A reconstruction of Holocene rainfall is presented for southwest Chinaan area prone to drought and flooding due to variability in the East Asian monsoon. The reconstruction is derived by comparing a new high-resolution stalagmite δ 18 O record with an existing record from the same moisture transport pathway. The new record is from Heshang Cave (30°27′N, 110°25′E; 294 m) and shows no sign of kinetic or evaporative effects so can be reliably interpreted as a record of local rainfall composition and temperature. Heshang lies 600 km downwind from Dongge Cave which has a published high-resolution δ 18 O record (Wang, Y.

ABSTRACT:The present article is the second part of a study on the inter-decadal variability of the summer precipitation in East China, which mainly addresses the possible cause of this change. Firstly, an updated analysis of the long-term variations of snow cover, snow days and snow depth in the preceding winter and spring over the Tibetan Plateau (TP) was done by using station and satellite data. The abrupt increase in the winter and spring snow over the TP since around 1977 has been well documented. At that time, the inter-decadal variation of the atmospheric heating over the TP in spring and summer had been estimated. It has been revealed that the atmospheric heating fields in subsequent spring and summer over the TP assumed a significant weakening after the late 1970s. This weakening is closely related to the significantly reduced surface sensible heat flux into the atmosphere and subsequent cooling over the TP and its surrounding atmosphere. The latter was produced by the increase of surface albedo and soil hydrological effect of melting snow under the condition of abrupt increase in the preceding winter and spring snow over the TP. On the other hand, three phases of significant inter-decadal warming of the sea surface temperature (SST) in the tropical central and eastern Pacific, which occurred in the mid-1960s, the late 1970s and the early 1990s, respectively, have been found. The above inter-decadal variability of heating fields over the land area in the Asian region and neighbouring oceanic region of the West Pacific has consistently reduced the land-sea thermal contrast in summer in the Asian monsoon region based on the estimate of atmospheric heating fields. This cause is likely to lead to weakening of the Asian summer monsoon. In such case, the northward moisture transport in East Asia is greatly weakened and cannot reach North China, thus causing the condition of less precipitation or droughts. In contrast, the Yangtze River basin and South China receive a large amount of moisture supply and have strong upward motion, creating favourable conditions for frequent occurrence of heavy rainfall. In the process of the southward shift of the high-precipitation zone, two abrupt or rapid regime shifts observed in the late 1970s and the early 1990s were possibly in response to the increase in the winter and spring snow over the TP, and two major rapid warming events of the SST in the tropical central and eastern Pacific in the late 1970s and the early 1990s.Correlative analysis has further confirmed that high TP snow and oceanic forcing factors have a positive correlation with the subsequent summer precipitation in the Yangtze River basin and most of South China, and a negative correlation with the summer precipitation in North China. This correlative relationship implies that if the TP has excessive (deficient) snow in the preceding winter and spring and the tropical central and eastern Pacific anomalously warms up (cools down), North China will have decreasing (increasing) summer precipitation, whereas the ...

[1] Summer thermal structure and winds over Asia show a larger land-ocean thermal gradient in the upper than in the lower troposphere, implying a bigger role of the upper troposphere in driving the Asian summer monsoon circulation. Using data from atmospheric re-analyses and model simulations, we show that the land-ocean thermal contrast in the mid-upper (200-500 hPa) troposphere (TCupper) contributes about three times as much as the thermal contrast in the mid-lower (500-850 hPa) troposphere (TClower) in determining both the strength and variations of Asian summer monsoon circulations. Tropical sea surface temperature anomalies associated with the annual cycle, El Niño-Southern Oscillation, decadal changes, and global warming all are accompanied with much larger variations and changes in TCupper than in TClower, partly due to enhanced latent heating aloft from convection. The variations and changes in TCupper and TClower are highly correlated with the strength of the South Asian Summer Monsoon (SASM) and the East Asian Summer Monsoon (EASM) in their respective sectors during the past 50-60 years. In particular, the weakening of the EASM since the 1950s is caused by the weakening mainly in TCupper and secondarily in TClower induced mainly by recent tropical surface warming, although spurious cooling over East Asia seen in reanalysis data may have enhanced this weakening. However, the strength of the SASM and EASM monsoons follows TCupper but decouples with TClower in the global warming case in the 21 st century. The results suggest that the TCupper plays a dominant role and provides an efficient mechanism through which tropical oceans can influence extratropical monsoons.

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The future potential changes in precipitation and monsoon circulation in the summer in East Asia are projected using the latest generation of coupled climate models under Intergovernmental Panel on Climate Change (IPCC) Special Report on Emission Scenarios (SRES) A1B scenario (a medium emission scenario). The multi-model ensemble means show that during the period of 2010-2099, the summer precipitation in East Asia will increase and experience a prominent change around the 2040s, with a small increase (~1%) before the end of the 2040s and a large increase (~9%) afterward. This kind of two-stage evolution characteristic of precipitation change can be seen most clearly in North China, and then in South China and in the mid and lower Yangtze River Valley. In 2010-2099, the projected precipitation pattern will be dominated by a pattern of "wet East China" that explains 33.6% of EOF total variance. The corresponded time coefficient will markedly increase after the 2040s, indicating a great contribution from this mode to the enhanced precipitation across all East China. Other precipitation patterns that prevail in the current climate only contribute a small proportion to the total variance, with no prominent liner trend in the future. By the late 21st century, the monsoon circulation will be stronger in East Asia. At low level, this is due to the intensification of southwesterly airflow north of the anticyclone over the western Pacific and the SCS, and at high level, it is caused by the increased northeasterly airflow east of the anticyclone over South Asia. The enhanced monsoon circulation will also experience a two-stage evolution in 2010-2099, with a prominent increase (by ~0.6 m s −1 ) after the 2040s. The atmospheric water vapor content over East Asia will greatly increase (by ~9%) at the end of 21st century. The water vapor transported northward into East China will be intensified and display a prominent increase around the 2040s similar to other examined variables. These indicate that the enhanced precipitation over East Asia is caused by the increases in both monsoon circulation and water vapor, which is greatly different from South Asia. Both the dynamical and thermal dynamic variables will evolve consistently in response to the global warming in East Asia, i.e., the intensified southwesterly monsoon airflow corresponding to the increased water vapor and southwesterly moisture transport.East Asian summer monsoon, precipitation, climate models, future projection Citation:Sun Y, Ding Y H. A projection of future changes in summer precipitation and monsoon in East Asia.