Geomagnetic evidence for fluid upwelling at the core-mantle boundary

Summary. Previous studies, both geomagnetic and seismic, have been unable to show conclusively whether or not there is fluid upwelling at the core-mantle boundary. Here a new method is developed, in which an attempt is made to invert geomagnetic secular variation data measured at the Earth's surface for a frozen-flux purely toroidal core-mantle boundary (CMB) velocity field, under the assumption that the mantle is electrically insulating and flux is frozen in at the CMB. These data have previously been inverted for the core-mantle boundary radial secular variation, from which the appropriate fit between model and data is known. Two different main field models were used to assess the effect of uncertainty in its radial component at the CMB. The conclusions were the same in both cases: frozen-flux purely toroidal motions provide a poor fit. A statistical test allows very firm rejection of the hypothesis that the residuals are not significantly larger, whereas there is no statistical difference between the residuals of inversions for radial secular variation and frozen-flux velocity fields at the CMB if upwelling and down-welling is included. The inherent non-uniqueness in the velocity field obtained is not of concern, since only their statistical properties are utilized and no physical significance is attached to the flows obtained.     

Surface wave tomography of the western United States from ambient seismic noise: Rayleigh and Love wave phase velocity maps

We present the results of Rayleigh wave and Love wave phase velocity tomography in the western United States using ambient seismic noise observed at over 250 broad-band stations from the EarthScope/USArray Transportable Array and regional networks. All available three-component time-series for the 12-month span between 2005 November 1 and 2006 October 31 have been cross-correlated to yield estimated empirical Rayleigh and Love wave Green's functions. The Love wave signals were observed with higher average signal-to-noise ratio (SNR) than Rayleigh wave signals and hence cannot be fully explained by the scattering of Rayleigh waves. Phase velocity dispersion curves for both Rayleigh and Love waves between 5 and 40 speriod were measured for each interstation path by applying frequency–time analysis. The average uncertainty and systematic bias of the measurements are estimated using a method based on analysing thousands of nearly linearly aligned station-triplets. We find that empirical Green's functions can be estimated accurately from the negative time derivative of the symmetric component ambient noise cross-correlation without explicit knowledge of the source distribution. The average traveltime uncertainty is less than 1 s at periods shorter than 24 s. We present Rayleigh and Love wave phase speed maps at periods of 8, 12, 16,and 20 s. The maps show clear correlations with major geological structures and qualitative agreement with previous results based on Rayleigh wave group speeds.     

Bootstrapped discrete scale invariance analysis of geomagnetic dipole intensity

The technique of bootstrapped discrete scale invariance allows multiple time-series of different observables to be normalized in terms of observed and predicted characteristic timescales. A case study is presented using the SINT2000 time-series of virtual axial dipole moment, which spans the past 2 Myr. It is shown that this sequence not only bears a clear signature of a preferred timescale of about 55.6 Ka, but additionally predicts similar features (of shorter and longer duration) that are actually observed on the timescales of historical secular variation and dipole reversals, respectively. In turn, the latter two empirical sources both predict the characteristic timescale found in the dipole intensity sequence. These communal scaling characteristics suggest that a single underlying process could be driving dynamo fluctuations across all three observed timescales, from years to millions of years.     

A geomagnetic secular variation study (31000–19 500 bp) in Western Canada

Summary. Palaeomagnetic results from 212 horizons spread evenly through an 18 m sedimentary sequence in southern British Columbia are reported. Radiocarbon ages suggest that the sequence spans the interval from 31 200 to 19 500 yr bp. No evidence for any large geomagnetic excursions (such as the so-called Mono Lake Excursion) is found, but a distinctive pattern of 'normal' secular variation is observed with declination and inclination swings of 45° and 25° peak to peak amplitude respectively. For the most part the secular variation consists of low amplitude oscillations about the field vector of a geocentric axial dipole expected at the site latitude, but three relatively large perturbations occur at approximately 4000 yr intervals. These perturbations systematically bias the overall mean to shallow inclinations and easterly declinations in a manner reminiscent of the spatially non-isotropic secular variation model proposed by Cox. The bias involved is about 6° in declination and 3° in inclination (overall mean D = 5.8°E, I = 64.2°, α95 = 0.9°, N = 212 horizons), which leads to a pole which is both 'far-sided' and 'right-handed'. If the horizons involved in the three major perturbations are eliminated the mean direction ( D = 1.2°E, I = 67.2°, α95 = 0.8°, N = 125 horizons) does not differ from that of a geocentric axial dipole despite the small cone of confidence.     

1960―2012年安徽省降水和气温变化特征及其与ENSO的相关性分析

使用安徽省1960―2012年气温与降水资料,采用线性倾向估计法、反距离加权插值法、最小二乘法和相关分析法分析安徽省近53年来气温和降水的时空变化特征及其与ENSO的关系。结果表明：1）近53年来安徽省年降水量和年平均气温分别以0.93 mm/a和0.02℃/a的倾向率呈增加趋势;季节变化表明,夏季和冬季降水量呈明显的增加趋势,而春、秋两季则呈减小趋势;四季气温均有所升高,春季气温增幅最大。2）不同季节降水量的年代际变化特征并不明显,降水主要集中在夏季,约占全年降水量的45.95%;与降水量年代际变化不同,年均温和四季气温的年代际变化呈波动上升趋势。3）降水存在显著的空间差异,夏、冬两季降水量由北向南减小幅度逐渐增大,春季降水量由北向南增加幅度逐渐增大;气温的空间变化并无一定规律,但宿州是四季以及全年增温幅度最大的地区。4）不同时间尺度的降水和气温均与Nino 3.4区海表温度距平和南方涛动均存在一定的相关性,其中3、9和11月的降水与Nino 3.4区海表温度距平以及南方涛动有较为显著的相关性,而在9月,气温受Nino 3.4区海表温度距平和南方涛动影响较显著;Nino 3.4区海表温度距平对年降水量和年均温的影响更明显。5）近53年来,El Nino事件和La Nina事件的出现频数分别为16和15次,La Nina事件对降水的增加的影响强于El Nino事件,而El Nino和La Nina事件对气温的影响均不显著。     

Deciphering chrysophyte responses to climate seasonality

Climate change involves alterations in seasonality as well as shifts in mean annual temperature. Cold temperate lakes show strong seasonality, with winter ice cover and alternating mixing and stratification periods during the ice-free season. These physical changes are ultimately related to seasonal weather variation and also drive annual phytoplankton succession. Therefore, phytoplankton remains in lake sediment records are potentially useful for reconstructing past seasonal climate signals. With the exception of investigations on varved sediments, however, little research has been carried out on the subject. Here we present two lines of evidence demonstrating that chrysophyte stomatocysts can be useful for inferring past climatic seasonality. First, we show that marked seasonal stomatocyst replacement is related to periods of the main physical processes in the lake. Second, using instrumental climate data and microfossils in sediment cores, we show that two main components of stomatocyst variability over the last ~150 years are related to seasonal fluctuations. The first of these components is related to stomatocysts present during summer stratification that respond to general warming trends, particularly in July and August. The second component relates to stomatocysts typically present in the spring and autumn mixing periods. Numbers of these two stomatocyst types vary inversely in response to variation in spring (April–May) temperatures. The number of stomatocyst types affected by spring temperatures is much greater than those related to summer temperatures. We provide evidence that chrysophyte stomatocysts are an excellent proxy for spring air temperature reconstructions, with little influence from summer or general annual trends. We also show that with relatively little effort, it is possible to categorise the chrysophyte stomatocyst assemblage of a lake to carry out detailed investigation of the record of seasonal changes preserved within the sediment.     

华北平原降水的长期趋势分析(英文)

The North China Plain (NCP) is the most important food grain producing area in China and has suffered from serious water shortages. To capture variation water availability, it is necessary to have an analysis of changing trends in precipitation. This study, based on daily precipitation data from 47 representative stations in NCP records passed the homogeneity test, analyzed the trend and amplitude of variation in monthly, seasonal and annual precipitation, annual maximum continuous no-rain days, annual rain days, rainfall intensity, and rainfall extremes from 1960 to 2007, using the MannKendall (M-K) test and Sen’s slope estimator. It was found that monthly precipitation in winter had a significant increasing trend in most parts, while monthly precipitation in July to September showed a decreasing trend in some parts of NCP. No significant changing trend was found for the annual, dry and wet season precipitation and rainfall extremes in the majority of NCP.A significant decreasing trend was detected for the maximum no-rain duration and annual rain days in the major part of NCP. It was concluded that the changing trend of precipitation in NCP had an apparent seasonal and regional pattern, i.e., precipitation showed an obvious increasing trend in winter, but a decreasing trend in the rainy season (July to September), and the changing trend was more apparent in the northern part than in the southern and middle parts. This implies that with global warming, seasonal variation of precipitation in NCP tends to decline with an increasing of precipitation in winter season, and a decreasing in rainy season, particularly in the sub-humid northern part.     

大面积开荒扰动下的三江平原近45年气候变化

黑龙江省三江平原近45年来，经过大规模的开荒，农田已取代原来的沼泽和沼泽化湿地，成为现今三江平原的主要景观类型，其下垫面发生了巨大变化，采用1955－1999年三江平原地区21个气象台站的气温，降水，日照时数和气压4个要素的资料，分析近45年来三江平原的气候变化和发展趋势：用Yamamoto法和Mann-Kendall法对5年滑动平均的区域季节和年时间序列进行突变检测，定量地认述了三江平原地区的气候变化事实，对于各气候要素出现在20世纪60年代，70年代和80年代的突变事实，尤其是20世纪70年代以来的阶梯式的持续变暖事实，认为三江平原区域气候系统内部的变化可能是引起这些突变的直接原因。     

Time series analysis of the 10 000 yr geomagnetic secular variation record from SE Australia

Summary. A 10 000 yr continuous secular variation record from intensively dated lake sediments in SE Australia has been subjected to periodogram and maximum entropy method analysis. Tests on synthetic data reveal some of the limitations of the latter method, particularly when applied to complex number series. Anticlockwise precession of the magnetic vector at a period of 5000 ± 1000 yr is tentatively ascribed to dipole precession, and clockwise precession at a period of about 2800 yr is probably due to westward drift of features of the non-dipole field.
The effect of calibrating the radiocarbon time-scale is important and results in periodicity shifts of up to 25 per cent. Even for well-dated lacustrine sequences power spectra are poorly constrained: it is thus possible that the geomagnetic secular variation on a time-scale of thousands of years is more uniform than often supposed. Mismatches between declination and inclination spectra can arise as a natural consequence of certain types of source mechanism and should not be simply attributed to figments of the analysis employed.     

Ambient noise Rayleigh wave tomography of New Zealand

We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.     

2007—2020年西藏草面温度时空分布特征

利用2007—2020年西藏38个气象站点平均草面温度（简称草温）、平均气温、平均地表温度、云量、降水量等观测资料,采用气候统计诊断方法分析了西藏草面温度的时空分异特征及其影响因素,以期科学研究当地草地生态系统和开展专业气象服务。结果表明：西藏年平均草温呈自东南向西北递减的分布。草温与海拔高度存在显著的负相关,海拔高度每升高100 m,季平均草温降低0.44~0.70 ℃,年平均草温降低0.58 ℃;与纬度有着显著的曲线关系,29.3°N以南（北）地区,随着纬度增加,草温随之升高（降低）。各站草温呈一峰一谷的日变化特征,日最低值出现在07:00—08:00（北京时间）,日最高值均出现在14:00;草温月平均最低值都出现在1月,月平均最高值出现在6月或7月;76%的站点草温的变化为夏季>春季>秋季>冬季的气候特征。西藏草温年较差为21.4 ℃,较气温年较差偏大3.1 ℃;草温日较差达35.7 ℃,远高于气温日较差,偏大21.6 ℃。草温与气温之差以夏季最大,其次是春季、冬季两者比较接近;草温与地表温度之差以春季最大,夏季次之,冬季最小。在空间分布上,月平均草温与气温、地表温度均呈显著的正相关,与平均风速、积雪日呈显著的负相关;积雪深度对草温的影响,除冬季外二者存在显著的负相关;大部分月份平均草温与总云量、低云量、降水量的关系不显著。86.8%的站点5—9月平均逐小时草温与降水量存在显著的负相关关系。     

Long-term features of drifting and standing non-dipole fields as determined from Holocene palaeomagnetic secular variations

Geomagnetic field motions of Holocene secular variations are investigated using a separation method. The palaeomagnetic secular variations from Britain, North America and Australia have been subjected to maximum-entropy method analyses. Based on the results of spectral analyses, the secular variations are separated by band-pass filters into low-frequency components, generally including the period band 1800-3600 yr, and high-frequency components, generally including the period band 1000-1200 yr. There is an interval, from 4200 to 1700 yr BP, which shows clockwise rotational motions in the low-frequency components of all three sites. Westward drifting of geomagnetic fields may be globally dominant. Swinging or elliptical looping motions constrained to a certain direction were observed in the low-frequency components of the British data. The time duration for the persistence of the swinging motion constrained to a certain direction was 3500 years or so, which could be the lifetime of an oscillating stationary field. The duration of the transitional motion was 1000-1300 years, which may indicate the recurrence time of a stationary field.     

基于SWAT-MODFLOW耦合模型的巴音河中游泽令沟盆地地表水−地下水转换关系研究

为准确刻画资料缺乏、地表水–地下水转换频繁的泽令沟盆地地表水?地下水不同时空尺度上的转换关系,利用河道径流数据、遥感蒸散发数据、地下水位观测数据等对SWAT-MODFLOW耦合模型进行率定和验证,在此基础上对泽令沟盆地水循环过程进行模拟分析。结果显示：① SWAT-MODFLOW耦合模型模拟效果较好,率定期和验证期月径流量的R²≥0.73,NSE≥0.67,PBIAS在?20%~20%。其各子流域实际蒸散发的R²均大于0.91,NSE均大于0.85,PBIAS在?10%~10%。此外,模拟地下水位与实测值误差在0.6 m以内,R²为0.94。② 地表水补给地下水的河道占整个河道长度90.31%,年平均补给量占年总交换水量的50.20%;季节尺度上,补给的最大值出现在7月,为331043.31 m³/d,最小值出现在12月份,为41208.33 m³/d。地下水对地表水的补给量较为稳定,季节性变幅在?2.5%~2.5%,年际变幅处于?6.5%~0.6%之间。     

中国内陆热带地区近40年气候变化特征

用西双版纳6个气象站40余年观测资料,探讨中国内陆热带地区气候变化特征及趋势。结果表明:西双版纳地区的平均气温、平均最低气温、平均最高气温总体上呈逐年增暖的趋势,其中平均气温上升率0.016 5～0.033 4℃/a,平均最低气温上升率0.008 6～0.038 7℃/a,平均最高气温上升率-0.001 4～0.018 6℃/a;降水长期变化特征则较复杂,规律不如气温明显,但总体趋势减少,年降水量变化主要决定于雨季降水量变化;相对湿度呈现逐年降低趋势。说明该地区气候向干热型转变。     

不同时间尺度下降水中氧稳定同位素的时空变化

The spatial and temporal variations of stable oxygen isotope in precipitation on different time scales are analyzed according to the data from the IAEA/WMO stations with long survey series in the Northern Hemisphere. Temperature effect is mainly distributed in mid-high latitudes on seasonal scale except for Bamako and Addisababa stations. The δ18O/temperature slope displays the positive correlation against altitude for most of the statistical stations. Amount effect appears primarily in the region south of 30°N and coastal areas. The δ18O/precipitation slope is indirectly proportional to precipitation amount. For some of the sampling stations at mid-high latitudes where their seasonal distribution of precipitation is contrary to that of temperature, coupled with temperature effect, the amount effect appears synchronistically. Either the temperature effect or the amount effect on seasonal scale, there are positive correlations to a certain extent between the annual weighted mean δ18O and the annual mean temperature for almost all the stations. The correlation between composite δ18O and temperature on spatial scale is much more marked, compared with that of individual station. There is a good agreement between 10-year moving average temperature curves I and Ⅱ, with the values of the former all markedly smaller than corresponding ones of the latter, calculated by the monthly mean series group I and the annual mean series group Ⅱ, respectively. However, two calculated dδ18O/dT curves display the distinct difference: the variation amplitude of slope series Ⅱ is larger than that of slope series I. Both curves had similar ascending trend from the 1960s to the 1970s, and then, their variations display the anti-phase. Moreover, the analyses show that there is negative correlation between slope series Ⅱ and temperature series Ⅱ. However, the status is different for slope series I and temperature series I. Both series have contrary trend from the 1960s to the 1970s, whereas the same trend since the 1980s.     

中国年和季节极端降水时空特征 及极值分布函数上尾部性质

基于热带气旋时间、路径、强度数据和中国728个气象站点1951~2014年日降水数据,分析了年和季节极端降水广义极值（GEV）分布函数特征及受热带气旋的影响。通过检查年和各季节极端降水的非一致性,发现具有变异点或显著时间趋势的站点占总站点数的比例较低。仅考虑满足一致性的站点,年和各季节极端降水GEV分布上尾部在全国大部分区域表现出厚尾特征,且不具有上边界。总体来看北方厚尾特征重于南方,秋季和冬季明显高于年和夏季。年极端降水厚尾特征受到不同季节极端降水机制的混合影响。而且,热带气旋对中国沿海区域极端降水有重要影响,往往引发大量级极端降水。东南沿海地区最大10场极端降水由热带气旋引发的比例达到60%以上。因此热带气旋趋向于增加沿海区域年极端降水GEV分布形状参数的大小,并控制着曲线上尾部的形状。     

Little Ice Age recorded in summer temperature reconstruction from vared sediments of Donard Lake, Baffin Island, Canada

Clastic varved sediments from Donard Lake, in the Cape Dyer region of Baffin Island, provide a 1250 yr record of decadal-to-centennial scale climate variability. Donard Lake experiences strong seasonal fluctuations in runoff and sediment fluxes due to the summer melting of the Caribou Glacier, which presently dominates its catchment. The seasonal variation in sediment supply results in the annual deposition of laminae couplets. A radiocarbon date measured on moss fragments, with a calibrated age of 860 ± 80 yrs before present (BP), is in close agreement with the age based on paired-layer counts. Together with the fabric of the laminae determined from microscope analysis, the age agreement demonstrates that the laminae couplets are annually deposited varves. Comparisons of varve thickness and average summer temperature from nearby Cape Dyer show a significant positive correlation (r= 0.57 for annual records, r = 0.82 for 3-yr averages), indicating that varve thickness reflects changes in average summer temperature. Varve thickness was used to reconstruct average summer temperatures for the past 1250 yrs, and shows abrupt shifts and large amplitude decadal-to-centennial scale variability throughout the record. The most prominent feature of the record is a period of elevated summer temperatures from 1200-1375 AD, followed by cooler conditions from 1375-1820 AD, coincident with the Little Ice Age.     

Annual and seasonal variation of NDVI explained by current and previous precipitation across Northern Patagonia

Temporal variation of aboveground net primary production (ANPP) of arid ecosystems has been associated with precipitation regimes with different results. The objective of this paper was to characterize the relationship between interannual variation of annual and seasonal Normalized Difference Vegetation Index (NDVI), as a surrogate for ANPP, and precipitation in the steppes of Northern Patagonia. In 11 sites encompassing a wide range of conditions and vegetation physiognomies, we studied a 20-year monthly data set of NDVI and precipitation. We took into account the precipitation of current, as well as previous periods of variable length. Interannual variation of annual NDVI was little correlated with annual precipitation, either current or previous. In contrast, it was highly and widely correlated with precipitation accumulated during a few months of the previous growing season. Interannual variation of seasonal NDVI was little correlated with current seasonal precipitation. In contrast, it was significantly correlated with precipitation accumulated during previous periods of variable length according to the season and site under consideration. NDVI was more tightly coupled with precipitation in drier ecosystems. Lags of response between NDVI and precipitation provide an opportunity for forecasting ANPP and suggest even longer lags between climatic variation and herbivore performance.     

罗斯海和普里兹湾海域海冰范围变化对比分析

海冰范围的变化对气候变化、生态系统以及人类活动都会产生重大的影响,近年来极地海冰范围的变化受到广泛关注。对南极罗斯海与普里兹湾海域海冰范围进行时间序列分析,研究发现海冰范围季节性变化在罗斯海与普里兹湾海域差异较大,罗斯海地区表现出"快速缩小、迅速扩大"的特性,普里兹湾海域表现出"快速缩小、缓慢扩大"的特性。两地区的海冰范围在年际变化上都表现出扩大的趋势,2003—2014年罗斯海地区海冰变化趋势为(1.39±1.12)×104km2·a~(-1),普里兹湾海域海冰变化趋势为(0.61±0.26)×104km2·a~(-1)。罗斯海地区夏季的年际变化为减少趋势。     

极隙区Pc3频段脉动的传播特性

南极中山站和戴维斯站 (不变磁纬 74.5°S)白天中午位于磁层极隙区 ,夜间位于极盖区或极光带的极向边沿。两站均安装了完全相同的感应式磁力计。选择两站 1 997年 3月和 1 996年 6、9、1 2月的数据 ,运用快速傅立叶变换和波形检查方法选择 Pc3频段脉动事件 ,然后用信号互谱技术进行统计分析 ,结果如下 :在中山站 -戴维斯站 ,Pc3频段脉动主要出现于白天中午 /磁中午和磁午夜附近 ;白天 ,Pc3频段脉动振幅、出现率和出现的时间范围均有一定的季节变化 ,冬季最小 ,但在夜间 ,Pc3频段脉动没有这种变化 ;夜间 ,Pc3频段脉动振幅比在白天大许多 ;Pc3频段脉动传播方向 ,白天主要向西 ,夜间不太规则。这些可能反映了电离层电导率和日侧电离层电流系统对 Pc3频段脉动的影响。