The Horn of Africa (HOA) is one of the most drought-prone regions in the world with many arid and semiarid areas, and even some extremely arid areas. Primarily affected by the marine continental climate systems, this zone is really sensitive to global warming. Drought is the main type of natural disaster affecting this region, which triggers famine, civil conflict, and even deterioration of food security. The present study examined changes in droughts in the HOA during 1979–2019 based on Standardized Precipitation Evapotranspiration Index (SPEI). Results show that frequency, duration, and intensity of droughts exhibited an increasing trend over the past decades. Moreover, in October to December (called locate “short rains”) 2016 and March to May (called locate “long rains”) 2017, the HOA experienced the most severe drought. Based on Generalized Extreme Value (GEV) fitting, the 2016/2017 SPEI index corresponds to a drought that occurs every 250 years in the observational records. However, considering the precipitation in short rains of 2016 (long rains of 2017) was only 27.1% (11.8%) less than normal, it is hard to explain the formation of this extreme drought only from precipitation anomalies. Further statistical result shows that the evaporation in 2016/2017 corresponds to a 1-in-131 years event in the observed records. The abnormally high temperature (1.02 °C higher than normal) as well as the greatest potential evaporation since 1979 are the more important causes for the formation of drought. Thus, the extreme drought in 2016/2017, probably caused by the combined effect of dry condition and high temperature simultaneously, and the latter, played a leading role. In other words, droughts can be exacerbated by the co-occurrence of extreme high temperature. With continuous warming caused by anthropogenic activities in the next decades, the Horn of Africa may be a hotspot of the compound droughts and, therefore, it is especially important to considering the combined impacts from less precipitation and high temperature when predicting the future drought trend and making adaptation measures.

The anticyclonic anomaly over the Ural Mountains, or the Ural high (UH), has recently received much attention as a factor related to weather anomalies across Eurasia. Here we studied how the UH affects the occurrence of cold wintertime episodes over eastern Europe and northern China. By employing three methods to identify the UH, we found that a method based on the sea level pressure anomaly captures a stronger cooling signal over eastern Europe and this method includes nonblocking cases associated with low-level anticyclones that do not affect the upper troposphere. However, with UH occurrence, a stronger cooling over northern China is detected by a method based on 500-hPa geopotential height fields. Cold events over eastern Europe typically occur when UH formation is associated with a Rossby wave breaking in the upper level. Our results show that the horizontal temperature advection plays an important role in formation of cold episodes in both eastern Europe and northern China. The advection is balanced by diabatic processes, which show an opposite sign to the temperature advection in both regions. Also adiabatic warming contributes to balancing the advection in northern China. We find that the exact location of the positive SLP anomaly during UH episodes is the most important factor controlling whether or not eastern Europe or northern China will experience a cold episode. If the positive SLP anomaly develops more northwest than usual, eastern Europe will experience a cold episode. When the anomaly moves eastward, northern China will be cold.