For fifty years, the Tibetan Plateau has been recognized as the largest topographic feature that perturbs atmospheric circulation. It serves as an ideal field laboratory for understanding the geodynamic processes that build high terrain. Accordingly, the growth of the plateau should have altered atmospheric circulation and therefore written an evolving paleoclimatic signature not only on eastern Asian regional climates, but on global climate as well. Despite many recent studies, we still do not know precisely when the Tibetan Plateau reached its current dimensions and how it perturbs atmospheric circulation. This project brings together geodynamicists, atmospheric scientists, and paleoclimatologists in a multidisciplinary study of the when and the how.
One of the major goals of the project is to quantify the extent to which Tibet has grown by crustal thickening, by thrust faulting and folding, by flow within the crust that redistributes material there, or by replacement of cold mantle lithosphere with hotter material (all in a state of isostatic equilibrium). Such quantification will take big steps toward the understanding of how high plateaus are built and how continental lithosphere deforms, topics at the forefront of geodynamics.
Determining how Tibet has grown will require determining when crustal shortening and thickening occurred, using basic field methods and modern laboratory techniques, and quantifying paleoaltitudes with new isotopic tools. Applying such paleoaltimetric techniques, however, requires an understanding not only of how the atmosphere transports isotopes, but how the evolving high terrain affected surface temperatures at times in the past. Even if the project?s focus were solely on how Tibet has grown, a meteorological component of the study, focused particularly on eastern Asia?s hydrological cycle, would be necessary.
Most continental paleoclimatic indicators are thought to be more sensitive to precipitation than to temperature, and among the unknowns of future climate, the hydrological cycle stands out. Accordingly, a major focus will be on understanding how high terrain like Tibet affects the hydrological cycle of eastern Asia, and China in particular. These studies will focus on: (1) how the plateau, as both a topographic obstacle and a sink for solar radiation, affects atmospheric circulation; (2) how the atmosphere transports stable isotopes (ä18O and äD); (3) how it affects mid-latitude climate variability, including how, via lee cyclogenesis, it lofts and transports dust, and (4) how vegetation feeds back on atmospheric circulation and the hydrological cycle. As links from geologic processes occurring at multi-Myr time scales to those on human time scales, the Principal Investigators plan studies that specifically examine paleoprecipitation over the past few hundred thousand years, using both loess deposition and speleothems that quantify paleoclimate.
The goal of the project is to understand the role of the Tibetan Plateau in determining summer precipitation regimes in Asia. The classical definition of "monsoon" describes a shift in wind directions, and not in precipitation regimes. Precipitation is linked to convection, which could be determined by local buoyancy or by low-level convergence of the winds. Even though the Indian Summer Monsoon and the East Asia Summer Monsoon are both called 'monsoons", their dynamics and hence the characteristics of precipitation are very different. The typical turnover time of water vapor in the global atmosphere is of order 10 days, and is shorter for regions of intense precipitation, and so the isotopic composition of precipitation reflects not only the isotopic composition of the source water but also of insitu evaporation and condensation along transport paths, and exchanges between raindrops and the ambient vapor. In this study, we explain a long-standing observation in the isotopic composition of precipitation. We demonstrate that precipitation around the southern coast of China is isotopically depleted because the vapor emanates from flow associated with the Indian Monsoon and has a long history of evaporation and precipitation. In contrast, precipitation in the east flanks of the Tibetan Plateau is isotopically enriched because it is derived from local water vapor and there is substantial evaporation of falling raindrops. The analysis shows clearly that the traditional monsoon view is insufficient to explain the dynamics of precipitation in East Asia. The deflection of the jet stream by the Tibetan Plateau is central in determining the convergence, convection and precipitation over East Asia. We applied our understanding and model to precipitation changes in past climates as recorded by speleothems in China. The results show, surprisingly, precipitation over China was higher during the Last Glacial Maximum (LGM) than present day. This can be traced to the topographic changes during the LGM and the redistribution of airmass from the continental atmosphere to the oceanic atmosphere. This redistribution increased the east-west pressure gradient between China and the Pacific, leading to enhanced southerly flow that brings moisture to eastern China. The study provides a clear insight into the influence of topogaphy on the patterns of atmospheric circulation and precipitation.
