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61.
近500年来长江上游亚热带山地中低山植被的演替 总被引:2,自引:1,他引:1
从人口分布变迁入手探讨植被变迁,发现距今500年以前,长江上游亚热带山地海拔3000~1500m的中低山地区多是乔木林为主而间与草甸混交景观,而非现在的以草甸、灌草坡为主的景观。人类主要居住在海拔1500m以下的平坝、台地和丘陵地区,人类活动对海拔1500m以上的山地地区影响甚小。现代地理学界静态描述的长江上游地区海拔1500~3000m的中低山湿地草坡带,在500多年前仍多是以冷杉、云杉为主的针阔叶林与草甸混交景观。近500年来人类垦殖、商业砍伐、皇木采办等活动是造成这种变化的主要原因。皇木采办、商业采办影响了长江上游中低山的森林资源的品质,主要对高大的楠杉的砍伐,较明显地影响了森林的郁闭度,但并不给森林资源带来不可回归的影响。人类垦殖活动,特别是固定的农耕活动,大量砍挖树根,使森林资源的自我恢复失去了可能。气候变化的影响可能在其中,一时还难以区分,需继续关注。 相似文献
62.
南印度洋副热带偶极模(Subtropical Dipole Pattern,SDP)是印度洋存在的另一种很明显的偶极型海温差异现象,在年际和年代际尺度上均有十分明显的表现.而目前有关印度洋海气相互作用的研究主要集中在赤道印度洋地区,针对南印度洋地区的工作还比较少,特别是有关南印度洋海温与ENSO(El NiDo-Southern Oscillation)事件关系的研究.本文初步探讨了年际尺度上南印度洋副热带偶极型海温变化差异与ENSO事件的关系,发现SDP与ENSO事件有密切的联系,SDP事件就像连接正负ENSO位相转换的一个中间环节,SDP事件前后期ENSO的位相刚好完全相反.进一步,本文通过分析SDP事件前后期海温、高低层风、低层辐合辐散、高空云量和辐射等的变化特征研究了南印度洋偶极型海温异常在ENSO事件中的作用,结果表明:SDP在ENSO事件中的作用不仅涉及海气相互作用的正负反馈过程,还与热带和副热带大气环流之间的相互作用有关,特别是与东南印度洋海温变化所引起的异常纬向风由赤道印度洋向赤道太平洋传播的过程等有十分直接的关系;同时,SDP对ENSO事件的影响在很大程度上还依赖于大尺度平均气流随季节的变换. 相似文献
63.
鄂西南岩溶地区表层岩溶带发育强度变化规律研究 总被引:5,自引:5,他引:0
鄂西南岩溶地区表层岩溶个体形态复杂多样,表层岩溶带发育与鄂西南地区岩溶地质环境条件及岩溶发育演变的历史过程密切相关.因此,通过野外地质调查与收集"沪蓉西"高速公路勘察资料.采用系统工程地质分析为主的方法,研究了表层岩溶带在不同的地质构造部位、地层岩性、地形地貌部位及深部岩溶水补径排区的发育强度及其变化规律.详细论述了表层岩溶带的空间发育强度.首次定量界定了该区表层岩溶带的最大发育深度为56.54m,最浅为4.5m,阐明了表层岩溶带从山顶、山脊到山脚、沟谷由弱到强的总体变化规律. 相似文献
64.
YUAN LinWang YU ZhaoYuan XIE ZhiRen SONG ZhiYao & Lü GuoNian School of Geographical Science Nanjing Normal University Nanjing China 《中国科学D辑(英文版)》2009,(6)
Sea-level is closely linked to the Earth's climate and its change is important as a metric for global and regional climate change. Identifying, extracting, and revealing such information through detailed analysis is the prerequisite for understanding the mechanisms of sea-level change. The monthlyaverage tide-change records reported and examined in this paper are from 10 tide gauge stations distributed in the northwest margin of the Pacific Ocean, registered during the period 1965-2005. In particular, we ha... 相似文献
65.
Cropland cover change in Northeast China during the past 300 years 总被引:15,自引:0,他引:15
Land use/cover change induced by human activities has emerged as a “global” phenomenon with Earth system consequences. Northeast
China is an area where the largest land cultivation activities by migrants have happened in China during the past 300 years.
In this paper, methods including documentary data calibration and multi-sourced data conversion model are used to reconstruct
historical cropland cover change in Northeast China during the past 300 years. It is concluded that human beings have remarkably
changed the natural landscape of the region by land cultivation in the past 300 years. Cropland area has increased almost
exponentially during the past 300 years, especially during the past 100 years when the ratio of cropland cover changed from
10% to 20%. Until the middle of the 19th century, the agricultural area was still mainly restricted in Liaoning Province.
From the late 19th century to the early 20th century, dramatic changes took place when the northern boundary of cultivation
had extended to the middle of Heilongjiang Province. During the 20th century, three agricultural regions with high ratio of
cropland cover were formed after the two phases of spatial expansion of cropland area in 1900s–1930s and 1950s–1980s. Since
1930s–1940s, the expansion of new cultivated area have invaded the forest lands especially in Jilin and Heilongjiang Provinces.
Supported by National Natural Science Foundation of China (Grant No. 40571165) and Innovation Foundation of Chinese Academy
of Sciences (Grant No. KZCX2-YW-315) 相似文献
66.
Sea-level is closely linked to the Earth’s climate and its change is important as a metric for global and regional climate change. Identifying, extracting, and revealing such information through detailed analysis is the prerequisite for understanding the mechanisms of sea-level change. The monthly- average tide-change records reported and examined in this paper are from 10 tide gauge stations distributed in the northwest margin of the Pacific Ocean, registered during the period 1965-2005. In particular, we have utilized the Second Order Blind Identification (SOBI) method to identify and the Empirical Mode Decomposition (EMD) method to extract the El Niño-Southern Oscillation (ENSO) signals imprinted in those tide gauge records; and subsequently, we have investigated the spatial-temporal characteristics of the extracted ENSO signals using wavelet analysis methods. The following results are obtained: (1) the ENSO events recorded by each tide gauge series are of different types and intensity, which show considerable temporal-spatial variation characteristics, with sea-level responses to ENSO signals remarkably stronger in low latitude areas than in medium-high latitude areas; (2) due to the influences of ocean currents, topographical conditions, and other factors, there exist variations in the type of relationship between the sea-level changes and the recorded ENSO events at different latitudes; (3) sea-level changes can also denote scale-variation characteristics of ENSO events, and particularly, since 1980s, all the tide gauges located south of Kanmen show intense responses to ENSO and the timescale of ENSO events extended gradually from around 4 years to 2–8 years, reflecting variations in the intensity and frequency of ENSO events; and (4) reverse changes of certain scale were noted before and after strong ENSO events recorded by sea-level changes, supporting the research findings about the interaction (mutual coupling and superimposition) between the probability of occurrence of strong ENSO events and their time scale. 相似文献
67.
The Gray Fossil Site (GFS) includes a small (<2 ha) paleosinkhole lake fill with an exceptionally well-preserved record of
sedimentation and fossils from the latest Miocene to earliest Pliocene. The uppermost lacustrine stratigraphy is characterized
by rhythmites that regularly alternate between coarse-grained and organic-rich (A) laminae and fine-grained, silty clay (B)
laminae. Both the A and B components are almost exclusively comprised of exogenic sediment (including organic matter). Periodicities
of 24 and 4.4 are recorded within a continuous 96 interpreted year sequence of rhythmite sediment. In a small lake with a
poorly oxygenated bottom, the presence of laterally continuous laminated sediment that includes well-known periodicities in
rhythmite thickness is interpreted as representing annually generated varves that correspond to seasonal variations in sedimentation.
The distinctly larger fraction of medium sand-size quartz grains present within the A laminae, as well as the abrupt transitions
between A and B components suggest that the rhythmites represent deposition during alternating high-energy and lower-energy
seasons, which is consistent with a monsoonal precipitation pattern. The seasonal climate may relate to changes in the ocean
circulation pattern prior to 4.6 Ma that resulted in an increased temperature and atmospheric pressure gradient between the
east coast of North America and the Atlantic Ocean, but this climate phase seems to be only a temporary condition, as underlying
and overlying sediment are both consistent with drier conditions. The periodicity at 24 interpreted years is consistent with
the well-known Hale solar cycle. The 4.4 interpreted-year periodicity occurs within the ENSO frequency band, and if this documentation
of ENSO-like interannual climate change is correct, then it suggests that ENSO operated at times during the warm Earth conditions
characterizing the late Tertiary. 相似文献
68.
东北地区近百年降水时间序列变化规律的小波分析 总被引:30,自引:2,他引:28
利用1905~2005年东北地区哈尔滨、长春、沈阳和大连的降水时间序列资料,采用距平和Morlet小波分析方法,研究了东北地区降水变化的多时间尺度的周期性性变化规律,并对东北地区近期降水状况进行了预测。结果表明:近百年来东北地区年降水量呈现较显著下降趋势,整个东北地区降幅为-5.2mm/10a;长春为-12.7mm/10a;哈尔滨为-7.1mm/10a;大连为-2.7mm/10a;沈阳略为上升趋势为1.3mm/10a。东北地区的年降水量存在着区域性的多重时间尺度下的周期变化特征,2a~3a、5a~6a,10a和50a左右的长期振荡周期具有全域性;长春、哈尔滨年降水的主要控制周期是20a左右;5a~6a的短周期和50年的长周期变化也对年降水有较大影响。 相似文献
69.
70.
南海、孟加拉湾不对称环流变化对2009年西南特大秋旱的影响 总被引:5,自引:1,他引:4
利用NCEP再分析资料及我国160站降水资料,分析了2009年秋季东亚中、低纬环流特征和水汽输送特征及其对西南干旱的影响。同时讨论了秋季不同ENSO状态下东亚地区水汽输送差异,并与2009年进行比较。结果表明:孟加拉湾(简称孟湾)和南海之间环流形势在2009年秋季发生不对称变化,造成两地上空气压梯度减小,孟湾和南海上空分别出现一个反气旋式和气旋式距平环流中心,我国西南至中南半岛处于两距平环流中心之间偏北距平风控制之下,使得进入我国的西南气流异常减弱。水汽输送随之出现变化,南海南部季风低压水汽环流圈异常偏强,孟湾和南海水汽主体经中南半岛重回南海而未进入我国,最终造成我国西南降水异常偏少,出现干旱。这段时间内,西南地区上空出现异常下沉运动,对流活动受到抑制,加剧了干旱程度。在El Ni o年,我国西南及江南地区秋季水汽通量比La Ni a年明显增大,西北及华北则减少。2009年秋季我国的降水分布及南海一带水汽输送特征与普通El Ni o年特征不符,甚至出现相反状态,经对2009年秋季东亚El Ni o影响特征作简单模拟还原和分析,认为上述差异可能与El Ni o反气旋环流影响位置偏北有关。 相似文献