新仙女木事件及全新世早中期降温事件——来自洱海湖泊沉积的记录

Three cold events (the Younger Dryas, 9.4 ka cal BP, 5.8 ka cal BP) since the 13 ka cal BP in Erhai (EH) Lake catchment, Yunnan Province, were analyzed using the Total Organic Carbon (TOC) series of the EH core. By comparison of the EH core, Qinghai Lake core and Guliya ice core, differences of these cold events were determined. Erhai Lake's responses to the global cold events were lagged in time and weakened in intensity in comparison with Qinghai Lake's. The latitude location of Erhai Lake and the obstruction of Tibetan Plateau may in part explain the differences. However, the remarkable cold event of 8.2 ka cal BP in the Guliya ice core was absent in the records of Erhai Lake and Qinghai Lake. Power spectrum analysis of the TOC proxy series shows that there were three kinds of millennial cycles, i.e. 5 ka, 2.3 ka, and 1.5 ka, in climate changes in Erhai Lake, which reveal the responses of climate to suborbit cycles.     

全新世气候

全新世指我们目前所在的间冰期.一般把新仙女木事件(YD)结束的时间(11.5 kaBP)作为全新世的开始.当前国际上对全新世的气候变化有3点共识:1)进入全新世温度大幅度回升,中全新世以后逐渐下降,20世纪由于人类活动的影响全球变暖;2)早全新世北半球亚非季风区气候湿润,5 kaBP以后逐渐变干,但是ENSO加强;3)在以上变化的背景下发生了若干次冷干事件,这些事件开始与结束均较迅速,所以也被为气候突变.     

8.2ka BP冷事件

全新世11.5 ka BP以来遭受的最强的一次冷事件是8.2 ka BP事件.全新世气候的基调是温暖湿润.但是,大量的古气候资料表明,全新世气候也有不稳定性[1],至今可能已发生过8～9次冷事件,8.2 kaBP事件就是其中的一次[2].     

早全新世气候振荡

早全新世气候振荡,一般指全新世中8.2 kaBP事件及其以前的气候振荡.根据Bond等[1]的研究,相应的北大西洋冷事件分别出现于8.1,9.4,10.3及11.1 kaBP,分别编号为5,6,7,8.     

全新世北大西洋冷事件

对北大西洋冷事件的研究至今不过10a左右,这个问题之所以令人关注,主要是因为它一,能反映了北大西洋经向翻转环流（MOC）的强度变化。MOC足全球热盐环流（THC）的重要组成部分。发生冷事件时,北大西洋表层为融冰淡水所控制,这抑制了北大西洋深水（NADW）的形成,使得MOC减弱,向北输送的热量大为减少,给欧洲带来冷干气候。近年来对北大西洋冷事件有了比较详细的了解,得到了较为一致的年表。     

The global response to Younger Dryas boundary conditions in an AGCM simulation

 Geological evidence points to a global Younger Dryas (YD) climatic oscillation during the last glacial/ present interglacial transition phase. A convincing mechanism to explain this global YD climatic oscillation is not yet available. Nevertheless, a profound understanding of the mechanism behind the YD climate would lead to a better understanding of climate variability. Therefore, the Hamburg atmospheric circulation model was used to perform four numerical experiments on the YD climate. The objective of this study is to improve the understanding of different forcings influencing climate during the last glacial/interglacial transition and to investigate to what extent the model response agrees with global geological evidence of YD climate change. The following boundary conditions were altered: sea surface conditions, ice sheets, insolation and atmospheric CO₂ concentration. Sea surface temperatures based on foraminiferal assemblages proved to produce insufficient winter cooling in the N Atlantic Ocean in two experiments. It is proposed that this discrepancy is caused by uncertainties in the reconstruction method of sea surface temperatures. Therefore, a model-derived set of Atlantic surface ocean conditions was prescribed in a subsequent simulation. However, the latter set represented an Atlantic Ocean without a thermohaline circulation, which is not in agreement with evidence from ocean cores. The global response to the boundary conditions was analysed using three variables, namely surface temperature, zonal wind speed and precipitation. The statistical significance of the changes was tested with a two-tailed t-test. Moreover, the significant responses to cooled oceans were compared with geological evidence of a YD oscillation. This comparison revealed a good match in Europe, Greenland, Atlantic Canada and the N Pacific region, explaining the YD oscillation in these regions as a response to cooled N Atlantic and N Pacific Oceans. However, the results leave the YD climate in other regions completely unexplained. This reflects either an insufficient set of boundary conditions or the important role played by feedbacks within the coupled atmosphere-ocean-ice system. These feedbacks are poorly represented in the used atmospheric model, since ice sheets and the ocean surface conditions have to be prescribed. Received: 30 July 1996 / Accepted: 12 February 1997     

Climatic and limnological changes associated with the Younger Dryas in Atlantic Canada

Pollen, diatom and chironomid fossils from the sediments of a core from Brier Island Bog Lake, Nova Scotia were studied in an attempt to relate changes in microfossil composition to a climatic cooling in Atlantic Canada correlative with the European Younger Dryas ca. 10 to 11 ka. Our paleolimnological data were then compared to similar types of data from Splan Pond, New Brunswick to determine if there were any significant differences between a coastal and a more inland site. Nonarboreal pollen was dominant throughout the Brier Island core and the interval 10.0–11.0 ka did not show the typical decline in Picea and increases in tundra-like vegetation characteristic of many sites in Atlantic Canada. However, the limnological indicators did undergo marked changes in taxon composition. The chironomid assemblage was initially dominated by shallow-water, warm-adapted chironomid taxa followed by abundant Sergentia (a cold stenotherm) during 10–11 ka. Sergentia disappeared in the post Younger Dryas interval and the warm-adapted genera resumed dominance. Chironomid-inferred paleotemperature reconstructions revealed that at both Brier Island Bog Lake and Splan Pond, summer surface-water temperatures dropped abruptly to between 13 and 17°C during the 10–11 ka interval, suggesting that a cooler climate was present in Atlantic Canada correlative with the European Younger Dryas. Diatom assemblage changes during the same period corroborate the occurrence of limnological fluctuations.     

The atmospheric winter circulation during the Younger Dryas stadial in the Atlantic/European sector

The Younger Dryas (YD) stadial signified an interruption of the warming during the transition from the last glacial to the present interglacial. The mechanism responsible for this cooling is still uncertain, so valuable information concerning climate variability can be obtained by numerical simulation of the YD climate. We performed four experiments on the Younger Dryas climate with the Hamburg atmospheric general circulation model. Here we use the results of these experiments, which differed in prescribed boundary conditions, to characterize the atmospheric winter circulation during the YD stadial in the North Atlantic/European sector. The 10 year means of the following variables are presented: sea level pressure, 500 hPa geopotential heights and 200 hPa winds. In addition, we used daily values to calculate an index to assess the occurrence of blocking and strong zonal flow and to compute storm tracks. Our results show that the YD cooling in Europe was present with a strong and stable westerly circulation without blocking. This is in conflict with an earlier study suggesting frequent easterly winds over NW-Europe. In our experiments the sea-ice cover in the North Atlantic Ocean was the crucial factor forcing this specific YD circulation. Moreover, the jet stream over the North Atlantic was strengthened considerably, causing an enhanced cyclonic activity over the Eurasian continent. The YD winter circulation was different from the circulation found in most simulation studies on the Last Glacial Maximum, since no glacial anticyclones were present and no split of the jet stream occurred. Received: 1 November 1995 / Accepted: 29 May 1996     

Modelling the concentration of atmospheric CO2 during the Younger Dryas climate event

 The Younger Dryas (YD, dated between 12.7–11.6 ky BP in the GRIP ice core, Central Greenland) is a distinct cold period in the North Atlantic region during the last deglaciation. A popular, but controversial hypothesis to explain the cooling is a reduction of the Atlantic thermohaline circulation (THC) and associated northward heat flux as triggered by glacial meltwater. Recently, a CH₄-based synchronization of GRIP δ¹⁸O and Byrd CO₂ records (West Antarctica) indicated that the concentration of atmospheric CO₂ (CO^atm ₂) rose steadily during the YD, suggesting a minor influence of the THC on CO^atm ₂ at that time. Here we show that the CO^atm ₂ change in a zonally averaged, circulation-biogeochemistry ocean model when THC is collapsed by freshwater flux anomaly is consistent with the Byrd record. Cooling in the North Atlantic has a small effect on CO^atm ₂ in this model, because it is spatially limited and compensated by far-field changes such as a warming in the Southern Ocean. The modelled Southern Ocean warming is in agreement with the anti-phase evolution of isotopic temperature records from GRIP (Northern Hemisphere) and from Byrd and Vostok (East Antarctica) during the YD. δ¹³C depletion and PO₄ enrichment are predicted at depth in the North Atlantic, but not in the Southern Ocean. This could explain a part of the controversy about the intensity of the THC during the YD. Potential weaknesses in our interpretation of the Byrd CO₂ record in terms of THC changes are discussed. Received: 27 May 1998 / Accepted: 5 November 1998     

Surface temperature in NW Europe during the Younger Dryas: AGCM simulation compared with temperature reconstructions

 During the Younger Dryas (YD) the climate in NW Europe returned to near-glacial conditions. To improve our understanding of climate variability during this cold interval, we compare an AGCM simulation of this climate, performed with the ECHAM model, with temperature reconstructions for NW Europe based on geological and paleoecological records. Maps for the mean winter, summer and annual temperature are presented. The simulated winters are consistent with reconstructions in the northern part of the study area. A strong deviation is noted in Ireland and England, where the simulation is too warm by at least 10 °C. It appears that the N Atlantic was cooler than prescribed in the YD simulation, including a southward expansion of the sea-ice margin. The comparison for the summer shows a too warm continental Europe in the simulation. Supposedly, these anomalously warm conditions are caused by the AGCM’s response to the prescribed increased summer insolation. The region of maximum summer cooling is similar in both the simulation and reconstruction, i.e., S Sweden. We suggest that this is due to the local cooling effect of the Scandinavian ice sheet. Compared to the present climate a considerable increase of the annual temperature range is inferred, especially for regions close to the Atlantic Ocean. Received: 20 November 1996 / Accepted: 8 July 1997     

The Definition and Classification of Extensive and Persistent Extreme Cold Events in China

Using the observed daily temperatures from 756 stations in China during the period from 1951 to 2009 extensive and persistent extreme cold events (EPECEs) were defined according to the following three steps:1) a station was defined as an extreme cold station (ECS) if the observed temperature was lower than its 10th percentile threshold;2) an extensive extreme cold event was determined to be present if the approximated area occupied by the ECSs was more than 10% of the total area of China (83rd percentile) on its starting day and the maximum area occupied by the ECSs was at least 20% of the total area of China (96th percentile);and 3) an EPECE was determined to be present if the extensive extreme cold event lasted for at least for eight days.52 EPECEs were identified in this manner,and these identification results were also verified using other reliable data.On the basis of cluster analysis,five types of EPECEs were classified according to the spatial distribution of ECSs at their most extensive time over the course of the EPECE.     

新疆"4.23"强寒潮降温特征分析

利用常规天气观测资料、美国NCEP 1°×1°逐6 h再分析资料,应用天气学原理和天气动力诊断分析方法,对2014年4月22—24日一场春季强寒潮降温天气("4·23"强寒朝)进行分析。结果表明:(1)前期北疆500 hPa上空被弱脊控制,欧洲脊强烈发展,强北风带建立,中高纬环流由纬向转为经向。中期欧洲脊衰退,引导冷空气向东南爆发,长波槽迅速东移南下侵入北疆,造成了本次寒潮天气。(2)寒潮爆发前期,日平均气温较历年同期偏高以及地形因素是导致冷空气强烈爆发、气温陡降的有利因素。(3)沿46°N的垂直温度平流、θ_(se)剖面图可以分析得出,地面冷锋先与高空锋面进入新疆境内,冷空气在6 h内迅速移动并且强度明显加强。(4)锋生函数分析表明强降温的区域、寒潮的路径和起始时间与其中心变化有很好的对应关系,可以清楚地分析出冷空气的时空变化。