Spatial distributions and temporal variabilities of the recent Indian Summer Monsoon Northern Boundaries in Tibetan Plateau: analysis of outgoing longwave radiation dataset and precipitation isotopes

As the "Asian Water Tower" region, the Tibetan Plateau (TP) has recently experienced a series of unbalanced hydrological cycles, with significant spatial heterogeneities. Large-scale circulations or moisture sources are the key drivers of these changes. The Indian Summer Monsoon (ISM) and the Westerlies are the two main atmospheric circulations that affect the climate of the TP, and considerable attention has been paid to their precise boundaries or the northern boundaries of ISM (M-W boundaries). Nonetheless, these boundaries remain unclear, meriting further investigation. Here we present our initial attempts to delineate the spatio-temporal variabilities of M-W boundaries in the TP, using outgoing longwave radiation (OLR) dataset for 1975-2020. Ground-based precipitation isotopes (1991-2008) at 7 sites and downward shortwave radiation (srad) reanalysis (1979-2018) at 14 sites along a southwest-northeast transect in the TP are used to verify the M-W boundaries with OLR. Our principal findings are as follows: (1) the M-W boundaries are generally framed within 30 degrees-35 degrees N, and appear ladder-shaped from western to eastern TP; and (2) the M-W boundaries experienced significant northward (or southward) shifts in 1999-2020 and 1975-2020 (or 1975-1998), true for all grids counted and for grids in each column; and (3) both the locations and the northward shifts of M-W boundaries are verified by precipitation d(18)O and srad reanalysis. These results will improve our understanding of moisture source-related hydrology and aid a more accurate comprehension of ecology or paleoclimate reconstructions in the TP.