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1.
High‐resolution marine palynological data have been obtained from two very long sediment cores (MD952009 and MD952010) retrieved from the southern Norwegian Sea. The dinoflagellate cyst assemblages show pronounced fluctuations in composition, which correlate strongly with magnetic susceptibility records and also mimic the δ18O signal of the GISP2 Greenland ice‐core. If focusing on the period from 48 to 30 cal. kyr BP, this correlation suggests a paradoxical response of the sea‐surface environments to the atmospheric conditions over Greenland: when the Greenland δ18O signal reflects warm interstadial conditions, the Norwegian Sea depicts cold sea‐surface temperatures with quasi‐perennial sea‐ice cover (based on dinoflagellate cysts). In contrast, when the Greenland δ18O records cold stadial periods, the Norwegian Sea‐surface temperatures are warm (based on dinoflagellate cysts), probably linked to inflow of the North Atlantic Drift. These results, similar in both cores, are contrary to those of previous studies and shed light on a possible decoupling of Norwegian sea surface‐water conditions and atmospheric conditions over Greenland. This decoupling could be linked to an atmosphere–ocean system behaving similar to that which the Northern Hemisphere is experiencing at present, i.e. strongly variable owing to the North Atlantic Oscillation. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

2.
A pollen-based quantitative climate reconstruction from a lake-sediment core on the Norwegian Barents Sea coast provides insights about climatic change over the Greenland stadial 1 (GS-1) to early-Holocene transition. GS-1 was characterized by low July mean temperatures ( c . 6.0°C) and dry conditions probably resembling modern arctic deserts. The increase in July mean temperatures to the Holocene level (10.0-12.0°C) took place in a two-step pattern interrupted by a short cool period with July mean temperatures of c . 8.0°C during the early Preboreal at c . 11450-11200 cal. yr BP. The reconstruction also suggests two other early-Holocene coolings of c . 1.5°C, dating to 10900-10800 cal. yr BP and 10400-10200 cal. yr BP, synchronously with short-term decreases in δ18 O values in the Greenland ice cores. These results reflect the highly unstable nature of the early-Holocene climate in northernmost Fennoscandia. Apart from the cooling at 10900-10800 cal. yr BP, the reconstructed cold events correlate with fluxes of fresh water to the North Atlantic and related reductions of North Atlantic deep-water formation, suggesting that the rapid climate changes resulted from the dynamics of the North Atlantic thermohaline circulation and oceanic energy transport during the GS-1 to early-Holocene transition.  相似文献   

3.
千年尺度气候变率的研究   总被引：5，自引：0，他引：5  
2 0世纪后期 ,千年 (ka)尺度气候变率的研究取得了重要的进展 ,这表现在以下几个方面 :(1)格陵兰冰芯及深海沉积证明 ,在末次冰期中普遍存在平均周期为 1 5ka的循环 ,有人认为全新世也存在这种循环 ,小冰期就是最近一个循环的冷期。 (2 )每个循环由 1个相对暖期 (间冰阶 )及 1个冷期(冰阶 )组成 ,称为Dansgaard/Oeshger循环 (D/O循环 )。 (3)连续几个D/O循环的冰阶气温愈来愈低 ,海因里希事件 (H事件 )就发生在最冷的冰阶之后。 (4 )自 15kaBP到 6 8kaBP共确定出 6次H事件 ,分别称为H1…H6,有的作者认为新仙女木事件 (YD)与H事件形成机制近似 ,可以称为H0 。 (5 )D/O循环与H事件的成因 ,目前尚无定论。但热盐环流 (THC)变化的学说得到了较多作者的承认。这个学说认为 :北大西洋北部的大量融冰使海面为冷的淡水控制 ,影响了大传送带中海水的下沉 ,从而削弱了深水的形成。北大西洋THC减弱 ,使向北输送的热量减少 ,使北大西洋气候更寒冷。一旦深水形成再次增加 ,完成一个D/O循环。H事件形成的机制与之类似 ,不过过程变化更为激烈。 (6 )这样 ,THC有 3种模态 :现代模 (北大西洋有两个泵 )、冰阶模 (一个泵 ) ,及H事件 (无泵 )。 (7)海洋环流模式已经对THC的变化及模态之间的转换进行了模拟 ,至少在一定程度?  相似文献   

4.
Using results from coupled climate model simulations of the 8.2 ka climate event that produced a cold period over Greenland in agreement with the reconstructed cooling from ice cores, we investigate the typical pattern of climate anomalies (fingerprint) to provide a framework for the interpretation of global proxy data for the 8.2 ka climate event. For this purpose we developed an analysis method that isolates the forced temperature response and provides information on spatial variations in magnitude, timing and duration that characterise the detectable climate event in proxy archives. Our analysis shows that delays in the temperature response to the freshwater forcing are present, mostly in the order of decades (30 a over central Greenland). The North Atlantic Ocean initially cools in response to the freshwater perturbation, followed in certain parts by a warm response. This delay, occurring more than 200 a after the freshwater pulse, hints at an overshoot in the recovery from the freshwater perturbation. The South Atlantic and the Southern Ocean show a warm response reflecting the bipolar seesaw effect. The duration of the simulated event varies for different areas, and the highest probability of recording the event in proxy archives is in the North Atlantic Ocean area north of 40° N. Our results may facilitate the interpretation of proxy archives recording the 8.2 ka event, as they show that timing and duration cannot be assumed to correspond with the timing and duration of the event as recorded in Greenland ice cores. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

5.
We present a high-resolution terrestrial archive of Central American rainfall over the period 100–24 and 8.1–6.5 ka, based on δ18O time series from U-series dated stalagmites collected from a cave on the Pacific Coast of Costa Rica. Our results indicate substantial δ18O variability on millennial to orbital time scales that is interpreted to reflect rainfall variations over the cave site. Correlations with other paleoclimate proxy records suggest that the rainfall variations are forced by sea surface temperatures (SST) in the Atlantic and Pacific Oceans in a fashion analogous to the modern climate cycle. Higher rainfall is associated with periods of a warm tropical North Atlantic Ocean and large SST gradients between the Atlantic and Pacific Oceans. Rainfall variability is likely linked to the intensity and/or latitudinal position of the intertropical convergence zone (ITCZ). Periods of higher rainfall in Costa Rica are also associated with an enhanced sea surface salinity gradient on either side of the isthmus, suggesting greater freshwater export from the Atlantic Basin when the ITCZ is stronger and/or in a more northerly position. Further, wet periods in Central America coincide with high deuterium excess values in Greenland ice, suggesting a direct link between low latitude SSTs, tropical rainfall, and moisture delivery to Greenland. Our results indicate that a stronger tropical hydrological cycle during warm periods and large inter-ocean SST gradients enhanced the delivery of low latitude moisture to Greenland.  相似文献   

6.
Models of atmospheric circulation in the North Atlantic sector during glacial inception can be expanded to a hemispheric scale with the aid of diagnostic studies of the present climate. The present “Greenland Above” (GA) atmospheric circulation type may be a candidate for the atmospheric circulation type required during glacial inception. The pattern is an amplification, with only minor phase shifts, of the present average winter circulation pattern in the extratropical Northern Hemisphere. Southerly flow in the northwest Atlantic is associated with warm ocean temperatures, low sea ice in the Davis Strait, and increased precipitation over northeast Canada. Evidence from modeling of the present climate indicates that the GA pattern could be maintained by increased snow cover over eastern North America. Enhanced snow cover, due to decreased Northern Hemisphere summer insolation, could cause a similar response on an ice-age time scale.  相似文献   

7.
萨拉乌苏河流域末次间冰阶气候——以米浪沟湾剖面为例   总被引：1，自引：0，他引：1  
米浪沟湾剖面末次间冰阶层序粒度和化学元素波动韵律与由古流动砂丘砂和上覆河湖相或古土壤构成的沉积旋回颇为一致。古流动砂丘砂犹如现代流动砂丘砂,是东亚冬季风主导下干冷气候的产物;河湖相和古土壤颗粒细化,化学、生物等地球风化程度增强,含较多喜暖的软体动物化石,指示其偏南夏季风主导下的温暖湿润气候。据此,末次间冰阶萨拉乌苏河流域至少经历了10次温湿(W事件)和9次冷干(C事件)气候波动,且可划分为MIS3e(58.85~48.98kaBP)、MIS3d(48.98~39.55kaBP)、MIS3c(39.55~34.59kaBP)、MIS3b(34.59~26.47kaBP)和MIS3a(26.47~23.07kaBP)等5个亚段。其中,19次冷/暖波动可与格陵兰GRIP冰心δ18O冰段/间冰段相对应,5个亚段与我国古里雅冰心在波动性质和相位上都极为一致,与V23-81冷性浮游有孔虫数代表的北大西洋地区气候也具有较好的可比性。谱分析显示出21.70ka、1.05ka、0.64ka、0.50ka等显著周期,即该地千百年尺度气候主要受与北大西洋热盐环流波动引起的东亚冬、夏季风强弱有关,而万年尺度上则受控于岁差周期所导致的太阳辐射变化。  相似文献   

8.
《Quaternary Science Reviews》2005,24(10-11):1159-1182
A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics.  相似文献   

9.
《Comptes Rendus Geoscience》2005,337(1-2):87-95
Holocene millennial climate variability is smaller than that of the last glaciation, due to the disappearance of large unstable ice sheets. Marine records show that the sea-surface temperature (SST) exhibited small variations, mainly in the high and low latitudes. They may be interpreted as a linear response to the mean annual insolation. Major changes in the hydrological cycle have been evidenced in the Asian and African monsoon area, resulting in enhanced precipitation and large river outflow in the Bay of Bengal and the Gulf of Niger. Enhanced rainfall over the Mediterranean Basin resulted in a weak circulation and sapropel formation below 800-m water depth in the eastern Mediterranean Sea. Finally small changes in the thermohaline circulation and the warm North Atlantic Drift have been detected in the Nordic Seas. The Holocene climatic variability is therefore similar to that of the Quaternary, but with small amplitude, while continents experienced large rainfall variations. To cite this article: J.-C. Duplessy et al., C. R. Geoscience 337 (2005).  相似文献   

10.
早全新世降温事件的湖泊沉积证据   总被引：12，自引：1，他引：12  
我国华北干旱－半干旱区封闭湖泊流域化学风化历史记录了全新世以来次级的气候环境波动过程。高精度的沉积物地球化学、物理及生物参数变化表明，在全新世早－中期过渡阶段存在一次强降温气候事件，具体表现为流域化学风化减弱（高Rb/Sr比）、湖泊产生力减弱（低有机碳）以及湖泊水位下降。虽然该事件的寒冷程度比Younger Dryas弱，但是其与来自湖沼（包括北极、非洲、北美、西欧、青藏高原、祁连山等）、海洋（比北大西洋、地中海、加勒比海等）、欧－美大陆生物组合、极地冰芯等在内的环境记录的冷事件发生时间基本一致，集中发生于8.0-8.5ka B.P.之间。  相似文献   

11.
We propose that prior to the Younger Dryas period, the Arctic Ocean supported extremely thick multi-year fast ice overlain by superimposed ice and firn. We re-introduce the historical term paleocrystic ice to describe this. The ice was independent of continental (glacier) ice and formed a massive floating body trapped within the almost closed Arctic Basin, when sea-level was lower during the last glacial maximum. As sea-level rose and the Barents Sea Shelf became deglaciated, the volume of warm Atlantic water entering the Arctic Ocean increased, as did the corresponding egress, driving the paleocrystic ice towards Fram Strait. New evidence shows that Bering Strait was resubmerged around the same time, providing further dynamical forcing of the ice as the Transpolar Drift became established. Additional freshwater entered the Arctic Basin from Siberia and North America, from proglacial lakes and meltwater derived from the Laurentide Ice Sheet. Collectively, these forces drove large volumes of thick paleocrystic ice and relatively fresh water from the Arctic Ocean into the Greenland Sea, shutting down deepwater formation and creating conditions conducive for extensive sea-ice to form and persist as far south as 60°N. We propose that the forcing responsible for the Younger Dryas cold episode was thus the result of extremely thick sea-ice being driven from the Arctic Ocean, dampening or shutting off the thermohaline circulation, as sea-level rose and Atlantic and Pacific waters entered the Arctic Basin. This hypothesis focuses attention on the potential role of Arctic sea-ice in causing the Younger Dryas episode, but does not preclude other factors that may also have played a role.  相似文献   

12.
Ocean Drilling Project (ODP) site 882 (50°22′N, 167°36′E) provides the first high-resolution GRAPE density, magnetic susceptibility, carbonate, opal and foraminifera (planktonic and benthic) stable isotopes records between 3.2 and 2.4 Ma in the Northwest Pacific. We observed a dramatic increase in ice rafting debris at site 882 at 2.75 Ma, which is coeval with that found in the Norwegian Sea, suggesting that the Eurasian Arctic and Northeast Asia were significantly glaciated from 2.75 Ma onwards. Prior to 2.75 Ma planktonic foraminifera δ18O records indicate a warming or freshening trend of 4°C or 2‰ over 80 ka. If this is interpreted as a warm pre-glacial Pliocene North Pacific, it may have provided the additional moisture required to initially build up the northern hemisphere continental ice sheet. The dramatic drop in sea surface temperatures (SST>7.5°C) at 2.75 Ma ended this suggested period of enhanced SST and thus the proposed moisture pump. Moreover, at 2.79 and 2.73 Ma opal mass accumulation rates (MAR) decrease in two steps by five fold and is accompanied by a more gradual long-term decrease in CaCO3 MARs. Evidence from the Southern Ocean (ODP site 704) indicates that just prior to 2.6 Ma there is a massive increase in opal MARs, the opposite to what is found in the North Pacific. This indicates that the intensification of northern hemisphere glaciation was accompanied by a major reorganisation of global oceanic chemical budget, possibly caused by changes in deep ocean circulation. The initiation of northern hemisphere glaciation occurred in the late Miocene with a significant build up of ice on southern Greenland. However, the progressive intensification did not occur until 3.5–3 Ma when the Greenland ice sheet expanded to include northern Greenland. Following this stage we suggest that the Eurasian Arctic and Northeast Asia glaciated at 2.75 Ma, approximately 100 ka before the glaciation of Alaska (2.65 Ma) and 200 ka before the glaciation of the North East American continent (2.54 Ma).  相似文献   

13.
Variations of orbital-driven insolation, ice volume, and greenhouse gas effects have been proposed as major controlling factors in determining the timing and amplitude of Quaternary cyclic climate changes. However, it remains to be determined how the internal feedback in the tropical atmosphere and ocean and the coupling between the low- and high-latitude systems may have produced instability or non-cyclic changes in the long-term climate evolution. Such “abnormalities” have been reported increasingly from paleoclimatic reconstructions in East Asia and the western Pacific for a prolonged warm and humid climate interval during marine isotope stage (MIS) 13–15, ～475–610 kya. To better address the climate abnormality in MIS 13–15 that has been observed in the western Pacific, here we report high-resolution late Quaternary planktic foraminifer faunal abundance and faunal sea surface temperature (SST) records from the International Marine Past Global Change (IMAGES) program core MD972142, which was retrieved from the southeastern South China Sea (SCS). Our results indicate that the faunal assemblages and SSTs in the southeastern SCS express a substantially prolonged, unusual warm interglacial-type climate condition in MIS 13–15. The climate was abnormally warm during the cold MIS 14. Our study also suggests a lowering of sea surface salinity (SSS) during MIS 13–15. While the western Pacific climate experienced a persistently warm and humid period at MIS 14, a “normal” cooling (～2 °C) condition on the surface of the eastern equatorial Pacific existed concomitantly. While assessing possible interpretations of this “abnormal” climate interval in MIS 13–15, our study indicates that an enhanced interhemispheric and/or longitudinal temperature gradient across the basin-wide Pacific cannot be ruled out. A change in the sensitivity of the East Asian Monsoon (EAM) that controlled temperature and precipitation patterns in East Asia and the western Pacific is probably also important or responsible for this climate abnormality. Based on our evidence, we suggest that the tropical dynamics would have played a role in the climate abnormality in MIS 13–15, through maintaining or even increasing the longitudinal SST gradient in the equatorial Pacific, which may have intensified the low-latitude trade winds in the eastern component of the Walker Circulation that drove a longer duration and/or stronger intensity of the summer vs. winter EAM.  相似文献   

14.
Kjellström, E., Brandefelt, J., Näslund, J.‐O., Smith, B., Strandberg, G., Voelker, A. H. L. & Wohlfarth, B. 2010: Simulated climate conditions in Europe during the Marine Isotope Stage 3 stadial. Boreas, 10.1111/j.1502‐3885.2010.00143.x. ISSN 0300‐9483. State‐of‐the‐art climate models were used to simulate climate conditions in Europe during Greenland Stadial (GS) 12 at 44 ka BP. The models employed for these simulations were: (i) a fully coupled atmosphere–ocean global climate model (AOGCM), and (ii) a regional atmospheric climate model (RCM) to dynamically downscale results from the global model for a more detailed investigation of European climate conditions. The vegetation was simulated off‐line by a dynamic vegetation model forced by the climate from the RCM. The resulting vegetation was then compared with the a priori vegetation used in the first simulation. In a subsequent step, the RCM was rerun to yield a new climate more consistent with the simulated vegetation. Forcing conditions included orbital forcing, land–sea distribution, ice‐sheet configuration, and atmospheric greenhouse gas concentrations representative for 44 ka BP. The results show a cold climate on the global scale, with global annual mean surface temperatures 5 °C colder than the modern climate. This is still significantly warmer than temperatures derived from the same model system for the Last Glacial Maximum (LGM). Regional, northern European climate is much colder than today, but still significantly warmer than during the LGM. Comparisons between the simulated climate and proxy‐based sea‐surface temperature reconstructions show that the results are in broad agreement, albeit with a possible cold bias in parts of the North Atlantic in summer. Given a prescribed restricted Marine Isotope Stage 3 ice‐sheet configuration, with large ice‐free regions in Sweden and Finland, the AOGCM and RCM model simulations produce a cold and dry climate in line with the restricted ice‐sheet configuration during GS 12. The simulated temperature climate, with prescribed ice‐free conditions in south‐central Fennoscandia, is favourable for the development of permafrost, but does not allow local ice‐sheet formation as all snow melts during summer.  相似文献   

15.
研究冰量较大时期千年尺度事件的区域响应和结构特征有助于进一步理解和验证千年尺度事件的驱动机制。基于湖北永兴洞一支石笋的5个高精度230Th年龄和323个氧同位素数据,重建了Dansgaard-Oeschger（DO）18事件发生时高分辨率的东亚水文循环变化的过程和特征。结合同一洞穴另两支石笋记录,发现永兴洞石笋δ18O在~64.7 ka迅速负偏1.5‰,指示在MIS4阶段东亚季风区确实发生了持续时间较短的DO18事件。石笋δ18O结果精确标定了DO18事件的结束时间为~63.0 ka,结束过程呈现出5个百年尺度的季风增强振荡。在形态特征上,不同于格陵兰冰芯DO18事件的快速开始和快速结束（~60 a）的对称变化,季风区DO18事件呈现出快速增强但缓慢结束（持续达1500 a）的显著不对称特征。但是,石笋δ18O与北大西洋温盐环流（AMOC）变化在形态特征和持续时间上高度一致,暗示了AMOC对东亚季风区DO事件的重要调控作用。DO18事件时东亚季风与北大西洋气候的不同结构特征可归于高低纬气候系统对AMOC响应方式的差异。石笋δ18O功率谱结果呈现出208 a、150 a、52 a和14~12 a的变化周期,分别接近于太阳活动的207 a、149 a、57 a和11 a周期,说明在冰量较大的MIS4阶段太阳活动仍控制着东亚季风短尺度的波动。  相似文献   

16.
Many sediment records from the margins of the Californias (Alta and Baja) collected in water depths between 60 and 1200 m contain anoxic intervals (laminated sediments) that can be correlated with interstadial intervals as defined by the oxygen-isotope composition of Greenland ice (Dansgaard–Oeschger, D–O, cycles). These intervals include all or parts of Oxygen Isotope Stage 3 (OIS3; 60–24 cal ka), the Bölling/Alleröd warm interval (B/A; 15–13 cal ka), and the Holocene. This study uses organic carbon (Corg) and trace-element proxies for anoxia and productivity, namely elevated concentrations and accumulation rates of molybdenum and cadmium, in these laminated sediments to suggest that productivity may be more important than ventilation in producing changes in bottom-water oxygen (BWO) conditions on open, highly productive continental margins. The main conclusion from these proxies is that during the last glacial interval (LGI; 24–15 cal ka) and the Younger Dryas cold interval (YD; 13–11.6 cal ka) productivity was lower and BWO levels were higher than during OIS3, the B/A, and the Holocene on all margins of the Californias. The Corg and trace-element profiles in the LGI–B/A–Holocene transition in the Cariaco Basin on the margin of northern Venezuela are remarkably similar to those in the transition on the northern California margin. Correlation between D–O cycles in Greenland ice with gray-scale measurements in varved sediments in the Cariaco Basin also is well established. Synchronous climate-driven changes as recorded in the sediments on the margins of the Californias, sediments from the Cariaco Basin, and in the GISP-2 Greenland ice core support the hypothesis that changes in atmospheric dynamics played a major role in abrupt climate change during the last 60 ka. Millennial-scale cycles in productivity and oxygen depletion on the margins of the Californias demonstrate that the California Current System was poised at a threshold whereby perturbations of atmospheric circulation produced rapid changes in circulation in the eastern North Pacific Ocean. It is likely that the Pacific and Atlantic Oceans were linked through the atmosphere. Warmer air temperatures during interstadials would have strengthened Hadley and Walker circulations, which, in turn, would have strengthened the subtropical high pressure systems in both the North Pacific and the North Atlantic, producing increased rainfall over the Cariaco Basin and increased upwelling along the margins of the Californias.  相似文献   

17.
A new centennial scale benthic foraminiferal record of late Holocene climate variability and oceanographic changes off West Greenland (Disko Bugt) highlights substantial subsurface water mass changes (e.g. temperature and salinity) of the West Greenland Current (WGC) over the past 3.6 ka BP. Benthic foraminifera reveal a long-term late Holocene cooling trend, which may be attributed to increased advection of cold, low-salinity water masses derived from the East Greenland Current (EGC). Cooling becomes most pronounced from c. 1.7 ka BP onwards. At this point the calcareous Atlantic benthic foraminiferal fauna decrease significantly and is replaced by an agglutinated Arctic fauna. Superimposed on this cooling trend, centennial scale variability in the WGC reveals a marked cold phase at c. 2.5 ka BP, which may correspond to the 2.7 ka BP cooling-event recorded in marine and terrestrial archives elsewhere in the North Atlantic region. A warm phase recognized at c. 1.8 ka BP is likely to correspond to the ‘Roman Warm Period’ and represents the warmest bottom water conditions. During the time period of the ‘Medieval Climate Anomaly’ we observe only a slight warming of the WGC. A progressively more dominant cold water contribution from the EGC on the WGC is documented by the prominent rise in abundance of agglutinated Arctic water species from 0.9 ka BP onwards. This cooling event culminates at c. 0.3 ka BP and represents the coldest episode of the ‘Little Ice Age’.Gradually increased influence of cold, low-salinity water masses derived from the EGC may be linked to enhanced advection of Polar and Arctic water by the EGC. These changes are possibly associated with a reported shift in the large-scale North Atlantic Oscillation atmospheric circulation pattern towards a more frequent negative North Atlantic Oscillation mode during the late Holocene.  相似文献   

18.
《Quaternary Science Reviews》2004,23(20-22):2231-2246
Palaeoclimatic changes through the last 1200 calibrated years have been documented by high-resolution multi-proxy studies of three cores from about 400 m water depth on the North Icelandic shelf. Benthic and planktonic foraminiferal assemblages and stable isotope values, as well as ice rafted debris (IRD) concentrations, are compared with diatom-based sea-surface water temperatures and the reconstructed mean temperature for the Northern Hemisphere. Changes in surface and bottom water characteristics are mainly due to variations in the strength of the relatively warm, high-salinity Irminger Current and the cold East Icelandic Current. The time period between 1200 and around 7–800 cal. (years) BP, including the Medieval Warm Period, was characterized by relatively high bottom and surface water temperatures due to the inflow of Atlantic water masses. After that, a general temperature decrease in the area marks the transition to a period with increased influence of the East Icelandic Current and, at the sea floor, the Norwegian Sea Deep Water. This corresponds to the transition to the Little Ice Age. After about 3–400 cal. BP, the inflow of cold East Icelandic Current was further enhanced. In particular, this had a strong influence on the surface waters, while the sea floor was under some influence of Atlantic water masses, resulting in stratification of the water masses. There is no clear indication of any warming in the area during the last decades.  相似文献   

19.
An objective classification of paleoclimatic proxies from 75 lake and peat records is used to re-evaluate data quality and inferred climatic patterns in Beringia during the Younger Dryas. Mapped data reveal coherent but spatially-complex regional patterns, suggesting the Younger Dryas was characterized by: (1) cooling in Southern Alaska, Eastern Siberia, and portions of Northeastern Siberia; and (2) uniform to warmer-than-present conditions through most of Central Alaska, Northeastern Siberia, and possibly the Russian Far East and Northern Alaska. The Beringian patterns correspond to distinctive large-scale climatic forcings, although in some locations further modified by more local influences, such as topography. General circulation models and modern synoptic climatology provide a conceptual framework for exploring possible mechanisms responsible for the observed changes. Forcings and associated climatic responses consistent with the proxy data include: (1) lowered sea-surface temperatures in the North Atlantic and Pacific reducing temperature and precipitation in Eastern Siberia; (2) intensified Aleutian low and lowered sea-surface temperatures causing cooler summers and higher winter precipitation in Southern Alaska; (3) stronger Pacific Subtropical High and an eastward shift of the East Asian Trough reducing summer temperatures in Southern Alaska and causing relative warmth in Northeastern Siberia; and (4) strong high pressure system producing warm, dry conditions in interior Alaska.  相似文献   

20.
黄翀  张强  陈晓宏  肖名忠 《水文》2017,37(5):12-20
利用模糊C-均值聚类算法、皮尔逊相关和滑动相关分析等方法,对珠江流域做了气候一致性分析,在此基础上,研究了珠江流域不同分区年降水和干湿季降水变化的时空特征,分析了区域干湿变化与厄尔尼诺-南方涛动(ENSO)、北大西洋涛动(NAO)、印度洋偶极子(IOD)和太平洋10年涛动(PDO)等主要气候因子的遥相关关系,探讨了珠江流域干湿变化的气候成因。在此基础上,进一步研究上述气候指标对不同时间尺度干湿变化影响的平稳性与差异性。除此之外,还研究了气候指标的冷暖期对基于6个月SPI值的珠江流域干湿状态的影响。研究表明:(1)IOD、NAO和ENSO分别是导致珠江流域年降水、湿季降水和干季降水发生变化的主要影响因素,且对当年及下一年降水的影响是相反的。(2)珠江流域不同时间尺度的降水与对其有显著影响的气候指标(年降水与IOD,湿季降水与NAO,干季降水与ENSO),两者之间不同时期的滑动相关往往具有较强的相关性和前后相关一致性。(3)各气候指标对珠江流域不同时间尺度降水的影响在空间分布上不太均匀。(4)不同位相下气候指标对珠江流域干湿状态的影响存在较大差异。总体而言,当处于各气候指标暖期时珠江流域出现湿润期的概率较冷期时更大且在空间分布上更均匀。  相似文献   

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