Spatiotemporal variabilities of the recent Indian Summer Monsoon activities in the Tibetan Plateau: a reanalysis of outgoing longwave radiation datasets

Unstable hydrological cycles and water resource instability over and around the Tibetan Plateau (TP) are a topic of wide concern. The Indian Summer Monsoon (ISM) is one of the TP’s most important moisture sources; as such, its behavior is key to any changes in precipitation and water-related environments. However, there have been relatively few thorough investigations into ISM activities. Here we primarily explore ISM activities using outgoing longwave radiation (OLR) datasets in TP, and precipitation isotopes recorded at Lhasa, for the period 1975-2020. Our major findings are that: (1) the ISM onset (retreat date) is between ~May 31-July 19 (~August 8-September 27), with ISM duration of ~40-110 days; (2) significant spatial inhomogeneous patterns are evident in ISM activities, i.e., the western part of our study area experiences earlier ISM onset, delayed retreat, longer duration, and greater intensity and strength; the inverse is true in the eastern sector of the study area; (3) the ISM activities that dominate the 1975-1998 period determine their general patterns over the entire 1975-2020 period, taking into account evident discrepancies in subperiods; (4) the negative relations between precipitation δ¹⁸O and ISM intensity/strength at Lhasa confirm the ISM activities defined using OLR. These results will improve our understanding of hydrological cycles in TP, and provide insights into hydrological studies in the “Asian Water Tower” region.