Historical droughts manifest an abrupt shift to a wetter Tibetan Plateau

Abstract

The Tibetan Plateau (TP) plays a vital role in Asian and even global atmospheric circulation, through the interactions between land and atmosphere. It has experienced significant climate warming and spatially and temporally variant wetting over the past half century. Because of the importance of land surface status to the interactions, determining the wetting and drying of the TP from individual changes in precipitation (Prep) or temperature is difficult. Soil moisture (SM) is the water synthesis of the surface status. The persistent deficit of SM (SM drought) is more sensitive to climate change than normal SM. This study first explored the climate wetting and drying of the TP from variations in historical SM droughts over 1961–2014, with a focus on spatiotemporal patterns, long-term variations, and climate causes of summer (May–September) SM droughts based on multiple observation and reanalysis data. The results showed comparatively frequent and severe droughts in the central and southern area, particularly in the semiarid and subhumid regions. SM drought exhibited an abrupt and significant (p < 0.05) alleviation in the interior and central-west TP in the middle to late 1990s. The prominent drought alleviation indicated a hydroclimate shift to a wetter plateau, not merely steady trends as given in the literature. We demonstrated that the wetting shift was dominated by Prep over potential evapotranspiration (PET). By contrast, the in-phase trends were combined forces of Prep and PET, with increased forces of PET after the wetting shift. Furthermore, the Prep dominance was largely attributed to a phase transition of the Atlantic multi-decadal oscillation from cold to warm since the mid-1990s. The PET impacts on the wetting trends were likely dominated by solar radiation, wind speed, and vapor pressure deficit. Regionally, the wetting shift was distinct from the arid to semiarid and semiarid to subhumid climate. Such spatiotemporal changes may affect the TP's atmospheric circulation and, subsequently, the Asian monsoon and global circulation, in addition to the fragile ecosystem in the TP.