Mechanism analysis of the summer dry-wet interdecadal transition in the Tarim Basin, Northwest China

Using the ERA-5 monthly reanalysis data and precipitation observations, this paper analyses the interdacal dry-wet transition and its mechanism in the Tarim Basin(TB) during summer. From the long-term perspective(1961–2021), TB is in a trend of "wet in the west and dry in the east". By taking a nine-year moving average of the precipitation(PRE), the research period is divided into three subperiods: P1(1961–1987), P2(1988–2009) and P3(2010−2021). From P1 to P3, the regional averaged Standardized Precipitation Evapotranspiration Index(SPEI) demonstrates an interdecadal turning of "wetting to drying". During P2(1988–2009), the dynamic component of water vapor increases the zonal moisture transport, causing a slight increase in PRE. The potential evapotranspiration(PET) is weakened by the increase in PRE and relative humidity(RH), the decrease in 2-m temperature(T2m) and 10-m wind speed(10UV), resulting in the humidification of the TB. In the P3(2010–2021), the non-linear component of water vapor increases moisture convergence in the western TB, causing a significant increase in PRE. However, the decrease in RH, accompanied by increases in T2m and UV10, results in a rapid increase in PET. The TB gets dry. The South Asian high(SAH) develops southeastward, guiding more water vapor transport from the upstream westerly and the tropical Indian Ocean. The Western Pacific subtropical high(WPSH) extends northwestward, increasing water vapor convergence in the western TB. The main reason for the increased precipitation changes from increased water vapor advection to enhanced water vapor convergence. The differences in PRE formation and T2m amplitude cause the interdecadal change in RH, deriving an interdecadal transition of wet and dry in the TB. Our findings may deepen the understanding of climate change, and provide a scientific support for ecological development, and global climate governance in arid/semi-arid regions.