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21.
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
22.
THE RELATIONSHIP OF THE PRECEDING WINTER MJO ACTIVITIES AND THE SUMMER PRECIPITATION IN YANGTZE-HUAIHE RIVER BASIN OF CHINA 总被引:2,自引:0,他引:2
The first two series(RMM1 and RMM2) of RMM Index(all-Season Real-time Multivariate MJO Index) are computed to obtain the interannual variation of the preceding winter(preceding December to current February) MJO strength,according to which active(or inactive) years of preceding winter MJO are divided.By utilizing the data provided by NCEP/NCAR,CMAP and China’s 160 stations from 1979 to 2008,we studied the preceding winter MJO strength and discovered that the summer precipitation in the basin are of significantly negative correlation,i.e.when the preceding winter MJO is relatively active,the summer precipitation in the basin decreases,and vise verse.We also analyzed the causes.When the preceding winter MJO is relatively active,its release of potential heat facilities Inter-Tropical Convergence Zone(ITCZ) to strengthen and locate northward in winter and propagate northeastward.This abnormal situation lasts from winter to summer.In mid-May,ITCZ jumps northward to the South China Sea,the western Pacific subtropical high withdraws eastward,and the South China Sea summer monsoon sets off and strengthens.In summer,ITCZ propagates to South China Sea-subtropical western Pacific,the zonal circulation of subtropical Pacific strengthens,and a local meridional circulation of the South China Sea to the basin area forms,giving rise to the East Asia Pacific teleconnection wave-train.An East Asian monsoon trough and the Meiyu front show opposite features from south to north,the East Asian summer monsoon strengthens and advances northward.As a result,the summer monsoon is weakened as the basin is controlled by the subtropical high continually,with less rain in summer.On the contrary,when the preceding winter MJO is inactive,ITCZ weakens and is located southward,the subtropical high is located southward in summer,and the basin is in a region of ascending airflow with prevailing southwest wind.The East Asian monsoon trough and EASM weaken so that summer monsoon is reduced in the basin where precipitation increases. 相似文献
23.
BAO Qing 《大气和海洋科学快报》2012,5(1):43-48
Responses of the Asian Summer Monsoon(ASM) in future projections have been studied based on two core future projections of phase five of the Coupled Model Intercomparison Project(CMIP5) coordinated experiments with the IAP-coupled model FGOALS_s2(the Flexible Global Ocean-Atmosphere-Land System Model).The projected changes of the ASM in climatological mean and interannual variability were respectively reported.Both the South Asian Summer Monsoon(SASM) and the East Asian Summer Monsoon(EASM) were intensified in their climatology,featuring increased monsoon precipitation and an enhanced monsoon lower-level westerly jet flow.Accordingly,the amplitude of the annual cycle of rainfall over East Asia(EA) is enhanced,thereby indicating a more abrupt monsoon onset.After the EA monsoon onset,the EASM marched farther northward in the future scenarios than in the historical runs.In the interannual variability,the leading pattern of the EASM,defined by the first multi-variable EOF analysis over EA,explains more of the total variances in the warmest future scenario,specifically,Representative Concentration Pathway(RCP8.5).Also,the correlation coefficients analysis suggests that the relationship between the EASM interannual variations and ENSO was significantly strengthened in the future projections,which may indicate improved predictability of the EASM interannual variations. 相似文献
24.
Monsoon water cycle is the lifeline to over 60 per cent of the world’s population. Throughout history, the monsoon-related calamities of droughts and floods have determined the life pattern of people. The association of Green House Gases (GHGs) particularly Carbon dioxide (CO2) with monsoon has been greatly debated amongst the scientific community in the past. The effect of CO2 on the monsoon rainfall over the Indian–Indonesian region (8–30°N, 65°–100°E) is being investigated using satellite data. The correlation coefficient (Rxy) between CO2 and monsoon is analysed. The Rxy is not significantly positive over a greater part of the study region, except a few regions. The inter-annual anomalies of CO2 is identified for playing a secondary role to influencing monsoon while other phenomenon like ENSO might be exerting a much greater influence. 相似文献
25.
This paper attempts to analyze in detail the remote influence of the Indian Ocean Basin warming on the Northwest Pacific (NWP) during the year of decaying E1 Nifio. Observation data and the Fast Ocean- Atmosphere coupled Model 1.5 were used to investigate the triggering conditions under which the remote influence is formed between the positive sea surface temperature (SST) anomaly in the North Indian Ocean and the Anomalous Northwest Pacific anticyclone (ANWPA). Our research show that it is only when there is a contributory background wind field over the Indian Ocean, i,e., when the Indian Summer Monsoon (ISM) reaches its peak, that the warmer SST anomaly in the North Indian Ocean incites significant easterly wind anomalies in the lower atmosphere of the Indo-West tropical Pacific. This then produces the remote influence on the ANWPA. Therefore, the SST anomaly in the North Indian Ocean might interfere with the prediction of the East Asia Summer Monsoon in the year of decaying E1 Nifio. Both the sustaining effect of local negative SST anomalies in the NWP, and the remote effect of positive SST anomalies in the North Indian Ocean on the ANWPA, should be considered in further research. 相似文献
26.
Zr/Rb比值因不受后期成壤作用影响,在黄土高原黄土堆积中可作为较好的冬季风气候替代性指标。与黄土沉积相比,中国北方沙地的气候变化研究相对较弱,能够敏感反映沙地气候变化的替代性指标仍处于探索阶段。本文选取东亚季风控制区的毛乌素沙地、浑善达克沙地和科尔沁沙地11个典型的风成沉积序列,对Zr/Rb比值及其意义进行了研究。通过全岩样品Zr、Rb元素含量、Zr/Rb比值与平均粒径和>63 μm的粗颗粒百分含量对比分析,发现在毛乌素沙地,Zr/Rb比值与粗颗粒百分含量和平均粒径成正相关关系,与黄土高原的变化趋势相似,可作为冬季风强度的替代性指标。而在浑善达克沙地和科尔沁沙地,虽然Rb含量在砂质古土壤层中随着细颗粒物质的增加而增大,与毛乌素沙地的变化相同;但是,Zr含量也表现出增加趋势,导致Zr/Rb比值变化趋势与毛乌素沙地相反。其主要原因可能是沙地所处的构造地貌单元不同,物源区存在差异。因此,Zr/Rb比值作为冬季风的替代性指标在不同沙地应用需要慎重分析。 相似文献
27.
选择12个树木年轮宽度年表和28个气象站的降水量资料分别建立甘肃树轮宽度场和降水量场。树轮气候响应分析发现树轮宽度场第一主成分与甘肃28个气象站上年7月至当年6月降水量第一主成分有较好相关性,相关系数为0.580 (p<0.001)。利用线性回归模型重建甘肃上年7月至当年6月降水量第一主成分,方差解释量为33.6% (1961—2006年)。在1768—2006年期间甘肃有11个降水量较多的时期(高于多年平均值)和11个降水量较少的时期(低于多年平均值),分别反映了东亚夏季风较强和较弱的时期。同时,降水量重建序列的低值与西北地区极端干旱历史事件有着良好的一致性。空间相关分析表明该重建序列能够表征甘肃大范围的降水量变化,与实测降水量资料的第一主成分空间变化较为一致。多窗谱分析发现,甘肃降水量具有36.6年(95%)、11.2年 (95%)、4.8年(95%)、3.4年 (95%)、3.1年(95%)、2.6年(95%)和2.4年(99%)的周期变化。甘肃降水量与亚洲中纬度西风区、季风区树轮降水量序列之间存在一定的关联性,甘肃降水量变化与季风降水量的关联要大于与西风区降水量。 相似文献
28.
29.
Diatom communities are influenced by environmental perturbations, such as the monsoon system that impact the niche opportunities of species. To discern the influence of the monsoon system on diatom community structure, we sampled during two consecutive post-monsoons (2001 and 2002) and the intervening pre-monsoon at Mumbai and Jawaharlal Nehru ports along the central west coast of India. Characteristic temporal shifts in diatom community structure were observed across the sampling periods; these were mainly driven by temperature, salinity and dissolved oxygen saturation. The nutrient-poor pre-monsoon period supported low abundance yet high species richness and diversity of diatoms. Coscinodiscus, Cyclotella, Thalassiosira, Triceratium, Pleurosigma, Skeletonema and Surirella were the most dominant genera. Both the post-monsoon periods, following dissimilar monsoon events, were dominated by Skeletonema costatum, but differed in some of the residual species. Thalassiosira and Thalassionema spp. dominated mostly during post-monsoon I whereas Triceratium and Pleurosigma spp. dominated during post-monsoon II. To understand the underlying ecological mechanisms involved in such dynamics, we focus on the dominant diatom species in post-monsoon periods, S. costatum, that contributes up to 60% to total diatom cell numbers. This research is relevant in light of the fluctuating monsoon regimes over the Asian continent, the confounding effects of anthropogenic eutrophication and the resulting cascading effects on trophic web dynamics. 相似文献
30.
Spatio-temporal estimation of precipitation over a region is essential to the modeling of hydrologic processes for water resources management. The changes of magnitude and space–time heterogeneity of rainfall observations make space–time estimation of precipitation a challenging task. In this paper we propose a Box–Cox transformed hierarchical Bayesian multivariate spatio-temporal interpolation method for the skewed response variable. The proposed method is applied to estimate space–time monthly precipitation in the monsoon periods during 1974–2000, and 27-year monthly average precipitation data are obtained from 51 stations in Pakistan. The results of transformed hierarchical Bayesian multivariate spatio-temporal interpolation are compared to those of non-transformed hierarchical Bayesian interpolation by using cross-validation. The software developed by [11] is used for Bayesian non-stationary multivariate space–time interpolation. It is observed that the transformed hierarchical Bayesian method provides more accuracy than the non-transformed hierarchical Bayesian method. 相似文献