In the spring of 2023, a severe meteorological drought occurred in southwestern China, which had a serious impact on the local social economy. In order to deeply understand the causes of this drought event and further provide a basis for the prediction technology of spring drought in southwestern China, causes of the spring drought event were analyzed from the perspective of SST (sea surface temperature) and MJO (Madden-Julian Oscillation) by using station-observed data, NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data, NOAA (National Oceanic and Atmospheric Administration) SST and other data, choosing T-N wave activity flux, composite analysis and other methods. The results showed that: (1) The spring drought over southwestern China in 2023 was a compound dought-heatwave event occurred in the middle in March, developed and expanded westward in April, and maintained in May. (2) In March, the horseshoe-shaped SST anomalies in the North Pacific caused the westerly jet stream to drift southward and westward, suppressing the precipitation in southwestern China. (3) In April, the anticyclonic circulation anomaly near the Bay of Bengal induced by warm SST in the Indian Ocean through Kelvin wave and the cyclonic circulation anomaly from the South China Sea to the Philippines triggered by warm SST in the Northwest Pacific through Rossby wave resulted in northerly winds in the south of southwestern China, which caused the divergence of water vapor and the development of drought in southwestern China. (4) MJO, which maintained in the western Pacific for a long time in May, stimulated the cyclonic anomaly from the South China Sea to the Philippines in lower troposphere due to Gill response, reduced the transport of southerly water vapor, and maintained drought in southwestern China.

Improving the Temperature Vegetation Dryness Index (TVDI) and clarifying the agricultural drought grade threshold of TVDI is of great significance for improving the ability of TVDI to monitor agricultural drought. Based on MODIS (Moderate Resolution Imaging Spectroradiometer) remote sensing data in the past 19 years, several feature spaces are constructed by using the single-time and multi-time methods, including NDVI (Normalized Difference Vegetation Index) -LST (Land Surface Temperature), EVI (Enhanced Vegetation Index) -LST, RVI (Ratio Vegetation Index) - LST, and SAVI (Soil Adjusted Vegetation Index) -LST. The calculation methods of TVDI are discussed, the applicability of TVDI for agricultural drought monitoring in Gansu Province is analyzed, and classification standards for summer TVDI agricultural drought in Gansu Province are clarified. The research results are as follows: 1) The TVDI calculated from the SAVI-LST feature space is more suitable for agricultural drought monitoring in Gansu Province. The root mean squared error (RMSE) and mean absolute error (MAE) of its fitting relative soil moisture (RSM) decreased by 1%-5% compared with the RMSE and MAE of RSM fitted by NDVI-LST feature space TVDI for RSM, which is used more commonly. 2) TVDI is suitable for agricultural drought monitoring at shallow depths of 10 and 20 cm in non-arid areas such as semi-arid, semi-humid and humid areas in Gansu Province in summer. The RMSE and MAE are approximately 15.6% and 12.6%, and the fitting errors in humid areas are the least, and they are less in semi-humid areas than in semi-arid areas they are the largest. 3) Compared to TVDI drought grades divided by 0.2 intervals and TVDI with uncertain classification criteria, the TVDI agricultural drought grade determined by the linear relationship between TVDI and RSM is more conducive to improving the accuracy of TVDI monitoring agricultural drought.

From January to June 2023, meteorological droughts of varying degrees occurred in southwestern China, eastern North China, northern East China, central southern China, southern South China, and central Northeast China, seriously affecting agricultural production and restricting local economic development. To improve the ability to respond to drought disasters, timely carry out disaster prevention and reduction work, and conduct real-time summaries of drought situations. This article uses K index, MCI index, T-N flux and CABLE land surface model, as well as meteorological observation data, reanalysis data, soil moisture data to comprehensively analyze the spatiotemporal distribution characteristics and causes of regional drought events. The results are as follows: (1) In the first half of 2023, severe regional drought occurred in southwestern China and eastern Inner Mongolia. The southwestern region experienced a transition from sustained to sudden drought, while Inner Mongolia continued to experience drought. (2) During the same period, the 500 hPa geopotential height field showed a two trough and two ridge pattern at mid to high latitudes. The western Pacific subtropical high pressure abnormally extended westward and uplifted northward, and the Rossby waves at mid latitudes in Eurasia were abnormally weak, resulting in a weakening of the influence of flat westerly winds and cold air in mid to high latitudes, resulting in less precipitation in the southwest and eastern Inner Mongolia, leading to regional drought. (3) In the first half of 2023, the winter La Niña event shifted to the spring El Niño event, resulting in weak convective activity in the southwest region and triggering sustained hot and dry weather; The distribution of sea surface temperature sensitive areas in Inner Mongolia has led to the stability of its upstream high-pressure ridge, resulting in drought and limited rainfall in the eastern region of Inner Mongolia.

Drought is one of the natural disasters with the widest global impact. The anomalous drought and heatwave event that occurred in the Yangtze River Basin in summer of 2022 is not only of high intensity but also of long duration, it is a rare and significant drought event leading to very serious socioeconomic impacts in China. In view of the extreme nature of this event, this paper reveals the possible influence of atmospheric circulation and external forcing anomalies on this drought event based on an objective analysis of the evolutionary characteristics of this event. It is found that the meteorological drought index and soil moisture monitoring results consistently indicate that this drought event started to appear in June, developed rapidly in July, and further expanded and intensified in both extent and intensity in August. At the same time, the overall temperature in the basin was high, with the number of high temperature days exceeding 40 days in some areas. In addition, anomaly of evapotranspiration over the basin in summer was the second highest on record since 1960, second only to the high temperature drought event in 2013, which further exacerbated the degree of water deficit in the Yangtze River Basin. From the perspective of circulation characteristics, the abnormal intensifying and westward extension of the western Pacific subtropical high pressure, the small area and weak strength of the polar vortex and the intensifying and eastward shift of the South Asian high pressure in summer jointly led to weak water vapor transport conditions and prevailing sinking air currents in the Yangtze River Basin, making the overall conditions unfavorable for the occurrence of precipitation. The persistence of the La Niña event, the appearance of negative Indian Ocean Dipole (IOD) and the persistence of the negative snow cover anomaly in the northwestern Tibet Plateau in spring may be the main external forcing factors leading to the circulation anomaly in this summer.

The European Space Agency Climate Change Initiative soil moisture (ESA CCI_SM) product can provide soil volumetric water content data with high spatiotemporal coverage and a long time series. It can be used for drought monitoring on a regional scale. Gansu Province was selected as the study site for ESA CCI_SM testing. Soil volumetric water content data at the 0-10 cm depth from automatic station was used to evaluate the ESA CCI_SM product by analyzing the spatiotemporal variation characteristics of soil moisture in a long time series and construction of a soil moisture condition index (SMCI) for use in drought monitoring. The ESA CCI_SM product showed the best correlation (R=0.71) with observed values in the east of Gansu Province, followed by the middle, the south and Min county of Gansu Province. Overall, the 92% correlation coefficients of the sites were significant and passed 0.01 significant level. ESA CCI_SM indicated that it was more arid in the northwestern regions while it was more humid in the southeastern regions. Soil volumetric water content showed significant seasonal variation, which was relatively more humid in antumn, followed by summer, while in winter and spring it was drier. The SMCI constructed using the ESA CCI_SM product effectively exhibited the summer-autumn drought development from July to October 2002 and the spring-summer drought development from April to June 2009. This product had good drought monitoring capability in rain-fed agricultural areas in the middle and the east of Gansu Province.