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The freeze-up period is the pentad average temperature below or equal to 0 °C (≤0 °C), which belong to extreme low temperature events but more rigorous. And analyzing it will contribute to the sparse knowledge of the freeze-up period in the Tibetan Plateau and provide scientific base for animal husbandry production in pastoral area of Northern Tibet and agricultural production in Qaidam Oasis. This paper based on the daily average temperature from a 0.5° × 0.5° gridded data range from 1961 to 2015, the beginning pentad, the ending pentad and the pentads of freeze-up period over the Tibetan Plateau (TP) were calculated. In order to investigate the temporal–spatial variations of the freeze-up period, some methods were applied such as climatic trend rate, the non-parametric Mann–Kendall test, Kriging Interpolation method, wavelet transform and Power Spectrum, etc. The main results were as follows: the annual average beginning pentad had a delaying trend at the rate of 0.24 pentad/decade, the ending pentad was advanced at the rate of 0.35 pentad/decade, but the pentads of freeze-up period had a significant decreasing trend of 0.64 pentad/decade in recent 54 years, additionally, the most remarkable period appeared in 1990s when the freeze-up period shorten. The abrupt change revealed that the year of abrupt change for the beginning pentad and the pentads of freeze-up period was in 1994, and the year of abrupt change for the ending pentad was in 1994, 1995 and 1996. There existed significant regional differences for the freeze-up period, it is noteworthy that the freeze-up period showed a decreasing trend from northwest to southeast over the study region. The significant wavelet power spectra of the beginning pentad were 3.77 years, 4.57 years and 7.14 years and there were obvious periodic oscillations of 3.39 years, 6.49 years for the ending pentad, and the pentads of freeze-up period varied with period of 3.34 years, 4.57 years and 5.46 years, which were consistent with the atmospheric circulation quasi-period of 2–4 years. Furthermore, the correlation coefficient analysis presented that the freeze-up period were mainly controlled by the circulation factors, including the TPI, NHSHII, WPSHII, NHPVAI, NHPVII and APVAI. Moreover, altitude, latitude and longitude were the main influencing factors to the freeze-up period in the Tibetan Plateau. [ABSTRACT FROM AUTHOR]

The mechanisms and controls of the Middle-Late Miocene paleoenvironmental evolution of the northeastern Tibetan Plateau remain elusive. Here, we present an integrated multiproxy study of total organic carbon isotopes, lipid biomarkers, petrographic components and micropaleontological records in the well-dated Xigou section of the Xunhua Basin. Our data reveal a large decrease (−1.1 ‰) in bulk organic carbon isotopes (δ13C org) at ∼12.7 Ma, a broad minimum at ∼12.7–8.0 Ma, and a large increase (+1.3 ‰) at ∼8.0 Ma. The decline in δ13C org at 12.7 Ma was accompanied by pronounced changes in n -alkane indices, clay minerals, palynology, and ostracods, recording a major rise of lake level of paleo-Lake Xunhua due to increased precipitation in the regional catchment. Our inference of sustained elevated humidity in the Xunhua Basin at 12.7–8.0 Ma is contrary to previously reported cooling and drying trends on the NE Tibetan Plateau for the Middle-Late Miocene (∼14–8 Ma), thus precluding the role of global climate change as the dominant driver. Rather, we attribute the paleoenvironmental evolution of the Xunhua Basin to changes in regional rainfall patterns related to orogenic uplift. The Jishi Shan mountain range reached a critical threshold elevation at ∼12.7 Ma, becoming an orographic barrier to Westerlies-transported moisture and thus enhancing intensified precipitation within the Xunhua Basin. By ∼8 Ma, further uplift caused this orogen to become a barrier to the East Asian monsoon, resulting in aridification of the Xunhua Basin. • The Xunhua Basin sensitively recorded environmental changes on NE Tibetan Plateau. • Uplift of Jishi Shan since ∼13 Ma caused moister Xunhua Basin and drier Lixia Basin. • Regional humidity changes due to trapping of Westerlies moisture to west, rain shadow effect to east. • This environmental change recorded by d13C org , n -alkanes, ostracods, and palynology. • Two key environmental transition periods—at ∼12.7 Ma and ∼8.0 Ma. [ABSTRACT FROM AUTHOR]

Within the context of global warming, climate extremes, including extreme wet event and drought events, have become one of the most significant and attractive themes around the world. The target region of this study is confined to eastern China, with most of the country's population concentrated, where both the wet and drought climate extremes can cause considerable damages to the economy, particularly to agriculture. From the inter-annual and intraseasonal scale, temporal and spatial distributions of climate extremes for 27 stations in the Huaihe River Basin over the period 1960–2013, are examined rigorously by means of a modified FAO Penman-Monteith method and the standardized variables of the monthly Surface Humid Index. Morlet wavelet analysis is utilized to thoroughly investigate the oscillation and periodicity of extreme wet/drought events during four seasons, as well as the whole year. The results suggest that the frequency of extreme wet events has significantly increased by 0.0118 times/year, whereas the trend for extreme drought events has gradually decreased, at the rate of 0.0127 times/year, both of which are in accordance with inter-decadal variations of climate extremes. Comparative study reveals climate extremes in autumn shows great differences, in sharp contrast to other seasons and the general inter-annual tendency. Spatial distributions of inter-annual extreme climate events exhibit certain symmetry characteristics, from west to east, indicating the combined influences of topography and monsoon circulation. The major cycles of extreme wet and drought events are 14 years and 24 years, respectively. Finally, possible causes of the temporal and spatial distributions of climate extremes are tentatively analyzed, by correlation analysis of six indexes, namely, AOI, AAOI, EASMI, WNPMI, SASMI, and NAOI, with AOI and NAOI being the dominant indexes under the background of large-scale atmospheric circulation. Additionally, other factors such as total annual precipitation, northward movement and enhancement of the subtropical anticyclone, and anthropogenically induced greenhouse forcing can also contribute to the changes in extreme climate events. [ABSTRACT FROM AUTHOR]

Analyses of the spatial and temporal variation of reference evapotranspiration (ET 0 ) and aridity index (AI) can further understand climate change and its effect on hydrology. In this study, based on date from 58 standard meteorological stations, the spatio-temporal variations and trends of ET 0 and AI were calculated for the Loess Plateau Region (LPR) from 1961 to 2012 using the Mann–Kendall test, the moving t -test, and the Morlet wavelet methods. The results demonstrated: (1) the annual ET 0 displayed a statistically significant decrease of −10.3 mm/10 y ( P < 0.05) during 1961–2012. At that time, AI also had a decreasing trend of −0.013/10 y over the past 52 years. Compared with ET 0 , the inter-decadal variations of AI over the LPR seemed to show similar declined trends. (2) Spatially, about 81% stations displayed a decreasing trend for annual ET 0 during 1961–2012, and most of them were distributed in the western, northern and southeastern LPR. In addition, about 60% of stations showed statistically decreasing trends for AI, and the major decreases were mainly in the east Qinghai division and northern LPR. (3) Abrupt changes were detected in annual ET 0 series in the 1980s and in annual AI series in 1987. Wavelet analysis indicated significant 2 year and non-significant 7, 10 and 14 years in annual ET 0 series, and there existed periods of 4, 8 and 12 years for AI. (4) For ET 0 , wind speed was the most sensitive meteorological factor, followed by sunshine hours, precipitation, and relative humidity. For AI, ET 0 and precipitation were the sensitive factors. This study provides an understanding of the effect of recent climate change on hydrological cycle in the LPR. [ABSTRACT FROM AUTHOR]

• δ18O carb and δ13C alk records from a >5,000 m-thick sequence in the Qaidam Basin. • Regional moisture fluctuations over the Cenozoic inferred. • Linkage of moisture changes to global climate depends on the chosen chronology. The Cenozoic history of aridification in the Asian interior is thought to be intimately linked to uplift of the Tibetan Plateau and long-term global cooling. How fluctuating climatic conditions influenced regional moisture levels during the Cenozoic remains largely unknown. To infer moisture changes, we present oxygen isotope values of carbonates (δ18O carb) and carbon isotope values of long-chain n -alkanes (δ13C alk) from a continuous geological section in the Qaidam Basin, northwestern China. The section spans most of the Cenozoic Era, from ~52 to 5 million years ago (Ma), based on conventional chronology, whereas recently a drastically revised chronology has placed its basal age at ~26 Ma. The association of moisture changes with global climate largely depends on the chosen chronology. With the revised chronology, our records are inconsistent with existing moisture records from the Asian interior and do not show clear association with global climatic conditions. However, with the conventional chronology, our δ18O carb record shows a long-term aridification trend over the Cenozoic, whereas the δ13C alk suggests reduced moisture levels during the Oligocene. Both records consistently show enhanced aridity, notably during the Oligocene, ~20–17 Ma and after ~13 Ma, which largely corresponded to relatively cool global periods. Our records thus suggest that global climatic conditions during the Cenozoic exerted more influence on the moisture levels in the Asian interior than previously thought, if the conventional chronology is correct. Our study also highlights the importance of constructing a reliable chronology in order to make an association of Cenozoic terrestrial records with global climate. [ABSTRACT FROM AUTHOR]