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.

[1] Lake Superior summer (July -September) surface water temperatures have increased approximately 2.5°C over the interval 1979 -2006, equivalent to a rate of (11 ± 6) Â 10 À2°C yr À1 , significantly in excess of regional atmospheric warming. This discrepancy is caused by declining winter ice cover, which is causing the onset of the positively stratified season to occur earlier at a rate of roughly a half day per year. An earlier start of the stratified season significantly increases the period over which the lake warms during the summer months, leading to a stronger trend in mean summer temperatures than would be expected from changes in summer air temperature alone.Citation: Austin, J. A., and S. M. Colman (2007), Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback, Geophys. Res. Lett., 34, L06604,

Introduction Purpose Previous work Acknowledgments General description of weathering rinds on andesitic and basaltic stones Factors affecting weathering-rind thickness Sampling design and methods Structure of the data Areas and types of deposits sampled Sample site selection Sample collection Measurement procedures Environmental factors other than time Topographic position Parent material Vegetation Climate Summary The relation between weathering-rind development and time Weathering rinds as an indicator of relative age

No abstract

A 100-yr-long time series of water temperature measured just downstream of Lake Superior is used to produce proxy time series of open-lake temperature. This analysis suggests that open-water Lake Superior summer temperatures have increased by roughly 3.5uC over the last century, most of that warming occurring in the last three decades. Correspondingly, the length of the positively stratified season has increased from 145 d to 170 d. The observed amount of warming is greater than the observed change in regional temperature over the same time period by roughly a factor of two. The discrepancy can be understood in the context of reduced winter ice cover, and implies that spatially and temporally averaged ice cover in Lake Superior has decreased from 23% to 12% over the last century.Global average air temperatures have recently warmed beyond their natural limits in historic atmospheric records (IPCC 2007). However, the expected response of temperature of oceans and other water bodies is less clear. Water temperatures of large natural systems may respond to the atmospheric warming trend in unexpected ways, due to nonlinearities, geographic variability, and feedback mechanisms. Unlike air temperature records, such as the Goddard Institute for Space Science (GISS) database (Hansen et al. 1999), reliable century-scale records of measured water temperature are exceedingly rare (Nixon et al. 2004); long, continuous records in lakes more so.Some analyses of lake-water temperature trends over nearly a century have been performed in the hypolimnetic waters of tropical lakes such as Lake Tanganyika (O'Reilly et al. 2003;Verburg et al. 2003) and Lake Malawi (Vollmer et al. 2005). Due to the absence of strong seasonal variation in surface heat flux and a lack of seasonal overturn, the deep water of these lakes respond gradually, and roughly proportionally, to changes in climate, and can be reliably analyzed using relatively temporally sparse data. In contrast, mid-latitude lakes with large inter-annual and annual variability compared to the magnitude of a longterm trend (for instance, the Laurentian Great Lakes), require dense temporal coverage in order to extract a statistically significant trend. These sorts of long time series are especially important in large lakes, since it has been shown (Austin and Colman 2007) that the thermal response of a complex system like a large, seasonally ice-covered lake can significantly exceed the rate of temperature change experienced by the regional atmospheric climate. This can be explained in terms of a coincident reduction of winter ice cover, which in turns leads to earlier spring overturn and a longer warming season.One example of such a time series of daily water temperature has been collected in the St. Mary's River, just downstream of Lake Superior, at a pair of locations near Sault Ste. Marie (McCormick 1996) from 1906 to the present. These data (through 1992) were previously discussed (McCormick and Fahnenstiel 1999) along with several other long time series collected at ...

No abstract

Summer temperatures on the Tibetan Plateau (TP) significantly affect stability of glaciers that provide steady water resources to nearly half of the world population. However, lack of reliable, long‐term proxy records greatly impedes understanding of regional temperature sensitivity to climate forcings. Here we present a 16 ka long, alkenone‐based summer temperature record from Lake Qinghai, northeastern TP that demonstrates major regional temperature response to changes in summer insolation and Atlantic Meridional Overturning Circulation during the Holocene and late glacial. Importantly, we find a period of sustained summer temperature decline (>4°C) between 5 and 3.5 ka, which coincides with expansion of Barents Sea ice coverage and is likely driven by intensification of the Westerlies. This unusually long and pronounced regional cooling event likely delayed permanent human settlements on the high‐altitude regions (>3000 m) of the TP by at least 500 years.

The scarcity of documented numerical relations between rock weathering and time has led to a common assumption that rates of weathering are linear. This assumption has been strengthened by studies that have calculated long-term average rates. However, little theoretical or empirical evidence exists to support linear rates for most chemical-weathering processes, with the exception of congruent dissolution processes. The few previous studies of rock-weathering rates that contain quantitative documentation of the relation between chemical weathering and time suggest that the rates of most weathering processes decrease with time. Recent studies of weathering rinds on basaltic and andesitic stones in glacial deposits in the western United States also clearly demonstrate that rock-weathering processes slow with time. Some weathering processes appear to conform to exponential functions of time, such as the square-root time function for hydration of volcanic glass, which conforms to the theoretical predictions of diffusion kinetics. However, weathering of mineralogically heterogeneous rocks involves complex physical and chemical processes that generally can be expressed only empirically, commonly by way of logarithmic time functions. Incongruent dissolution and other weathering processes produce residues, which are commonly used as measures of weathering. These residues appear to slow movement of water to unaltered material and impede chemical transport away from it. If weathering residues impede weathering processes then rates of weathering and rates of residue production are inversely proportional to some function of the residue thickness. This results in simple mathematical analogs for weathering that imply nonlinear time functions. The rate of weathering becomes constant only when an equilibrium thickness of the residue is reached. Because weathering residues are relatively stable chemically, and because physical removal of residues below the ground surface is slight, many weathering features require considerable time to reach constant rates of change. For weathering rinds on volcanic stones in the western United States, this time is at least 0.5 my.