Wetting trend in Northwest China reversed by warmer temperature and drier air

• The relative contributions of meteorological variables to the dryness changes have been quantified in Northwest China. • Warmer temperature and drier air have resulted in significant increase of dryness in recent decades. • Dryness may further increase along with higher temperature, drier air, and higher wind speed. Northwest China has experienced a warming-wetting climate featured by increasing temperature and precipitation during the past six decades. Recent studies suggested that the climate may have shifted from wetting to drying after the 1990s if changes in energy supply were considered. However, how the change in energy supply affects the dryness remains unclear. In this study, we quantified the dryness in Northwest China based on the Standardized Precipitation Evapotranspiration Index (SPEI) which considers the impact of potential evapotranspiration estimated by the Penman-Monteith equation on dryness during 1961–2018. Then, the contributions of changes in meteorological variables (precipitation, temperature, relative humidity, wind speed, and net radiation) to the dryness changes were estimated using a detrending technique. The results indicated that Northwest China experienced a wetting period (1961–1989) and then a drying period (1989–2018). During 1961–1989, increased precipitation and slowed wind were the main drivers of the wetting trend. During 1989–2018, although the increased precipitation and decline in solar radiation favored a wetting condition, warmer temperature, drier air (decrease in relative humidity), together with rebounded wind speed offset the wetting trend and resulted in significantly increasing dryness in Northwest China. The dryness would further increase if current climate trends continue in the future, which may have important implications for water management in Northwest China.