A speleothem δ 18O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennialscale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial-interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the "land bridge" in the Maritime continents in the western equatorial Pacific.he Indian summer monsoon (ISM), a key component of tropical climate, provides vital precipitation to southern Asia. The ISM is characterized by two regions of precipitation maxima: a narrow coastal region along the Western Ghats, denoted by ISM A , with moisture from the Arabian Sea, and a broad "Monsoon Zone" around 20°N in northeastern India, denoted by ISM B , where storms emanate from the Bay of Bengal and whose rainfall variability is well correlated with that of "All India" rainfall (1). Multiple proxies obtained from Arabian Sea sediments have revealed the variability of summer monsoon winds on timescales of 10 1 to 10 5 y (e.g., refs. 2-6). Our understanding of the paleo-precipitation variability of ISM B remains incomplete, owing to the scarcity of long and high-resolution records. Here we present a 252,000-y-long speleothem δ 18 O record from Xiaobailong cave, southwest China and characterize variability in the ISM B precipitation on multiple timescales.Xiaobailong (XBL, "Little White Dragon") cave is located in Yunnan Province, southwestern China, near the southeastern edge of the Tibetan Plateau (103°21′E, 24°12′N, ∼1,500 m above sea level; SI Appendix, Fig. S1). Local climate is characterized by warm/wet summers and cool/dry winters. The mean annual precipitation of ∼960 mm falls mostly from June through September (∼80%) (SI Appendix, Fig. S2), indicating the summer monsoon rainfall dominates the annual precipitation at the cave site. The temperature in the cave is 17.2°C, close to local mean annual air temperature (17.3°C).Eight stalagmites were collected from the inner chamber (∼350 m from the entrance) of the cave, where humidity is ∼100% and ventilation is confined to a small crawl-in channel to the outer chamber. One hundred four 230 Th dates were determined on inductively coupled plasma mass spectrometers with typical relative error in age (2σ) of less than 1% (Methods and SI Appendix, Table S1 and Figs. S3 and S4). The ages vary monotonically with depth in the stalagmites (SI Appendix, Fig. S4...

24The summer rainfall climate of East Asia underwent large and abrupt changes during past 25 climates, in response to precessional forcing, glacial-interglacial cycles as well as abrupt changes 26 to the North Atlantic during the last glacial. However, current interpretations of said changes are 27 typically formulated in terms of modulation of summer monsoon intensity, and do not account 28 for the known complexity in the seasonal evolution of East Asian rainfall, which exhibits sharp 29 transition from the Spring regime to the Meiyu, and then again from the Meiyu to the Summer 30 regime. 31We explore the interpretation that East Asian rainfall climate undergoes a modulation of its 32 seasonality during said paleoclimate changes. Following previous suggestions we focus on role 33 of the westerly jet over Asia, namely that its latitude relative to Tibet is critical in determining 34 the stepwise transitions in East Asian rainfall seasons. In support of this linkage, we show from 35 observational data that the interannual co-variation of June (July-August) rainfall and upper 36 tropospheric zonal winds show properties consistent with an altered timing of the transition to 37 the Meiyu (Summer), and with more northward-shifted westerlies for earlier transitions. 38We similarly suggest that East Asian paleoclimate changes resulted from an altered timing in 39 the northward evolution of the jet and hence the seasonal transitions, in particular the transition 40 of the jet from south of the Plateau to the north that determines the seasonal transition from 41Spring rains to the Meiyu. In an extreme scenario -which we speculate the climate system 42 tended towards during stadial (cold) phases of D/O stadials and periods of low Northern 43Hemisphere summer insolation -the jet does not jump north of the Plateau, essentially keeping 44East Asia in prolonged Spring conditions. 45We argue that this hypothesis provides a viable explanation for a key paleoproxy signature of 46 3 D/O stadials over East Asia, namely the heavier mean δ 18 O of precipitation as recorded in 47 speleothem records. The southward jet position prevents the low-level monsoonal flow -which 48 is isotopically light -from penetrating into the interior of East Asia; as such, precipitation there 49 will be heavier, consistent with speleothem records. This hypothesis can also explain other key 50 evidences of East Asian paleoclimate changes, in particular the occurrence of dusty conditions 51 during North Atlantic stadials, and the southward migration of the Holocene optimal rainfall. 52Earth's orbit. Other records corroborate the sense of large and abrupt change in East Asia; for 60 example, paleoproxy dust records show East Asia to be dustier during cold stadials (and in 61 particular Heinrich stadials) Nagashima et al., 2011], and more generally during 62 glacial periods [An, 2000]. 63The dominant interpretation of variability in the speleothem records is as a record of 64 changes in East Asian summer monsoon intensity, with δ 18 O relatively light when m...

The topography and continental configuration of East Asia favor the year-round existence of storm tracks that extend thousands of kilometers from China into the northwestern Pacific Ocean, producing zonally elongated patterns of rainfall that we call "frontal rain events." In spring and early summer (known as "Meiyu Season"), frontal rainfall intensifies and shifts northward during a series of stages collectively known as the East Asian summer monsoon. Using a technique called the Frontal Rain Event Detection Algorithm, we create a daily catalog of all frontal rain events in east China during 1951-2007, quantify their attributes, and classify all rainfall on each day as either frontal, resulting from large-scale convergence, or nonfrontal, produced by local buoyancy, topography, or typhoons. Our climatology shows that the East Asian summer monsoon consists of a series of coupled changes in frontal rain event frequency, latitude, and daily accumulation. Furthermore, decadal changes in the amount and distribution of rainfall in east China are overwhelmingly due to changes in frontal rainfall. We attribute the "South Flood-North Drought" pattern observed beginning in the 1980s to changes in the frequency of frontal rain events, while the years 1994-2007 witnessed an uptick in event daily accumulation relative to the rest of the study years. This particular signature may reflect the relative impacts of global warming, aerosol loading, and natural variability on regional rainfall, potentially via shifting the East Asian jet stream.