisphere polar vortex area index. During 1995-2005, the annual-scale SPEI indicated a negative phase resonance with the PDO, WP, and the South China Sea subtropical high ridge position index at a 5-10 a period, lagging behind the PDO by approximately 3 months. During 1996-2002, the annual-scale SPEI exhibited a negative phase resonance with the SOI and the Northern Hemisphere polar vortex area index at a 3-5 a period, lagging behind polar vortex area changes by approximately 9 months.
