Late Pleistocene variations in Antarctic sea ice I: effect of orbital insolation changes

The effect of orbitally induced insolation changes on Antarctic sea-ice cover are examined by means of a dynamic-thermodynamic seaice model. Results are compared with modified CLIMAP 18 000 B.P. sea-ice reconstructions. Calculations suggest that changes in insolation receipt had only a minor influence on Pleistocene sea-ice distributions. The small response can be explained by a number of factors: albedo effects reduce the insolation perturbation at the surface; some of the shortwave radiation entering the ocean contributes to bottom ablation rather than lateral melting; the radiation perturbation at the upper surface of the ice must go to warming the surface to the melting point before melting ensues; and, finally, the relatively high heat capacity of open water dampens the surface temperature response to altered seasonal insolation perturbations.     

Late Pleistocene variations in Antarctic sea ice II: effect of interhemispheric deep-ocean heat exchange

Variations in production rates of warm North Atlantic Deep Water (NADW) have been proposed as a mechanism for linking climate fluctuations in the northern and southern hemispheres during the Pleistocene. We have tested this hypothesis by examining the sensitivity of a thermodynamic/dynamic model for Antarctic sea ice to changes in vertical ocean heat flux and comparing the simulations with modified CLIMAP sea-ice maps for 18 000 B.P. Results suggest that changes in NADW production rates, and the consequent changes in the vertical ocean heat flux in the Antarctic, can only account for about 20%–30% of the overall variance in Antarctic sea-ice extent. This conclusion has been validated against an independent geological data set involving a time series of sea-surface temperatures from the subantarctic. The latter comparison suggests that, although the overall influence of NADW is relatively minor, the linkage may be much more significant at the 41 000-year obliquity period. Despite some limitations in the models and geological data, we conclude that NADW variations may have played only a modest role in causing late Pleistocene climate change in the high latitudes of the southern hemisphere. Our conclusion is consistent with calculations by Manabe and Broccoli (1985) suggesting that atmospheric CO₂ changes may be more important for linking the two hemispheres.     

大气河对南极冰盖及海冰的影响

南极冰盖与海冰对全球气候具有重要影响。大气河作为高纬度地区经向水汽输送的重要途径,其对南极冰盖与海冰的影响在近年来愈发受到重视。南极大气河通常形成于高压脊(阻塞高压)与温带气旋之间的强向极经向输送带内。低频的大气河活动为南极带来强降雪,有利于冰盖质量增加。然而,强暖湿水汽侵入同时会导致表面融化、冰架崩解和极端高温,对冰盖质量存在潜在负贡献。大气河携带极端暖湿水汽与强风通过热力与动力过程导致海冰密集度下降。目前,大气河的识别算法仍不完善,其对液态降水的直接影响、与南大洋的相互作用等仍不清楚,需要进一步明晰大气河对南极冰盖与海冰的影响机制,以准确预估未来大气河对南极冰盖物质平衡与海冰变化的作用。     

Methanesulfonic acid and non-sea-salt sulfate in pacific air: Regional and seasonal variations

Concentrations of aerosol methanesulfonic acid (MSA) and non-sea-salt (nss) sulfate were measured at six island stations in the Pacific Ocean to investigate regional and seasonal patterns of organosulfur emissions and the origin of nss sulfate over the Pacific. The mean MSA concentrations, in g/m³, at the stations were: Shemya, 0.097±0.098; Midway, 0.029±0.021; Fanning, 0.044±0.012; American Samoa, 0.026±0.012; New Caledonia, 0.021±0.009; Norfolk, 0.024±0.019. The extremely high MSA levels found at Shemya indicate a major source of organosulfur emissions in the western North Pacific. Significant seasonal trends in MSA were observed, with higher MSA occurring during warm months. The amplitude of the seasonal variation was greatest at higher latitude stations. At Fanning and American Samoa, which have minimal input of continental material, there is a significant positive correlation between MSA and nss sulfate. MSA/nss sulfate ratios at other Pacific stations exhibit greater variability, which may be related to variations in: the input of continentally derived sulfate, the composition of oceanic organosulfur emissions, and atmospheric reaction pathways.     

Spatial and temporal variations of aridity indices in Iraq

In this paper, a nonhydrostatic Weather Research and Forecasting (WRF) model has been used to simulate the extreme precipitation event of 25 November 2009, over Jeddah, Saudi Arabia. The model is integrated in three nested (27, 9, and 3 km) domains with the initial and boundary forcing derived from the NCEP reanalysis datasets. As a control experiment, the model integrated for 48 h initiated at 0000 UTC on 24 November 2009. The simulated rainfall in the control experiment depicts in well agreement with Tropical Rainfall Measurement Mission rainfall estimates in terms of intensity as well as spatio-temporal distribution. Results indicate that a strong low-level (850 hPa) wind over Jeddah and surrounding regions enhanced the moisture and temperature gradient and created a conditionally unstable atmosphere that favored the development of the mesoscale system. The influences of topography and heat exchange process in the atmosphere were investigated on the development of extreme precipitation event; two sensitivity experiments are carried out: one without topography and another without exchange of surface heating to the atmosphere. The results depict that both surface heating and topography played crucial role in determining the spatial distribution and intensity of the extreme rainfall over Jeddah. The topography favored enhanced uplift motion that further strengthened the low-level jet and hence the rainfall over Jeddah and adjacent areas. On the other hand, the absence of surface heating considerably reduced the simulated rainfall by 30% as compared to the observations.     

Spatial and temporal variations of thunderstorm activity in the Northern Caucasus

Space distribution is considered of thunderstorm activity in Northern Caucasus during 1936–2006. Area distribution is presented of numbers of days with thunderstorm and of thunderstorm duration. Time variations are analyzed of mean numbers of the days with thunderstorms and of the thunderstorm mean duration, which probably represent a part of climate changes at the area under consideration. The largest variations of thunderstorm activity occur in the western part of the Northern Caucasus, where annual number of the days with thunderstorms reaches 70. Minimum changes in the thunderstorm characteristics occur in the eastern part of the area, with annual number of the days with thunderstorms equal to 25.     

Summer rain events in south-east Asia: Spatial and temporal variations

During the summer monsoon period in south-east Asia marine airstreams normally prevail at the south China coast. However, when tropical cyclones approach then polluted airmasses from south-eastern China can impart high ionic concentrations and high acidity to rainwater. This is illustrated by two examples and the small-scale horizontal variations in rainwater composition are minor during these episodes. Since long-term quality-assured studies of the composition of rainwater in south-east Asia are scarce, the results at three sampling sites in Hong Kong during summer monsoon periods are compared with previous data from the same season. The results for the mid- to end-1990's show a similar trend to those for the ambient concentrations of sulphate and nitrate in aerosol in Hong Kong, which show flattened trends from 1995 to 1999. A marked increase in acidity is found in the summer monsoon period of 2004 which is attributed to the greater proportion of approaching cyclone weather systems in the dataset, reflecting both the increased local emission sources and the burgeoning economic growth of the Pearl River Delta Region. Comparison of the rainwater composition with that at other south-east Asian cities during the summer monsoon period shows that it is most acidic in Hong Kong.     

北极海冰及其与气象要素年际关系的年代际变化

利用Hadley海冰密集度资料和NCEP/NCAR再分析资料,分析了北极海冰融冰量及其与大气变量年际关系的年代际变化。结果表明,北极海冰存在显著的年代际变化,且有较强的区域性。东西伯利亚海和波弗特海海冰融冰量的平均值变大且方差增大,格陵兰岛以东洋面海冰融冰量的量值和变率均在减弱。对3个不同气候时段内北极海冰融冰量进行EOF分解,前两个模态均在3个气候时段发生显著的年代际变化,东西伯利亚海海冰融冰量的增加与EOF第一模态年代际变化相关,而EOF第二模态则明显造成了波弗特海海冰的年代际消融。并且,与之相应的大气环流也出现了明显的年代际变化,它们与AO/NAO的年际关系也存在年代际转折,融冰量第二模态与AO的年际关系更为紧密,1960—1990年第二模态与AO的相关系数仅为0.186,而1980—2010年相关系数已升高至0.367。整个北冰洋的海冰融冰量与AO的年际关系也出现了年代际增强,尤其是东西伯利亚地区海冰融冰量与AO的年际关系发生了年代际增强,1980—2010年两者相关达到了0.4以上。而波弗特海融冰量与AO相关系数变化较大,1960—1990年其的相关系数高达-0.488,1980年后却减少至0.161。然而AO却未发生明显的年代际变化。造成北极海冰融冰量及其与大气变量年际关系发生年代际变化的主要因子之一是波弗特高压,其年代际减弱使得极区向东西伯利亚海和波弗特海的海冰输送减弱,导致这两个区域海冰减少,使得AO与北极海冰的年际关系发生了年代际转折。     

中国北方暴雪的年代际变化及其与大气环流和北极海冰的联系

采用相关、回归等统计方法,对中国北方暴雪的年代际变化特征及其与大气环流和北极海冰的联系进行探讨。结果表明:中国北方冬季暴雪发生频次较高区域主要位于东北,在空间分布上呈现由西北向东南增加的态势,且存在明显的年代际变化特征:1965—1980年为东北暴雪少发期; 2002—2011年为东北暴雪多发期。分析表明:暴雪少发期,输送至东北的水汽较少;暴雪多发期,更多的水汽输送来自于西北太平洋,同时偏北气流引导的极地冷空气与偏南风引导的太平洋暖湿空气在东北地区汇合,提供暴雪频发的动力条件,造成东北暴雪出现年代际增多。此外,研究发现:前期秋季北极海冰的年代际减少与东北暴雪的年代际增加存在很好的相关性;秋季北极海冰异常偏少导致的大气环流异常主要表现为纬向西风减弱和NAO负位相,由此导致大气经向活动增强,利于极地冷空气向南入侵,且冷空气与暖湿空气在东北地区汇合,这是东北暴雪年代际增加的重要原因之一。     

Light-absorbing particles in snow and ice: Measurement and modeling of climatic and hydrological impact

Light absorbing particles(LAP, e.g., black carbon, brown carbon, and dust) influence water and energy budgets of the atmosphere and snowpack in multiple ways. In addition to their effects associated with atmospheric heating by absorption of solar radiation and interactions with clouds, LAP in snow on land and ice can reduce the surface reflectance(a.k.a., surface darkening), which is likely to accelerate the snow aging process and further reduces snow albedo and increases the speed of snowpack melt. LAP in snow and ice(LAPSI) has been identified as one of major forcings affecting climate change, e.g.in the fourth and fifth assessment reports of IPCC. However, the uncertainty level in quantifying this effect remains very high. In this review paper, we document various technical methods of measuring LAPSI and review the progress made in measuring the LAPSI in Arctic, Tibetan Plateau and other mid-latitude regions. We also report the progress in modeling the mass concentrations, albedo reduction, radiative forcing, and climatic and hydrological impact of LAPSI at global and regional scales. Finally we identify some research needs for reducing the uncertainties in the impact of LAPSI on global and regional climate and the hydrological cycle.     

Spatial and temporal variations of blocking and cyclogenesis in the 1978 / 79 winter

Complex empirical orthogonal function (CEOF) and Fourier analyses are applied to 500 hPa geopotential height anomaly for two selected latitude belts in the Northern Hemisphere from Dec 1978 through Feb 1979 based on the ECMWF FGGE III-b data. The positive anomalies in the three leading CEOFs for the high-latitude belt mainly show the preferred locations for blocking activity in the North Atlantic, the North Pacific and to the west of the Ural Mountains. The negative anomalies in the three leading CEOFs for the mid-latitude belt mainly show the preferred locations for cyclogenesis in the east coasts of Asia and North America, and the Mediterranean; weak cyclogenesis is also seen in the western United States and off the coasts of Spain and Morocco. The travelling components of the positive anomalies in the high-latitude belt mainly propagate westward, weakening as approaching the east side of some mountain chains while intensifying to the west side. On the contrary, the travelling components of the negative anomalies in the mid-latitude belt mainly propagate eastward, intensifying over the lee side of mountain and / or approaching the east coasts of the two continents. These preferred locations for blocking and cyclogenesis are basically consistent with the climatological results, and related to some teleconnection patterns found earlier. The temporal variation of blocking highs seems to relate with the vacillation of the potential vorticity (PV) index defined by Weng (1992). There are two build-up stages of the PV index during the winter. Each build-up stage corresponds to a westward propagation of a large-scale positive anomaly in the high-latitude belt, resulting in the occurrence of a series of blocking highs over the western Eurasia, Scandinavia, Greenland and the Pacific. In general, the temporal variation of cyclogenesis is less reflected by the PV index than blocking highs. The duration of a PV index cycle of build-up and break-down is about 30-50 days. Within this low-frequency envelope, there is a global quasi-two-week vacillation of the PV index, reflecting one of the preferred time scales of mid-latitude cyclone and anticyclone activity in some preferred locations during the 1978 / 79 winter.     

中国东北地区中尺度对流复合体的时空分布特征

普查2005年5月1日至2008年9月30日FY-2C每隔30 min一次的红外云图,根据修正的MCC（Mesoscale Convective Complexes）标准,统计分析中国东北地区出现的MCC变化。结果表明：4 a共出现了22个MCC,其中19个出现在7-8月;东北地区MCC生成源地有明显的地域特点,MCC的生成与大兴安岭的地形密切相关,而辽宁和吉林东部极少生成MCC。中国东北地区MCC平均生命史为9.3 h,比美洲及中国南方地区都短,而≤-52℃和≤-32℃冷云罩面积明显大于以上两个区域;东北地区具有明显的夜发性特征,近85 %的MCC生成于午后到傍晚,15-24时是MCC对流最旺盛的时段。     

Interannual predictability of Arctic sea ice in a global climate model: regional contrasts and temporal evolution

The predictability of the Arctic sea ice is investigated at the interannual time scale using decadal experiments performed within the framework of the fifth phase of the Coupled Model Intercomparison Project with the CNRM-CM5.1 coupled atmosphere–ocean global climate model. The predictability of summer Arctic sea ice extent is found to be weak and not to exceed 2 years. In contrast, robust prognostic potential predictability (PPP) up to several years is found for winter sea ice extent and volume. This predictability is regionally contrasted. The marginal seas in the Atlantic sector and the central Arctic show the highest potential predictability, while the marginal seas in the Pacific sector are barely predictable. The PPP is shown to decrease drastically in the more recent period. Regarding sea ice extent, this decrease is explained by a strong reduction of its natural variability in the Greenland–Iceland–Norwegian Seas due to the quasi-disappearance of the marginal ice zone in the center of the Greenland Sea. In contrast, the decrease of predictability of sea ice volume arises from the combined effect of a reduction of its natural variability and an increase in its chaotic nature. The latter is attributed to a thinning of sea ice cover over the whole Arctic, making it more sensitive to atmospheric fluctuations. In contrast to the PPP assessment, the prediction skill as measured by the anomaly correlation coefficient is found to be mostly due to external forcing. Yet, in agreement with the PPP assessment, a weak added value of the initialization is found in the Atlantic sector. Nevertheless, the trend-independent component of this skill is not statistically significant beyond the forecast range of 3 months. These contrasted findings regarding potential predictability and prediction skill arising from the initialization suggest that substantial improvements can be made in order to enhance the prediction skill.     

近55 a中国西北地区夏季降水的时空演变特征

利用1961—2015年中国西北地区274个气象观测站点的日降水数据和再分析大气资料,采用EOF分析及累积距平等方法,研究了近55 a中国西北地区夏季降水的时空演变特征。结果表明：1）1961—2015年中国西北地区夏季降水的演变可分为三个时段,1961—1975年,该区域降水普遍偏少;1976—1996年,西北地区的东部降水偏多,西部降水偏少;1997—2015年,其东部降水偏少,而西部降水偏多。2）1976—1996年,西北地区东部降水偏多,是因为该地区夏季降水强度和降水频次明显增加,而西部降水偏少,则是该区域小雨与中雨的频次减少,降水强度偏弱造成的;1997—2015年,由于有效降水日数减少,降水强度偏弱等原因导致西北地区东部降水偏少,与此同时,西北地区的西部却因降水强度明显增强,持续降水日数和极端降水事件增加使得该区域降水呈现偏多的态势。3）降水区的转移,伴随着北半球对流层中层中纬度波列的演变,同时来自东欧与印度季风的水汽输送也对降水的异常起到了关键作用。     

1961年-2012年山西雾霾的时空变化特征及成因分析

利用1961年-2012年山西逐日天气现象、能见度、相对湿度和日平均气温资料,采用Kendall-tau方法和相关分析法研究山西雾霾日数的时空变化特征及成因。结果表明：雾多发区在中南部,北部雾日较少。霾、烟幕日数高值区出现在以大同、太原、临汾为中心线的带状区域。季节分布来看,轻雾、雾日数峰值出现在8、9月份,谷值在5月份出现;霾和烟幕日数的峰值出现在12、1月份,谷值在8、9月份出现。近50余年以来,山西雾霾日数呈现增多趋势,雾日增加趋势较弱,60、70年代为增多趋势,进入21世纪则为减少趋势;轻雾和霾日数均为显著单调上升趋势;烟幕日数也为显著增多趋势,但表现为抛物线型,90年代后期以前为增多,之后转为下降趋势。山西霾和烟幕日数与E1Nino事件有很好的对应关系,E1Nino事件发生年往往霾和烟幕日数较多,赤道中东太平洋的海温异常通过海气相互作用,引起东亚地区上空的大气环流异常,形成利于霾和烟幕出现的天气条件。山西冬季气温偏高往往导致霾和烟幕天气的增多,气候变暖对霾和烟幕天气的影响不容置疑。     

中国近50a积雪变化时空特征

利用1961～2012年中国1400个站点逐日积雪增量、积雪日数和气温稳定通过0℃日数资料,对我国积雪时空变化特征进行了分析研究。结果表明：我国积雪主要分布在新疆北部地区、东北和内蒙古东北部地区及青藏高原地区,年积雪增量均超过50era;在年代际变化中,1991～2000年我国大部分地区积雪增量偏少;在对我国5个区域的趋势分析中,新疆北部地区、东北和内蒙古东北部地区积雪量有显著增加趋势,积雪日数的变化趋势均不显著,气温稳定通过0oC日数均呈显著减少。     

Comments on the origin of dust in East Antarctica for present and ice age conditions

We have studied the distribution of 327 clay mineral particles retrieved from four Antaretic ice smaples corresponding to present and Last Glacial Maximum (LGM) climate conditions. Illite, chlorite, smectite and kaolinite were identified in all samples. Focusing on kaolinite, because of its use as a possible tracer of low latitude soils, we find a significantly smaller amount for LGM samples while the dust concentration in snow during the LGM was about 30 times higher than for present climate conditions. This can be interpreted as change in the contribution of the Australian source with climate.A second approach was based on the modeling of the desert dust cycle using an Atmospheric General Circulation Model (AGCM) under both present-day and ice age conditions. Unlike mineralogical results, the model suggests the prevalence of the Australian dust source in the deposits over East Antarctica under both present-day and LGM climate conditions. However the model fails to reproduce the strong increase in dust deposits during the LGM. This discrepancy could be partly due to the lack of a higher latitude dust source in the model.The stronger dust input recorded in ice cores for the LGM could be related to an additional active high latitude source (possibly close to South America) overlapping the atmospheric background coming from low latitude areas.