Late Holocene monsoon climate as evidenced by proxy records from a lacustrine sediment sequence in western Guangdong,South China

The study of a 300-cm-thick exposed lacustrine sediment section in the Hedong village in Zhaoqing area which is located in sub-tropical west Guangdong Province in South China, demonstrates that the lacustrine sedimentary sequence possibly contains evidence for exploring variation of Asian monsoon climate. Multi-proxy records, including the humification intensity, total organic carbon, and grain size fractions, reveal a general trend towards dry and cold conditions in the late Holocene that this is because of a decrease in solar insolation on an orbital scale. Three intensified Asian summer monsoon (ASM) intervals (∼3300–3000 cal yr BP, ∼2600–1600 cal yr BP, and ∼900–600 cal yr BP), and three weakened ASM intervals (∼4000–3300 cal yr BP, ∼3000–2600 cal yr BP, and ∼1600–900 cal yr BP) are identified. Our humification record (HD_cal) shows a good correlation on multi-centennial scale with the tree ring Δ¹⁴C record, a proxy of solar activity. A spectral analysis of HD_cal reveals four significant cycles, i.e., ∼1250 yr, 300 yr, 110 yr, and 70 yr, and most of these cycles are related to the solar activity. Our findings indicate that solar output and oceanic–atmospheric circulation probably have influenced the late Holocene climate variability in the study region.     

Bayesian modelling the retreat of the Irish Sea Ice Stream

We present an 8000‐year history spanning 650 km of ice margin retreat for the largest marine‐terminating ice stream draining the former British–Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0–25.3 ka, accelerating into the Celtic Sea to reach maximum limits 25.3–24.5 ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea‐level rise, mega‐tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (～550 m a^?1) that slowed (～100 m a^?1) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (～200 m a^?1) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer‐term (>1000 a) patterns and rates of ice‐marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present‐day ice streams. Copyright © 2013 John Wiley & Sons, Ltd.     

ERA-Interim和CMFD气象驱动数据在新疆额尔齐斯河流域的适用性评价

气象驱动数据质量是影响流域水文过程模拟精度的一个重要因素。基于新疆额尔齐斯河流域及周边区域8个气象站记录的数据,对ERA-Interim再分析资料和中国区域地面气象要素驱动数据集（CMFD）在流域的适用性进行了评价,并对比了ERA-Interim和CMFD气象要素年均值在流域的空间分布。结果表明：ERA-Interim和CMFD记录气温、相对湿度、向下短波辐射和向下长波辐射数据与观测数据具有较高的一致性,但降水和风速数据与观测数据的一致性比较差。小时尺度上ERA-Interim记录的气温、相对湿度、降水量、向下短波辐射准确度略高于CMFD数据,而日尺度上CMFD记录的所有气象要素的准确度均高于ERA-Interim数据,结合Noah-MP模型的模拟结果,认为CMFD数据在新疆额尔齐斯河流域的适用性整体优于ERA-Interim数据。从两种驱动数据获取的流域气象要素空间分布来看,ERA-Interim和CMFD获取的年平均气温、风速、相对湿度、降水量、向下长波辐射在流域空间具有高度一致性,但向下短波辐射空间分布差别较大。     

Generating non-stationary random fields of auto-correlated,normally distributed CPT profile by matrix decomposition method

This paper emphasises the true realisation of Cone Penetration Test (CPT) profiles considering non-stationary nature of the data. Formulation of stationary random field theory has been modified and adapted to non-stationary state in order to take into account the mean and variance variability for soil properties. Multi-variance correlation matrix along with the Cholesky decomposition technique was employed to produce realisations of non-homogenous and non-stationary random fields of CPT profiles. A piecewise and segmental data realisation according to the lithology and site class specifications acquired directly from CPT data is adopted in this study so as to render an accurate data simulation. For validation of proposed method 8 CPT test profiles collected from Urmia Lake site have been introduced and simulated by the stationary and non-stationary algorithms. The mean correlation coefficient between the actual CPT data profiles and related realisations along with some other important statistical parameters and their coefficients of variation strongly demonstrate that non-stationary random field generation technique gives quite better accuracy, by comparison to the conventional stationary random field generation scheme.     

Shortwave cloud radiative forcing on major stratus cloud regions in AMIP-type simulations of CMIP3 and CMIP5 models

Cloud and its radiative effects are major sources of uncertainty that lead to simulation discrepancies in climate models. In this study, shortwave cloud radiative forcing (SWCF) over major stratus regions is evaluated for Atmospheric Models Intercomparison Project (AMIP)-type simulations of models involved in the third and fifth phases of the Coupled Models Intercomparison Project (CMIP3 and CMIP5). Over stratus regions, large deviations in both climatological mean and seasonal cycle of SWCF are found among the models. An ambient field sorted by dynamic (vertical motion) and thermodynamic (inversion strength or stability) regimes is constructed and used to measure the response of SWCF to large-scale controls. In marine boundary layer regions, despite both CMIP3 and CMIP5 models being able to capture well the center and range of occurrence frequency for the ambient field, most of the models fail to simulate the dependence of SWCF on boundary layer inversion and the insensitivity of SWCF to vertical motion. For eastern China, there are large differences even in the simulated ambient fields. Moreover, almost no model can reproduce intense SWCF in rising motion and high stability regimes. It is also found that models with a finer grid resolution have no evident superiority than their lower resolution versions. The uncertainties relating to SWCF in state-of-the-art models may limit their performance in IPCC experiments.     

ANALYSES OF THE DYNAMIC EFFECTS ON WINTER CIRCULATION OF THE TWO MAIN MOUNTAINS IN THE NORTHERN HEMISPHERE——Ⅱ.VERTICAL PROPAGATION OF PLANETARY WAVES

A linear,hemispheric and stationary spectral model with multilayers in the vertical was employed to simulate thevertical propagation of waves triggered by mountains.Results show that,in cooperation with the East Asia zonal meanflow,Tibetan Plateau can excite a strong wavenumber 1 perturbation in the stratosphere with its ridge and trough lo-cated over the Pacific and Atlantic Oceans respectively.On the other hand,the stratospheric wavenumber 1 perturbationcaused by the mechanical forcing of the Rocky Mountains in cooperation with the North America zonal mean flow isvery weak.Calculations from observational data of the vertical profile of critical wavenumber for vertically propagatingwaves imply that the tropospheric wavenumber 1 perturbation can hardly penetrate the North America tropopause up-wards,whereas it can freely propagate through the East Asia tropopause into the stratosphere.Two-dimensional E-Pcross-sections obtained from both observational data and simulated results also demonstrate that waves excited by theRocky Mountains are refracted towards low latitudes in the troposphere during their upward propagation:whereas,inaddition to the above mentioned equatorward leaning branch,the wavenumber 1 and 2 planetary waves excited by theTibetan Plateau possess another branch which is refracted to high latitudes during upward propagation and penetratesthe tropopause into the stratosphere.It is therefore concluded that the difference in the horizontal and vertical wavepropagations in the two hemispheres is a result of the different dynamical forcing induced by the two main mountains inthe Northern Hemisphere.     

Climatological distribution of lightning density observed by satellites in China and its circumjacent regions

The 0.5°×0.5°grid resolution distribution of lightning density in China and its circumjacent regions have been analyzed by using the satellite-borne OTD (Apr 1995-Mar 2000) and LIS (Dec 1997-Mar 2003) databases. It is shown that: (i) Firstly, the variability of the lightning density (LD) is particularly pronounced over the different subareas, 9 times greater over the south than the north side of Himalayas Mountains, 2.5 times greater over the eastern than the western area of China. While the maximum and minimum LD are respectively 31.4fl/km2/a (in Guangzhou region) and less than 0.2fl/km2/a (in the desert of western China). Secondly, the LD of China's continent regularly varies with latitude and distance off coast, which is consistent with annual mean precipitation in varying trend. In conclusion, the Qinghai-Tibet Plateau, the China's three-step staircase topography and the latitude are three important factors affecting macro-scale characteristics of the LD distribution, (ii) The regional differences     

The effect of transient eddy on interannual meridional displacement of summer East Asian subtropical jet

Using ERA-40 reanalysis daily data for the period 1958-2002,this study investigated the effect of transient eddy(TE) on the interannual meridional displacement of summer East Asian subtropical jet(EASJ) by conducting a detailed dynamical diagnosis.The summer EASJ axis features a significant interannual coherent meridional displacement.Associated with such a meridional displacement,the TE vorticity forcing anomalies are characterized by a meridional dipole pattern asymmetric about the climatological EASJ axis.The TE vorticity forcing anomalies yield barotropic zonal wind tendencies with a phase meridionally leading the zonal wind anomalies,suggesting that they act to reinforce further meridional displacement of the EASJ and favor a positive feedback in the TE and time-mean flow interaction.However,The TE thermal forcing anomalies induce baroclinic zonal wind tendencies that reduce the vertical shear of zonal wind and atmospheric baroclinicity and eventually suppress the TE activity,favoring a negative feedback in the TE and time-mean flow interaction.Although the two types of TE forcing tend to have opposite feedback roles,the TE vorticity forcing appears to be dominant in the TE effect on the time-mean flow.