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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 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 ∼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 sector 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 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. 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 important moisture sources, the Indian summer monsoon (ISM) is key to changes in precipitation and water-related environments. 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 understanding of hydrology, meteorology, ecology, and paleoclimate reconstructions in the Asian Water Tower region. [ABSTRACT FROM AUTHOR]

Modern observations from Bay of Bengal (BOB) head have shown that precipitation δ18O value reaches the lowest stage during late postmonsoon when the summer monsoon has weakened or retreated, complicating the traditional interpretation of low δ18O value as strong monsoon. We show that this postmonsoon 18O depletion phenomenon exists from BOB periphery to the southeast and south Tibetan Plateau. Accumulative precipitation along back‐trajectory still explains daily precipitation δ18O variability during both the monsoon and nonmonsoon, thus indicating that longer‐term weighted‐mean δ18O is biased toward synoptic conditions during precipitation events. Further, we propose that transport of moisture from the east (west) of the India‐Burma Trough leads to depleted (enriched) 18O during the postmonsoon (premonsoon). Our results reconcile the apparent contradiction but beg the consideration of potential bias and influence of different precipitation regimes in paleo archives. Plain Language Summary: Oxygen isotope signal (δ18O) retrieved from various geological archives, for example, speleothems, reflects paleo‐precipitation δ18O variability and has been used as a proxy of monsoon intensity in South Asia based on the generally negative correlation between δ18O and rainfall amount. However, precipitation δ18O over the Bay of Bengal (BOB) head shows the lowest values during September–October where rainfall peaks during June–August, complicating the interpretation of δ18O records from paleo archives. We show that daily precipitation δ18O variability still correlates with accumulative rainfall along moisture transport paths. While averaged toward monthly means, precipitation δ18O shows a more robust relationship with rainfall intensity rather than the classical monthly total rainfall, indicating that longer‐term mean δ18O is biased toward precipitation events. We further propose that moisture transport pathways from the east of the trough embedded in the southern branch westerlies after the summer monsoon lead to stronger isotopic distillation and thus more depleted precipitation 18O, and the opposite before the monsoon. Key Points: Precipitation 18O shows a postmonsoon depletion over Bay of Bengal head, and this phenomenon extends to a large region in South AsiaAccumulative precipitation along back‐trajectory well explains daily and monthly precipitation δ18O variabilitiesTransport of moisture from the front of the India‐Burma Trough leads to the postmonsoon 18O depletion and opposite for the premonsoon [ABSTRACT FROM AUTHOR]

Recent studies have revealed robust in‐phase relationships between El Niño–Southern Oscillation (ENSO) and Asian Monsoon precipitation δ18O values (i.e., warm ENSO events with high δ18O values), and this relationship has been used in an attempt to reconstruct past ENSO activity. However, whether this relationship holds in the past is unknown. Here we use precipitation δ18O data from Hong Kong (East Asia) and Bangkok (Southeast Asia) and an ice core δ18O record from Dasuopu glacier (South Asia) to examine the δ18O‐ENSO relationship across two recent climate shifts that occurred during the winters of 1976/1977 and 1988/1989. On an annual scale, the δ18O‐ENSO relationship is weak prior to 1977 and strongest after 1988. We show that the changing δ18O‐ENSO relationship mainly originates from changes in the dry season isotope/climate relationship (which is significant only after 1988), whereas the rainy season relationship is relatively stable. We confirm that, consistent with earlier work on the rainy season, the significant δ18O‐ENSO relationship in the dry season post‐1988 is associated with ENSO's influence on regional convection (Bay of Bengal to South China Sea region). We suggest the insignificant dry season relationship prior to 1989 is due to limited ENSO impacts on convection in the Bay of Bengal to South China Sea region, which is supported by the insignificant relationship between ENSO and vertical velocity at 500 hPa. These findings suggest that without additional constraints, systematic variability in isotope/climate relationships will lead to large uncertainties in ENSO reconstructions based on Asian Monsoon region δ18O data. Key Points: The Asian Monsoon precipitation δ18O‐ENSO relationship shows interdecadal changeδ18O‐ENSO relationship is more stable for rainy season but changes through time for dry seasonChanging relationship between source region convection and ENSO leads to the unstable dry season δ18O‐ENSO relationship [ABSTRACT FROM AUTHOR]

Abstract Correlated seasonal variation in precipitation amount and its oxygen isotope ratios (δ 18 O) has long been observed and frequently invoked in the interpretation of paleo- δ 18 O records from the Asian Monsoon (AM) region. However, the underlying cause of the observed seasonal δ 18 O variation is still under debate. Precipitation δ 18 O values show a single, consistent seasonal pattern across the region, with high values in pre-monsoon (late-spring to early-summer) and low values in the monsoon mature (mid- to late-summer) periods. We tested three hypotheses that may give rise to the measured precipitation δ 18 O pattern, involving variation in: 1) local precipitation, 2) moisture sources, and 3) convection in moisture source regions. We show that seasonal precipitation amounts across the AM region exhibit two pattern types: pre-monsoon peak or monsoon mature peak, and thus do not provide a consistent explanation for the isotopic pattern. We test the hypotheses that changing moisture sources or moisture source isotope ratios drive the seasonal isotopic pattern using a combination of Lagrangian moisture source attribution and output from an isotope-enabled General Circulation Model (LMDZ4), but find little seasonal variation in moisture sources; that which does exist is not consistent with the observed isotopic change. Instead, we show that the precipitation δ 18 O transition from pre-monsoon to monsoon mature stage is correlated with intensification of convective activity in moisture source regions, which is the result of monsoon establishment and northward migration of the Intertropical Convergence Zone (ITCZ). We propose that light water vapor, transferred to the lower-troposphere through downdrafts and evaporation of rain in the vicinity of intense convection, is advected to the AM region and labels precipitation during the monsoon mature period. These results demonstrate that precipitation δ 18 O in the AM region more strongly reflect large-scale atmospheric dynamics than local precipitation amount or moisture source, guiding the interpretation of paleo-isotope data from this region. Highlights • Local "amount effect" not ubiquitous across Asian Monsoon region. • Land-ocean moisture source shift cannot explain spring-summer δ 18 O contrast. • Monthly δ 18 O values are correlated with upstream convection intensity. • Northward ITCZ/monsoon trough migration drives systematic δ 18 O variations. [ABSTRACT FROM AUTHOR]

Oxygen isotope signals ( δ 18 O) from paleo-archives are important proxies for past Asian Summer Monsoon (ASM) climate reconstruction. However, causes of interannual variation in the δ 18 O values of modern precipitation across the ASM region remain in argument. We report interannual δ 18 O variation in southern Tibetan Plateau precipitation based on long-term observations at Lhasa. These data, together with precipitation δ 18 O records from five Global Network of Isotopes in Precipitation (GNIP) stations and two ice core δ 18 O records, were used to define a regional metric of ASM precipitation δ 18 O (ASMOI). Back-trajectory analyses for rainy season precipitation events indicate that moisture sources vary little between years with relatively high and low δ 18 O values, a result that is consistent for the south (Lhasa), southeast (Bangkok), and east ASM regions (Hong Kong). In contrast, δ 18 O values at these three locations are significantly correlated with convection in the estimated source regions and along transport paths. These results suggest that upstream convection, rather than moisture source change, causes interannual variation in ASM precipitation δ 18 O values. Contrasting values of the ASMOI in El Niño and La Niña years reveal a positive isotope-El Niño Southern Oscillation (ENSO) response (e.g., high values corresponding to warm phases), which we interpret as a response to changes in regional convection. We show that the isotope-ENSO response is amplified at high elevation sites and during La Niña years. These findings should improve interpretations of paleo- δ 18 O data as a proxy for past ASM variation and provide new opportunities to use data from this region to study paleo-ENSO activity. [ABSTRACT FROM AUTHOR]

This project launched the first study to compare the stable isotopes (δO and δD) in daily precipitation at Kathmandu (located on the southern slope of the central Himalayas) and Tingri (located on the northern slope). The results show that low δO and δD values of summer precipitation at the two stations were closely related to intense convection of the Indian monsoon. However, summer δO and δD values at Tingri were lower than those at Kathmandu, a result of the lift effect of the Himalayas, coupled with convection disturbances and lower temperatures at Tingri. In winter, the relatively high δO and δD values at the two stations appears to have resulted from the influence of the westerlies. Compared with those during the summer, the subsidence of the westerlies and northerly winds resulted in relatively high δO and δD values of the winter precipitation at Tingri. Winter δO and δD values at Kathmandu far exceeded those at Tingri, due to more intense advection of the southern branch of the westerlies, and higher temperatures and relative humidity at Kathmandu. The detailed differences in stable isotopes between the two stations follow short-term variability in the onset date of the Indian monsoon and its retreat across the central Himalayas. During the sampling period, the Indian monsoon onset at Tingri occurred approximately 1 week later than that at Kathmandu. However, the retreat at Tingri began roughly 3 days earlier. Clearly, the duration of the Indian monsoon effects last longer at Kathmandu than that at Tingri. Our findings also indicate that the India monsoon travels slowly northward across the central Himalayas due to the blocking of the Himalayas, but retreats quickly. [ABSTRACT FROM AUTHOR]

Understanding variations in isotopic composition of precipitation from monsoon regions is crucial for its utilization in paleoclimate studies. This study explores the relationship between precipitation δ18O data for the East Asian monsoon (EAM) region archived in Global Network for Isotopes in Precipitation (GNIP) and the cloud data archived in ISCCP and their linkage with large-scale atmospheric circulation patterns. Results show that precipitation δ18O are significantly and positively correlated with cloud-top pressure (CTP) on both local and regional scales. Mechanically speaking, the stronger the monsoon convection precipitation, the higher the cloud and the lower the condensation temperature and thus the lower the precipitation δ18O. This result implies that the sharp drop in precipitation δ18O in the early summer in monsoonal Asia is related to the atmospheric circulation pattern rather than the different moisture sources, as was previously assumed. This result helps explain the processes leading to the observed 'amount effect.' A comparison of atmospheric circulation patterns with precipitation δ18O on an interannual scale shows that the positive CTP anomalies in the central Indo-Pacific within the weak Walker circulation (El Niño) can be associated with positive δ18O anomalies, while negative CTP anomalies in the central Indo-Pacific within the strong Walker circulation (La Niña) can be linked to negative δ18O anomalies. This result further confirms the aforementioned conclusion. This is important for understanding paleoclimatic change in monsoonal Asia, as interannual variations in stable isotopes in that region have received less attention in the past. [ABSTRACT FROM AUTHOR]

ABSTRACT δ18O variability in daily precipitation at two stations (Lanzhou and Zhangye) north of the Qinling Mountains at the eastern margin of the Tibetan Plateau, China, and monthly precipitation δ18O at four stations to the south of the Qinling Mountains were examined. The data show that the δ18O composition of precipitation south of the Qinling Mountains is influenced strongly by the prevailing westerlies during winter and by the summer Asian monsoon during its most active periods. To the north, the westerlies prevail in winter, and δ18O trends coincide strongly with temperature. In summer, the Qinling Mountains block or weaken the Asian monsoon, which crosses them only during the most active periods. In these periods, the stations to the north experience relatively depleted δ18O values (<−7‰). During weak monsoon periods, however, most of the δ18O values in summer precipitation at Lanzhou and Zhangye are relatively enriched (>−5‰, and even as high as +9.2‰). A positive northward weighted average δ18O gradient of summer precipitation with increasing distance from the coast suggests the influence of the prevailing westerlies and of continental recycling of moisture. Both the frequency of these heavily depleted δ18O values and their duration decrease from south to north, reaching minimums at Zhangye, which may have been in the monsoonal tail region during the sampling period. The data reveal that the Qinling Mountains act as an important climatic divide along the eastern margin of the Tibetan Plateau. The effects of the Asian summer monsoon and of different moisture sources should be considered for paleoclimatic studies. [ABSTRACT FROM AUTHOR]

The lapse rate of water isotopes is used in the study of the hydrologic cycle as well as in the estimation of uplift of the Tibetan Plateau. The greater elevation contrast in the Southern Himalayas allows for a detailed discussion about this lapse rate. We analyze variations of δO in precipitation and river water between 1320 m and 6700 m elevations in the Southern Himalayas, and calculate the specific lapse rate of water δO. The results show that the multi-year average lapse rate in precipitation over this region is 0.15‰/100 m. The one-year average lapse rate is 0.17‰/100 m from three sites along the Southern Himalayas. The two results agree, but are much lower than the global average of 0.28‰/100 m. This work also shows that there is a difference in precipitation δO lapse rate between the monsoon and non-monsoon seasons. The calculated precipitation lapse rate is much lower than that in surface water. [ABSTRACT FROM AUTHOR]

Abstract: Many works show the dependence of precipitation δ 18O on altitude and latitude as a result of air temperature change. The assumption that these quantitative relations can hold across a large geographical region can be used to give a rough estimation of local precipitation δ 18O where there is on direct observation of precipitation δ 18O. Although there truly exists uncertainty in the estimation, an effect was made in this paper to establish the precipitation δ 18O pattern over the whole China. Based on regional topography and the δ 18O of meteoric precipitation, this paper established a quantitative relationship between δ 18O in precipitation and latitude and altitude using the existing δ 18O data from 55 sites over China. The model describes as δ 18Oppt =−0.0073LAT2+0.3261LAT−0.0015ALT−9.7776, and was used to generate a higher-resolution map of mean annual δ 18O for precipitation over China using geographic information system software, integrating with latitude, altitude and the moisture cycle. This determination of the spatial distribution of δ 18O in modern precipitation of China provides important information for ancient climate and stable isotope hydrology studies. [Copyright &y& Elsevier]

This paper reveals the temporal and spatial variations of stable isotope in precipitation of the Yarlung Zangbo River Basin based on the variations of δ18O in precipitation at four stations (Lhaze, Nugesha, Yangcun and Nuxia) in 2005. The results show that δ18O of precipitation has distinct seasonal changes in the Yarlung Zangbo River Basin. The higher value of δ18O occurs in spring prior to monsoon precipitation, and the lower value occurs during monsoon precipitation. From the spatial variations, with the altitude-effect and rainout process during moisture transport along the Yarlung Zangbo River Valley, 18O of precipitation is gradually depleted. Thus, δ18O of precipitation decreases gradually from the downstream to the upstream, and the lapse rate of δ18O in precipitation is approximately 0.34‰/100m and 0.7‰/100km for the two reasons. During monsoon precipitation, spatial variation of δ18O in precipitation is dominated by the amount effect in the large scale synoptic condition. [ABSTRACT FROM AUTHOR]

Based on summer observations of stable isotope of precipitation at Muztagata, western China, during 2002-2003, this paper presents the relationship between δ18O in precipitation and air temperature, and discusses the effect of moisture transport on δ18O in precipitation. Results show that air temperature correlates positively with δ18O in precipitation, and the temperature effect controls the δ18O of precipitation in this area. The Muztagata region exhibits high δ18O values in summer precipitation, similar to those shown at stations in adjacent regions. According to the results of our model set up to trace the moisture trajectories, the westerlies and local moisture circulation contribute to variations of oxygen isotopes in precipitation. In addition, the impacts of the moisture transport distance, the moisture transport level, and the incursion of the polar air mass also influence the variations of δ18O in precipitation. The moisture origins and transport mechanisms also contribute to the variation of δ18O in precipitation at Muztagata. [ABSTRACT FROM AUTHOR]

Previous studies found extremely high d-excess in both ice core and glacial melt water in Dasuopu glacier, Xixiabangma, middle of Himalayas. These values are much higher than the global average and those measured in southwest monsoon precipitation. The d-excess variation in over one year at Nyalam station will clarify this phenomenon. Studies show that the high d-excess is related to the seasonal variation of moisture transport to this region. The d-excess values are low during the southwest monsoon active periods, when moisture originated from the humid ocean surface. The d-excess values are higher in non-monsoon months, when moisture is derived from westerly transport. Winter and spring precipitation accounts for a substantial portion of the annual precipitation, resulting in higher d-excess in the yearly precipitation in the middle of Himalayas than other parts of the southern Tibetan Plateau. This finding reveals that the precipitation In the middle of Himalayas is not purely from southwest monsoon, but a large portion from the westerly transport, which is very important for ice core study in this area. [ABSTRACT FROM AUTHOR]

Three sampling cross sections along the south path starting from the Tropics through the vapor passage in the Yunnan-Guizhou Plateau to the middle-low reaches of the Yangtze River, the north path from West China, via North China, to Japan under the westerlies, and the plateau path from South Asia over the Himalayas to the northern Tibetan Plateau, are set up, based on the IAEA (International Atomic Energy Agency)/WMO global survey network and sampling sites on the Tibetan Plateau. The variations, and the relationship with precipitation and temperature, of the δ18O in precipitation along the three cross sections are analyzed and compared. Along the south path, the seasonal differences of mean δ18O in precipitation are small at the stations located in the Tropics, but increase markedly from Bangkok towards the north, with the δ18O in the rainy season smaller than in the dry season. The δ18O values in precipitation fluctuate on the whole, which shows that there are different vapor sources. Along the north path, the seasonal differences of the mean δ18O in precipitation for the stations in the west of Zhengzhou are all greater than in the east of Zhengzhou. During the cold half of the year, the mean δ18O in precipitation reaches its minimum at Ürümqi with the lowest temperature due to the wide, cold high pressure over Mongolia, then increases gradually with longitude, and remains at roughly the same level at the stations eastward from Zhengzhou. During the warm half of the year, the δ18O values in precipitation are lower in the east than in the west, markedly influenced by the summer monsoon over East Asia. Along the plateau path, the mean δ18O values in precipitation in the rainy season are correspondingly high in the southern parts of the Indian subcontinent, and then decrease gradually with latitude. A sharp depletion of the stable isotopic compositions in precipitation takes place due to the very strong rainout of the stable isotopic compositions in vapor in the process of lifting over the southern slope of the Himalayas. The low level of the δ18O in precipitation is from Nyalam to the Tanggula Mountains during the rainy season, but δ18O increases persistently with increasing latitude from the Tanggula Mountains to the northern Tibetan Plateau because of the replenishment of vapor with relatively heavy stable isotopic compositions originating from the inner plateau. During the dry season, the mean δ18O values in precipitation basically decrease along the path from the south to the north. Generally, the mean δ18O in precipitation during the rainy season is lower than in the dry season for the regions controlled by the monsoons over South Asia or the plateau, and opposite for the regions without a monsoon or with a weak monsoon. [ABSTRACT FROM AUTHOR]

• Seasonal influences on water vapor isotopes in monsoon and non-monsoon seasons. • Strong seasonal dependence of vapor δ18O on large regional meteorological regime. • High frequency coherent changes in vapor δ18O and d-excess with precipitation events. • A weak seasonal change of vapor d-excess likely from vapor source climate. • Annual precipitation and annual vapor δ18O are not in equilibrium in southern TP. Water isotopes in precipitation provide new insight on the controlling factors for moisture source variations on the Tibetan Plateau. However, understanding the intra-annual water isotope cycle is difficult for a region lack of winter precipitation. Here, we reported water vapor δ18O and δ2H time series over two and half years at Lhasa on the southern Tibetan Plateau. We compare our values with concurrent daily precipitation isotopes to assess the seasonal controls of vapor isotopes and possible correlation to precipitation isotopes. Results show an intra-seasonal dependence of water vapor δ18O on large scale meteorological regime. During the winter months, vapor water δ18O shows positive dependence on regional-scale temperature, confirming a Rayleigh-distillation controlling moisture transport. During the summer months, lower vapor δ18O is weekly in association with the heavy precipitation over tropical Indian Ocean and on the Tibetan Plateau, and this relation is further enhanced by the evaporation of precipitation at the sampling site. We also found a weak seasonal variation of vapor d-excess , likely resulted from climate seasonality over the large moisture sources region. Moreover, the isotope dataset contains a high-frequency of fluctuations for both δ18O and d-excess lasting several days during the summer monsoon season. These high frequency fluctuations are in phase with precipitation events, confirming significant local evaporation effects on the short-term water cycle, which further complicated the control of near surface water vapor isotopes. These findings improve our regional scale understanding of hydrological cycle on the southern Tibetan Plateau, and will potentially improve our understanding of isotope variations in proxy archives in the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Abstract: This project collected and analyzed precipitation samples along a south to north transect of the Tibetan Plateau, from Lhasa in the south, Nagqu and Tuotuohe in the middle, to Delingha in the north, from 2000 to 2003. Results show that overall the positive correlation between δ 18O in precipitation and temperature gradually strengthens from south to north, with gradual weakening of monsoon activity. In particular, good linear relationships exist between δ 18O values of precipitation and air temperatures (T) at all the four stations from January to May over the years. In addition, our data show that when air temperature is low enough, the variations of δ 18O values in precipitation over the whole Tibetan Plateau are controlled mainly by temperature effects. Our data also show that T–δ 18O effects are of importance throughout the year along the transect, even in the south where the summer moisture mainly comes from monsoon activities. The stable oxygen isotope variations in precipitation along the transect, especially in the south, can reveal the long-term changes of δ 18O records in the south Tibetan ice cores. In summer, however, the monsoon activities and the great difference between the condensation temperature and the ground temperature weaken the δ 18O–T correlation in precipitation in the south and middle part of Tibetan Plateau. The stable oxygen isotope variations in precipitation can reveal the short-term changes of δ 18O records in the south Tibetan ice cores. Compared with the results from previous studies, the δ 18O–T correlations in precipitation in 2000–2003 on this transect are poorer than the ones reported from 1991–1999. This may result from higher average annual precipitation and the increasing trend of the monsoon activities in 2000–2003 compare to those in 1991–1999. [Copyright &y& Elsevier]

This study investigated the variations of stable isotopes (δ 18 O and δD) in precipitation at Deqin on the southeastern margin of the Tibetan Plateau. The results show that correlations between stable isotopes in precipitation and temperature vary during different seasons. In comparison, an apparent inverse correlation between stable isotopes and precipitation amount exists throughout different seasons. Hence, the “amount effect” appears to have significantly controlled the stable isotope variations in precipitation over the entire sampling period, rather than the “temperature effect”. The overall temporal variations of δ 18 O and δD in precipitation from Deqin are similar to those from a southern station at Xishuangbanna. Both stations demonstrate considerably depleted precipitation δ 18 O values in the summer as a consequence of intense Asian summer monsoon activities. However, at Xishuangbanna, the occurrences of the timing and duration of the depleted δ 18 O values, as well as the negative surface lift index values, occur earlier and last longer than those at Deqin. These results indicate that the Asian summer monsoon onset is later and that the duration of the monsoon activities is shorter at Deqin relative to Xishuangbanna. [ABSTRACT FROM AUTHOR]