Zhengguo Shi

Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka

Two atmospheric circulation systems, the mid-latitude Westerlies and the Asian summer monsoon (ASM), play key roles in northern-hemisphere climatic changes. However, the variability of the Westerlies in Asia and their relationship to the ASM remain unclear. Here, we present the longest and highest-resolution drill core from Lake Qinghai on the northeastern Tibetan Plateau (TP), which uniquely records the variability of both the Westerlies and the ASM since 32 ka, reflecting the interplay of these two systems. These records document the anti-phase relationship of the Westerlies and the ASM for both glacial-interglacial and glacial millennial timescales. During the last glaciation, the influence of the Westerlies dominated; prominent dust-rich intervals, correlated with Heinrich events, reflect intensified Westerlies linked to northern high-latitude climate. During the Holocene, the dominant ASM circulation, punctuated by weak events, indicates linkages of the ASM to orbital forcing, North Atlantic abrupt events, and perhaps solar activity changes.

Impact of Mongolian Plateau versus Tibetan Plateau on the westerly jet over North Pacific Ocean

Mountains have long been considered to play an important role in the formation of modern climate. Particularly in the Asia-Pacific sector, the existence of Tibetan Plateau (TP) is believed to maintain the stationary planetary wave patterns and to intensify the westerly jet over North Pacific. However, the potential role of Mongolian Plateau (MP) has often been neglected in these studies. With an atmospheric general circulation model, we show that the MP, despite its smaller size, exerts a great influence on the planetary-scale circulation and the subtropical westerly jet. The MP amplifies the meridional thermal gradient at the mid-upper troposphere and thus strengthens the jet, which is primarily facilitated by its high-latitude location. By blocking westerly winds, the MP forces their northern branch further northward, which allows the southward penetration of cold air at the lee side, and intensifies the East Asian trough. In contrast, the impact of the TP itself is not as large as expected. Hence, one should not simply ascribe all the mountain-induced climate change to the TP uplift, because other smaller topography might be also very important.

Distinct impacts of the Mongolian and Tibetan Plateaus on the evolution of the East Asian monsoon

The Mongolian Plateau (MP), which is relatively lower in altitude and smaller in extent than the Tibetan Plateau (TP), has received little attention about its climate effect. Building upon previous work in which we highlighted the role of the MP on the high-level westerly jet stream, the response of surface-level features of the Asian climate is examined in this study. The results show that the Indian and East Asian summer monsoonal and inland precipitation are mainly enhanced by the uplift of the TP. The precipitation during the onset of the summer monsoon is also intensified over India and eastern China. In addition, the East Asian monsoon domain is significantly expanded with the uplift of the TP, while the Indian summer monsoon domain does not change obviously. The MP plays a significant role in the strengthening of the East Asian winter monsoon, which is larger than the TP. With the uplift of the MP, the cold northerly wind in winter intensifies significantly in East Asia from higher latitudes to the South China Sea. The Siberian high is also enhanced and moves remarkably northward to its modern location. The strengthening of the Asian winter monsoon is related to the MP-induced diversion of westerly wind. The bypassing flows around the plateau modify the temperature advections over middle latitudes and the atmosphere thermal structure in winter, which leads to the strengthening of the East Asian winter monsoon.

Hydroclimatic contrasts over Asian monsoon areas and linkages to tropical Pacific SSTs.

Knowledge of spatial and temporal hydroclimatic differences is critical in understanding climatic mechanisms. Here we show striking hydroclimatic contrasts between northern and southern parts of the eastern margin of the Tibetan Plateau (ETP), and those between East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) areas during the past ~2,000 years. During the Medieval Period, and the last 100 to 200 years, the southern ETP (S-ETP) area was generally dry (on average), while the northern ETP (N-ETP) area was wet. During the Little Ice Age (LIA), hydroclimate over S-ETP areas was wet, while that over N-ETP area was dry (on average). Such hydroclimatic contrasts can be broadly extended to ISM and EASM areas. We contend that changes in sea surface temperatures (SSTs) of the tropical Pacific Ocean could have played important roles in producing these hydroclimatic contrasts, by forcing the north-south movement of the Intertropical Convergence Zone (ITCZ) and intensification/slowdown of Walker circulation. The results of sensitivity experiments also support such a proposition.

Late Cenozoic Climate Change in Monsoon-Arid Asia and Global Changes

Based on the geological and biological findings, and climate model simulations, current understanding of the Asian monsoon-arid environment has been synthesized here. This chapter addresses the evolution of the Asian monsoon-arid environment and how it is related to the growth of the Tibetan Plateau (TP), and global environmental change since the Cenozoic. Asian monsoon appearance may begin in the late Eocene. The basic structure of the monsoon-arid environment was established by late Oligocene, and the modern pattern of the monsoon-arid environment developed in the late Pliocene. Conceptual models for Asian monsoon-arid environmental change mechanisms at various time scales are proposed here. The occurrence of great Northern hemisphere glaciation has been discussed. Differentiation of the natural background and anthropogenic signals are examined. Policies and countermeasures for sustainable development on the Loess Plateau and arid areas are proposed for future reference.

Effect of Yunnan–Guizhou Topography at the Southeastern Tibetan Plateau on the Indian Monsoon

AbstractTopographic insulation is one of the primary origins for the influence of the Tibetan Plateau (TP) on Asian climate. The Yunnan–Guizhou (YG) Plateau, at the southeastern margin of the TP, is known to block the northern branch of the Indian monsoon circulation in summer. However, it is an open question whether this blocking feeds back to the monsoon. In this study, the effect of the YG topography on the Indian monsoon and its comparison with that of the TP were evaluated using general circulation model experiments. The results showed that the TP strengthens the monsoon precipitation, especially during the onset. However, the YG topography significantly weakens the monsoon. With the YG topography, strengthened low-level airflow around the YG Plateau induces anomalous anticyclonic winds to the southwest, and the changes remodulate the whole circulation structure over Asia. As a result, the Indian monsoon becomes weakened from the Bay of Bengal to the Indian subcontinent and Arabian Sea, as does the a...

Role of the Tian Shan Mountains and Pamir Plateau in Increasing Spatiotemporal Differentiation of Precipitation over Interior Asia

AbstractNumerical simulations were conducted to determine the impact of the Tian Shan Mountains and Pamir Plateau on arid conditions over interior Asia. These topographies are crucial for the diffe...

Coral record of variability in the upstream Kuroshio Current during 1953–2004

The Kuroshio Current (KC), one of the most important western boundary currents in the North Pacific Ocean, strongly affects regional hydroclimate in East Asia and upper-ocean thermal structure. Limited by few on-site observations, the responses of the KC to regional and remote climate forcings are still poorly understood. Here, we use monthly coral δ18O data to reconstruct a KC transport record with annual to interannual resolution for the interval 1953–2004. The field site is located in southern Taiwan on the western flank of the upstream KC. Increased (reduced) KC transport would generate strong (weak) upwelling, resulting in relatively high (low) local coral δ18O. The upstream KC transport and downstream transport, off Tatsukushi Bay, Japan, covary on interannual and decadal timescales. This suggests common forcings, such as meridional drift of the North Equatorial Current bifurcation, or zonal climatic oscillations in the Pacific. The intensities of KC transport off southeastern and northeastern Taiwan are in-phase before 1990 and anti-phase after 1990. This difference may be due to a poleward shift of the subtropical western boundary current as a response to global warming.