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 topics 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 thor-ough 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 -31 May and 19 July (-8 August-27 September), 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, and the inverse is true in the eastern sec-tor of the study area; 3) the ISM activities that dominate the 1975-98 period determine their general patterns over the entire 1975-2020 period, taking into account evident discrepancies in subperiods; and 4) the negative relations between precipitation d18O and ISM intensity/strength at Lhasa confirm the ISM activities defined using OLR. These results will improve our under-standing of hydrological cycles in TP and provide insights into hydrological studies in the "Asian Water Tower" region. SIGNIFICANCE STATEMENT: Over the recent decades, the Tibetan Plateau (TP) (Asian Water Tower) has undergone dramatic environmental changes, evinced by the instabilities of hydrological cycles. As one of TP's most im-portant moisture sources, the Indian summer monsoon (ISM) is key to changes in precipitation and water-related envi-ronments. To get thorough investigations into ISM activities, we primarily explore ISM activities using outgoing longwave radiation (OLR) datasets in TP and precipitation isotopes at Lhasa. Significant spatial inhomogeneous patterns are evident in ISM activities: the western part experiences earlier ISM onset, delayed retreat, longer duration, and stronger intensity and strength, and the inverse occurs in the eastern sector. These results will improve our under-standing of hydrology, meteorology, ecology, and paleoclimate reconstructions in the Asian Water Tower region.