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 daily precipitation and average temperature data at 18 meteorological stations of Ningxia from 1981 to 2016, prefectural and municipal disaster situations and annual climate impact assessment reports, CI and MCI were used as drought monitoring indicators, and their differences of application on meteorological drought monitoring in Ningxia from three aspects of drought days, drought intensity and typical drought events were compared and their applicability was explored. The results show that the annual change trends of drought days with moderate level and above monitored by CI and MCI were consistent in the past 36-year in Ningxia, they presented decreasing trend as a whole, but the drought days monitored by CI were more than MCI, and their spatial distribution characteristics were different, to certain extent, especially in northern arid region. The change trends of drought intensity monitored by CI and MCI were basically same, but the drought intensity of CI was stronger than MCI, and the less precipitation was, the stronger drought intensity of CI was. For typical drought events, the drought intensity monitored by CI was strong, the beginning was early, the duration was long, and the jumping phenomenon between drought levels was obvious, which wasn’t accordance with the evolution characteristics of drought. The monitoring effect of MCI was more stable and better sustainability, but the drought intensity was lighter. The precipitation was generally less in Ningxia, it concentrated from May to September, and the evaporation was large. Considering the influence of precipitation previous 150-day, the drought intensity monitored by MCI was lighter than actual situation. On the whole, CI was more suitable to daily meteorological drought monitoring than MCI in Ningxia, but two indexes could reflect the temporal evolution characteristics of meteorological drought days and intensity in central and southern Ningxia.

Based on the observation data from 20 meteorological stations in Ningxia during 1961-2018, variation characteristics of winter temperature in Ningxia were analyzed on the monthly timescale using S-EOF. The results show that the spatial patterns in each month of the first mode of S-EOF analysis were characterized by region-wide uniform. The spatial patterns of the second and third modes in each month were marked by an alternation of warm and cold phases. The spatial patterns of the second mode in December presented partial cold (warm), and in January and February it presented warm (cold). The spatial patterns of the third mode in December and January presented partial cold (warm), and in February it presented warm (cold). The temporal and spatial distributions of the second and third modes were in good correspondence with the intensity adjustment processes of the Ural mountain ridge, Siberia high alternation and East Asia trough on the monthly timescale from December to February. When the Ural high ridge and the East Asian trough were stronger, and the Siberia high was strengthened, the temperature in corresponding months was low, and vice versa.

Based on data such as first frost date in Ningxia, geopotential height, sea surface temperature (SST), snow cover area, and sea ice area from 1981 to 2019, the influence of external forcing factors including sea surface temperature, sea ice area, and snow cover area on the abnormally early and late first frost in Ningxia was studied. On the basis of above, a physical conceptual model and an objective prediction model for predicting first frost date were established. The results are as follows: (1) In the early years of first frost, the SST in the equatorial central and eastern Pacific continued to be significantly warmer in the early period, and the SST anomaly presented an obvious ENSO model. When the SST of the equatorial central and eastern Pacific was warmer in the early period, the east Asian trough was stronger and the subtropical high was weaker, which was conducive to cold air activity. So, first frost date was early, otherwise it was late. (2) The snow cover area in the northern hemisphere from May to August in the early period and the sea ice area of Greenland from January to July had a continuously and significantly negative correlation with the date of first frost. When the snow cover in the northern hemisphere decreased or the sea ice in Greenland decreased, the east Asian trough was weaker and the western Pacific subtropical high was relatively stronger, which was not conducive to active cold air, causing first frost to be late, and vice versa. (3) The main factors affecting the date of first frost in Ningxia were the intensity of the east Asian trough, the SST anomaly in the NINO3.4 area, the SST anomaly in the tropical south Atlantic, the snow area in the northern hemisphere, the intensity of the western Pacific subtropical high, and the area of Greenland sea ice. The objective prediction model established by using the multiple regression equation had a good prediction effect.

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.

The rainfall and its process prediction during wine grape harvest period were analyzed by using rainfall data in the east foothills region of  Helan Mountain located in Ningxia during 1972-2016 and the atmospheric circulation, sea ice and SST data. Prediction results had been evaluated by using anomaly symbol consistence rate. The results show that the anomaly symbol consistence rate of the model established based on 200 hPa geopotential height, sea temperature and sea ice prediction factors was 67.0%, especially in concurrent rich or seldom rainfall years, it could reach 86.0%. Further, pentad rainfall prediction model had been built, the forecast accuracy was 54.2%, false alarm rate was 32.3%, false negative probability was 13.5%.

Based on monthly temperature in winter from 20 meteorological stations in Ningxia, monthly sea ice concentration in autumn from the Hadley Centre of UK and monthly atmospheric reanalysis in winter from the NCEP/NCAR from 1961 to 2016, temperature anomaly and its causes in winter of 2016 in Ningxia were studied. The results show that the temperature in winter of 2016 was the highest in the same period since 1961 in Ningxia. In 2016, the 500 hPa zonal circulation was obvious over the middle and high latitudes of Eurasia, and the Ural mountains blocking high was unusually weak, and geopotential height over the mainland China was unusually higher, and position of polar vortex skewed Europe and North America. East Asian winter monsoon index was 1.3 m·s-1, which was the fifth low value since 1961, and Siberia high intensity anomaly was 1.5 hPa, which was the second low value since 2000. The sea ice concentration of  Greenland sea in autumn had a significant influence on temperature in winter in Ningxia. When the sea ice concentration was low, the East Asian winter wind was weaker,the wave pattern labeling “-+-”  on 500 hPa geopotential height in Arctic, Eurasia and Aleutian region enhanced the height difference between the Arctic and Eurasia in the middle and high latitudes, and the westerly airflow over the middle and high latitudes was stronger, and the zonal activity was strengthened. At the same time the weaker Siberian high in the sea level pressure field was not conducive to the Arctic cold air intruding into low latitude region. All above reasons resulted in  higher temperature anomaly in 2016 in Ningxia.