The use of different climatological normal periods means the change of the evaluation results of meteorological elements, the abnormal state of climate events and their change characteristics, which have a substantial impact on the climate monitoring and prediction operations. Using temperature and precipitation observation data from national meteorological stations in Ningxia from 1981 to 2021, this study conducted a comparative analysis of the temperature and precipitation characteristics between the old climatological normal period (1981-2010) and the new climatological normal period (1991-2020). Additionally, it explored the changes in extreme characteristics of these factors. The results are as follows: Compared to the old climatological normal period, the annual and seasonal average temperatures in Ningxia are generally higher in the new climatological normal period, which is particularly evident in spring, summer and winter, and the frequency of abnormally high (low) temperatures increases (decreases) accordingly. Yinchuan, the western part of Wuzhong, and the northern part of Zhongwei are areas experiencing significant temperature increase. The overall intensity of extreme high (low) temperature has intensified (weakened), and their frequency has increased (decreased). In summer, the threshold and intensity of extreme high-temperature rise across various regions, especially in the central and northern areas, while in winter, the intensity of extreme low-temperature weakens in most regions, the amplitude of extreme low-temperature varies significantly. The average annual precipitation, as well as the summer, autumn and winter average precipitation, are greater in the new climatological normal period compared to the old. There’s an increased frequency of abnormally more precipitation in summer and autumn, whereas the opposite trend is observed in spring and winter. Meanwhile, the frequency of abnormally less precipitation in all seasons has decreased to some extents. There are significant spatial differences in seasonal precipitation, with a general increase in precipitation in summer and autumn, and a pattern of “decreasing in the north and increasing in the south” in spring and winter. The overall trend of extreme precipitation in spring and autumn (summer and winter) is intensifying (weakening), albeit with fewer (more) extreme precipitation events. In summer, the threshold and intensity variations of extreme precipitation are greater in the north and south, and smaller in the central region, with a notable increase in extreme precipitation in Shizuishan.

In order to project the future climatic characteristics and their changing tendencies in different areas in Ningxia section of the Yellow River Basin, the performance of the CMIP6 models in simulating the annual mean air temperature in Ningxia are evaluated based on observation data at 19 national meteorological stations and the CMIP6 models data. Then the future air temperature changes in the Yellow River irrigation area, the middle arid area and the southern mountainous area of Ningxia under different scenarios are analyzed. The results are as follows: (1) Most models of the CMIP6 have a good simulation ability to annual mean air temperature in the Ningxia section of the Yellow River Basin, with spatial correlation coefficient of 0.603-0.930 and temporal correlation coefficient of 0.381-0.782. Meanwhile, the result of multi-model ensemble simulation is better than that of a single model. (2) Under the SSP1-2.6, SSP2-4.5, SSP3-7.0 and SSP5-8.5 scenarios, it is predicted that the annual mean air temperature in the Yellow River Basin Ningxia section will present a significant warming trend from 2021 to 2099, with a warming rate between 0.09 and 0.68 ℃·(10 a)^-1. The warming rates are obviously different in different scenarios, which shows a decreasing trend under the SSP1-2.6, and firstly increasing and then decreasing trend under the SSP2-4.5, an increasing-decreasing-increasing trend under the SSP3-7.0, and an increasing trend under the SSP5-8.5. (3) It is estimated that the annual mean air temperature in the Yellow River irrigation area, the middle arid area and the southern mountainous area will reach 10.91-11.29, 9.48-9.87, 7.47-7.84 ℃ in the 2030s, respectively, and 11.46-13.21, 10.00-11.75, 7.97-9.66 ℃ in the 2060s under the four scenarios, respectively.

With the global warming, the intensity and frequency of abnormal drought and flood are increasing, to improve the understanding of drought-flood transition events and the ability of precipitation prediction in the east region of Northwest China, the circulation characteristics of drought-to-flood transition over the east region of Northwest China from spring to summer are analyzed by establishing an index based on the observed monthly mean precipitation, sea surface temperature (SST), and NCEP/NCAR reanalysis datasets during 1979-2020, and the possible influence from the Atlantic SST anomalies is also discussed. The results are summarized as follows: In spring of drought-to-flood years, the polar vortex is weaker, the Ural blocking is stronger and deeper, and the east Asian trough is deeper, which result in less precipitation in the east region of Northwest China controlled by dry and cold northwest flow. In summer, the upstream low-pressure system is active, the south Asian high is stronger and the western Pacific subtropical high (WPSH) is stronger and westward. Such circulation anomalous can lead to warm and moisture air supplement, resulting in a sharp transition phenomenon occurring in the east region of Northwest China. While in flood-to-drought years, it presents an opposite feature. From the previous winter to summer, the Atlantic tripole SST anomalies are key factors affecting the difference of precipitation between spring and summer. In spring of drought-to-flood years, the negative phase of the Atlantic tripole SST pattern stimulates a zonal teleconnection wave train, passing through the central and western Europe, Lake Balkhash, and Northeast China to the Sea of Japan, and this circulation is conducive to less precipitation in the study area. With respect to summer, the intensity of the wave train is weakened and shifts westward. Therefore, the intensity and location of the key circulation system in the middle and high latitudes are adjusted comparing with that in spring, causing more precipitation. In flood-to-drought years the opposite occurs.

Based on the observational rainfall data in July of Ningxia, the NCEP/NCAR reanalysis data and the ERSSTv5 SST data from 1981 to 2018, the variation characteristics of precipitation in July in Ningxia and their formation causes were analyzed. This paper also discussed the influence of the equatorial Pacific SST anomaly on precipitation. The results are as follows: (1) In the past 38 years, the inter-annual variability of precipitation in July was significant, but the extremity and anomaly of precipitation both increased after 2000. (2) The 500 hPa geopotential height anomaly was low over the Balkash lake and it was high between North China, the Northeast China and the Sea of Japan in mid-high latitudes. The active low-pressure system from the Bay of Bengal to the South China Sea, coupled with water vapor transported by the southerly anomaly on 700 hPa from the Northwest Pacific, led to more precipitation in July in Ningxia. (3) Since the 21th century, the implication of equatorial Pacific SST on precipitation in July has strengthened, especially during late winter and early spring during the ENSO decaying phase. When the SST anomaly in the equatorial Pacific characterized  by a dipole pattern, namely, warm (cold) in the west and cold (warm) in the central, and the subtropical western Pacific anomaly cyclone (anticyclone) in July played an important role in transportation of water vapor. The pattern of lower in the west (east) and higher in the east (west) existed on 500 hPa height, which was favorable to more (less) precipitation and reflected the impact of the central pacific type of ENSO on precipitation in Ningxia to some extent.