Characteristics of the Spring Soil Moisture Evolution over the Tibetan Plateau from 1981 to 2020 and Its Relationship with the Plateau Monsoon

This study uses the daily soil moisture (soil volumetric water content), precipitation, geopotential height field, and wind field data from the European Center ERA5 reanalysis data focusing on the analysis of the shallow (0–7 cm) soil moisture of the plateau spring from 1981 to 2020. The study also determines the characteristics of the spring soil moisture evolution over the Tibetan Plateau and discusses its relationship with the plateau monsoon. The results show that the distribution characteristics of the spring soil moisture on the Tibetan Plateau are dry in the northwest and relatively wet in the southeast. The empirical orthogonal function (EOF) analysis of the spring soil moisture in the plateau shows that the first mode has the characteristics of reverse change in the middle, east, and west. This mode has a quasi 3-year (2–4 years) oscillation period, which was more pronounced from 2000 to 2010. The second mode has a north–south reverse distribution, which better characterizes the climatic zone and underlying surface coverage in the plateau area. This study identified a close inter-season correlation between the plateau summer monsoon and plateau spring soil moisture change. The abnormal change of the plateau summer monsoon is the main reason for the spring soil moisture change in the following spring.