The Seasonal Climate and Low Frequency Oscillation in the Simulated Mid-Holocene Megathermal Climate

l.IntroductionThegoalofPMIP(PaleoclimateModellingIntercomparisonProject)istocomparethestate-ofthe-artclimatemodels'simulationsforthepastclimateonthebasisofthepaleoclimatedata.Oneofthemostinterestingperiodsisthemid-Holoceneduring8.5-3.OkaBPwiththemaximumin7.2-6.OkaBP(beforepresent)inChina(Shietal.,l992).Itwasre-vealedthatthetemPeraturewasl-4'Chigherandtheprecipitationwas4O%-loO%largercomparedtothepresentoverChina(Shietal.,l992;Anetal..l99l;Kongetal.,l99O,l99l).Therehavebeensomesimula…     

中全新世时期中国地区水循环因子变化的模拟研究

利用全球气候模式CAEM3嵌套区域模式MM5模拟了现代和中全新世时的气候,从模拟结果可以发现中全新世有效降水变化中心随季节变化,最大的有效降水增加出现在夏季东北地区和内蒙古东部,最大值超过3 mm/d;同时,黄河与长江之间区域降水减少,最大变化超过2 mm/d.中国北方地区云量增加,同时,中国东部的长江流域云量减少.高云量变化较小,低云量变化最大,最大变化超过2成.夏季,对应着黄河与长江之间区域的云量减少,这个区域的温度升高最大.从水汽的变化可以看到长江流域地区水汽减少,相对湿度也减少,这与云量的变化一致;华南地区水汽的变化与季节有关;东北地区水汽增加,相对湿度增大,对应云量的增加和降水增多.从结果可以发现相对湿度最大的变化超过15%,不是一个常数.有些地区温度升高,但是水汽却减少.但是,在LGM的温度降低的区域,水汽一致减少.这说明温度降低水汽对应减少,但温度升高不一定对应水汽增加.这与全球尺度水汽相对湿度基本保持常数的结果不同.中全新世时,长江流域除春季外变得干燥、少雨和高温,东北和内蒙古东部变得多雨和潮湿.     

北半球大气环流能量循环的气候特征

利用1958—2011年NCEP/NCAR逐日再分析资料,根据Lorenz能量循环理论框架,分析了北半球大气能量循环的年变化特征,在此基础上给出了更具普适性的多年平均的大气能量循环框图.结果表明:北半球大气能量循环的年变化特征十分明显.大气能量及能量转换率均表现为冬季高、夏季低、春秋季过渡的演变特征;纬向平均有效位能、纬向平均动能和涡动动能中有少许能量在冬季时由南半球向北半球进行越赤道输送,夏季时则由北半球向南半球输送,而涡动有效位能的输送方向则与此相反;纬向平均有效位能的制造在秋季最大,涡动有效位能的制造在夏季最大;动能的耗散冬季最强,夏季最弱.就年平均而言,相较于能量转化过程,能量越赤道交换过程非常微弱.在经向上,纬向平均有效位能主要分布于高纬地区,纬向平均动能主要分布于中低纬地区,而涡动能量主要贮存在中纬和高纬地区;此外,能量转化过程一般在中纬度地区较活跃.     

Influence of the temperate and boreal forests on the Northern Hemisphere climate in the Météo-France climate model

A temperate and boreal deforestation experiment has been performed at Météo-France using the ARPEGE climate model. A first simulation was performed as a control with a present-day vegetation map, and another one with all forests north of 45 °N replaced by meadows. Prescribed monthly mean climatological SSTs were used in both integrations. The ARPEGE climate model includes a physically based land surface scheme, which has been tested both on snowfree and snow-covered sites, and has a relatively high horizontal resolution. Results of the 4-year integrations suggest that forests exert a strong influence on the surface climate of the temperate and boreal regions. Deforestation induces a significant cooling which modifies the atmospheric circulation simulated in the high latitudes, and also in the tropics. The most important impact is observed during the melting season which is delayed by the forest removal. This result is consistent with preliminary stand-alone experiments showing that the atmospheric boundary layer can be heated by the forest, even if the ground is covered by snow. The study confirms that vegetation feedbacks should be included when performing future climate studies such as doubled CO₂ experiments, eventhough many uncertainties still remain with regard to other physical aspects of the climate models. Received: 5 September 1995 / Accepted: 12 August 1996     

Understanding the impact of climate change on Northern Hemisphere extra-tropical cyclones

Extra-tropical cyclones strongly influence weather and climate in mid-latitudes and any future changes may have large impacts on the local scale. In this study Northern Hemisphere storms are analysed in ensembles of time-slice experiments carried out with an atmosphere only model with present day and future anthropogenic emissions. The present day experiment is forced by observed sea-surface temperature and sea-ice. The sea-surface temperatures and sea-ice for the future experiment are derived by adding anomalies, from parallel but lower resolution coupled model experiments, to the observed data. The storms in the present day simulation compare fairly well with observations in all seasons but some errors remain. In the future simulations there is some evidence of a poleward shift in the storm tracks in some seasons and regions. There are fewer cyclones in the Northern Hemisphere in winter and spring. The northeast end of the North Atlantic storm track is shifted south in winter giving more storms and increased frequency of strong winds over the British Isles. This shift is related to an increase in baroclinicity and a southward shift of the jet that occurs as a response to a minimum in ocean warming in the central North Atlantic. An increase in the frequency of storms over the UK is likely to cause enhanced levels of wind and flood damage. These results concur with those from some other models, however, large uncertainties remain.     

Representation of Northern Hemisphere winter storm tracks in climate models

Northern Hemisphere winter storm tracks are a key element of the winter weather and climate at mid-latitudes. Before projections of climate change are made for these regions, it is necessary to be sure that climate models are able to reproduce the main features of observed storm tracks. The simulated storm tracks are assessed for a variety of Hadley Centre models and are shown to be well modelled on the whole. The atmosphere-only model with the semi-Lagrangian dynamical core produces generally more realistic storm tracks than the model with the Eulerian dynamical core, provided the horizontal resolution is high enough. The two models respond in different ways to changes in horizontal resolution: the model with the semi-Lagrangian dynamical core has much reduced frequency and strength of cyclonic features at lower resolution due to reduced transient eddy kinetic energy. The model with Eulerian dynamical core displays much smaller changes in frequency and strength of features with changes in horizontal resolution, but the location of the storm tracks as well as secondary development are sensitive to resolution. Coupling the atmosphere-only model (with semi-Lagrangian dynamical core) to an ocean model seems to affect the storm tracks largely via errors in the tropical representation. For instance a cold SST bias in the Pacific and a lack of ENSO variability lead to large changes in the Pacific storm track. Extratropical SST biases appear to have a more localised effect on the storm tracks.     

High-frequency climate oscillations drive ice-off variability for Northern Hemisphere lakes and rivers

Climatic Change - This study examines the climatic drivers of ice-off dates for lakes and rivers across the Northern Hemisphere. Most lakes and rivers have trended toward earlier ice-off dates over...     

Notes on extended–range atmospheric prediction in the Northern Hemisphere winter

We examined the characteristic feature and predictability of low frequency variability (LFV) of the atmosphere in the Northern Hemisphere winter (January and February) by using the empirical orthogonal functions (EOFs) of the geopotential height at 500 hPa. In the discussion, we used the EOFs for geostrophic zonal wind (Uznl) and the height deviation from the zonal mean (Zeddy). The set of EOFs for Uznl and Zeddy was denoted as Uznl-1. Uznl-2, …, Zeddy-1, Zeddy-2…, respectively. We used the data samples of 396 pentads derived from 33 years of NMC, ECMWF and JMA analyses, from January 1963 to 1995. From the calculated scores for Uznl-1, Uznl-2, Zeddy-1, Zeddy-2 and so on we found that Uznl-1 and Zeddy-1 were statistically stable and their scores were more persistent than those of the other EOFs. A close relationship existed between the scores of Uznl-1 and those of Zeddy-1 30-day forecast experiments were carried out with the medium resolution version of JMA global spectral model for 20 cases in January and February for the period of 1984-1992. Results showed that Zeddy-1 was more predicta?ble than the other EOFs for Zeddy Considering these results, we argued that prediction of the Zeddy-1 was to be one of the main target of extended range forecasting     

Interdecadal change in the Northern Hemisphere seasonal climate prediction skill: part I. The leading forced mode of atmospheric circulation

Using observations and 1-month lead hindcast data from six coupled atmosphere–ocean climate models, this study investigates the interdecadal change in the leading maximum covariance analysis mode (MCA1) of atmospheric circulation in response to the changes in the El Niño and Southern Oscillation (ENSO) occurred around late 1970s. We focus on boreal winter climate variability and predictability over the North Pacific–North American (NPNA) region using December–January–February prediction initiated from November 1st in the period of 1960–1980 (P1) and 1981–2001 (P2). Observed analysis reveals that ENSO variability, the related tropical convective activity, and thus the MCA1 are considerably enhanced from P1 to P2. As a result, surface climate anomalies over the NPNA are more significantly correlated with the MCA1 in P2 than P1, particularly over North America. The six coupled models and their multi-model ensemble not only are capable of capturing the interdecadal change of the MCA1 and its relationship with surface air temperature and precipitation over the NPNA regions but also have significantly higher forecast skills for the MCA1 and the surface climate anomalies in P2 than P1. However, models have systematic biases in the spatial distribution of the MCA1. It is demonstrated that the interdecadal change in the MCA1 should contribute to the improved forecast skill of the NPNA climate during recent epoch.     

The effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model

An analysis is made of the effects of topography on the summer atmospheric energetics of the Northern Hemisphere in a low-resolution global spectral model. The numerical mode! is a global, spectral, primitive equation model with five equally spaced sigma levels in the vertical and triangular truncation at wavenumber 10 in the horizontal. The model includes comparatively full physical processes. Each term of the energy budget equations is calculated in four specific latitudinal belts (81.11°S–11.53°S; 11.53°S–11.53°N; 11.53°N–46.24°N; 46.24°N–81.11°N) from a five-year simulation with mountains and a one-year simulation without mountains, respectively. Differences between them are compared and statistically tested. The results show that synoptical scale waves transport available potential energy and kinetic energy to long waves and increase conversion from available potential energy of the zonal flow to eddy's and from the eddy kinetic energy to the zonal kinetic energy in region 3 (11.53°N-46.24°N) due to mountains; topography intensifies the atmospheric baroclinity in region 3, consequently the baroclinic conversion of atmosphere energy is increased. The seasonal characteristics associated with the summer atmospheric energy source in region 3 are caused by seasonal variation of the solar radiation and the land-ocean contrasts and independent of topographic effects. The mechanism of topographic effects on the increase of long wave kinetic energy is also discussed.     

Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation

The “Panama Hypothesis” states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2–2.7 Ma. Here we test this hypothesis using a fully coupled, fully dynamic ocean-atmosphere general circulation model (GCM) with boundary conditions specific to the Pliocene, and a high resolution dynamic ice sheet model. We carry out two GCM simulations with “closed” and “open” Panama Seaways, and use the simulated climatologies to force the ice sheet model. We find that the models support the “Panama Hypothesis” in as much as the closure of the seaway results in a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets. However, the volume difference between the ice sheets in the “closed” and “open” configurations is small, equivalent to about 5 cm of sea level. We conclude that although the closure of the Panama Seaway may have slightly enhanced or advanced the onset of NHG, it was not a major forcing mechanism. Future work must fully couple the ice sheet model and GCM, and investigate the role of orbital and CO₂ effects in controlling NHG.     

北方涛动同北半球温带大气环流的遥相关(一)——基本结构

本文利用1951—1980年逐季的平均值资料(共120个季)讨论了北方涛动和与其相联系的北太平洋海温与北半球海平面气压场、500hPa位势高度场遥相关的基本结构,并与南方涛动和赤道东太平洋海温的结果进行了对比分析.发现北太平洋Namias海区和加利福尼亚海流区海温的变化与北方涛动具有很密切的联系;北方涛动和这两个海区的海温同北半球中高纬度大气环流特别是PNA型和NAO型环流异常存在明显的遥相关关系;南方涛动和赤道太平洋海温同WP型或NPO型环流异常关系比较密切,而与PNA型和NAO型的关系不如北方涛动和Namias海区及加利福尼亚海流区海温的显著.     

北方涛动同北半球温带大气环流的遥相关(二)——季节变化

在以前工作的基础上,本文进一步分析了各个季节北方涛动和与其相联系的北太平洋海温同北半球温带大气环流遥相关的特征,并与南方涛动和赤道太平洋海温的结果进行了比较,发现它们同北半球温带大气环流的遥相关,不仅具有很强的地区性,而且具有明显的季节变化。 冬季,北方涛动和北太平洋海温与PNA型相关非常明显,且比南方涛动和赤道太平洋海温的更密切。除PNA型外,北方涛动还同500hPa高度的亚洲-北美(ANA)流型有联系。 夏季,PNA遥相关型的表现仍然非常清楚,但位置较冬季偏酉和偏北,并在北美西海岸具有特殊的分叉现象。     

欧亚土壤湿度异常对北半球大气环流的显著影响

用44 a ERA40再分析资料的土壤湿度和大气环流变量场, 研究持续性的欧亚大陆土壤湿度异常对后期北半球大尺度大气环流的反馈作用。首先,运用经验正交函数分解去除ENSO遥相关及趋势影响后,分析了欧亚大陆中高纬度土壤湿度变率主要模态的季节变化特征,及相对应主分量时间序列显示的土壤湿度异常的衰减时间,结果表明土壤湿度异常的主要模态在全年都表现出很好的连续性。其次,对不同季节的连续3个月的月平均土壤湿度和500 hPa高度场进行滞后最大协方差分析,研究欧亚地区中高纬度土壤湿度异常与北半球大气环流异常之间的线性耦合。第一最大协方差模态的结果表明:全年的主导信号是大气强迫土壤湿度的变化,但在冬季和夏季,大气中类似于负位相北极涛动的环流型与之前月份(最长达4个月)土壤湿度的持续变化显著相关。最后,基于土壤湿度变率中心的回归分析也证实了秋季和春季欧亚土壤湿度,特别是北非副热带,欧亚内陆和西伯利亚地区的土壤湿度异常,分别与其后的冬季和夏季的大气环流显著相关。欧亚大陆土壤湿度异常超前大气环流的信号,将有助于改善冬季和夏季北半球季节气候预报能力。     

Numerical simulation of Eurasian teleconnection pattern in atmospheric circulation during the Northern Hemisphere winter

In this paper, the anomaly of disturbance height field over Northern Hemisphere due to SST anomaly in the tropical Atlantic Ocean is simulated by using the general circulation model of IAP. A quasi-geostrophic, 34-level spherical coordinate model is also used to compute the distribution of atmospheric circulation anomaly when there is an anomaly of heat source over the tropical Atlantic. The computed results show that, owing to the heat source anomaly over the tropical Atlantic, the EU-pattern anomaly in the winter circulation may be caused. Namely, a ridge will be enhanced over the northwest Europe, a trough will be deepened over Siberia, but a positive anomaly of disturbance height field will be formed over the northeast China, Japan and other areas of East Asia. Moreover, the numerically simulated results show that the above-mentioned EU-pattern anomalies of circulation are due to the propagations of planetary wave train. About 15 days after an anomaly of the heat source over the tropical Atlantic is injected, the EU-pattern anomaly of atmospheric circulation is formed. This is in good agreement with the results analysed theoretically. On the leave from Geophysical Institute, Faculty of Science in Tokyo University, Japan.     

用一个耦合的全球格点大气环流模式-植被模式模拟中全新世气候变化

用一个耦合的全球格点大气环流模式－植被模式模拟中全新世的气候变化，模拟试验中考虑了地球轨道参数的变化，而其他强迫条件均取成现今值。结果表明，耦合的模式能够模拟出较今强的大尺度夏季风，特别是亚洲－非洲季风，而其他季节和区域的变化值一般都比较小。季风环流和季风降水都大幅度地增大了。结果还显示，耦合模式模拟的大尺度季风系统的变化同单纯大气环流模式模拟的结果非常相似，但是，在非洲北部季风区耦合模式模拟的降水和温度变化较单纯大气模式模拟的值要大，而且，耦合模式模拟的冬季降温值要比单纯大气模式模拟的结果小。     

Sensitivity of the Northern Hemisphere climate of the Last Glacial Maximum to sea surface temperatures

The role of prescribing sea surface temperature in paleoclimate atmospheric simulations has been investigated by comparing Last Glacial Maximum AGCMs experiments using different SSTs data sets as well as coupled atmosphere/oceanic mixed layer models. Changes in the SSTs and sea-ice margin generate different patterns of zonal asymmetries in the atmospheric circulation that are responsible for reorganisation of heat and moisture transport, leading to important variations of Northern Hemisphere regional climates, particularly in winter. Additional sensitivity experiments have been carried out to isolate the individual role of North Pacific and North Atlantic SSTs anomalies. We found that changes in North Pacific SSTs have a much stronger impact over all the northern continental surfaces, including Europe and Siberia, than changes in the North Atlantic SSTs. As these SSTs anomalies are of the order of the typical errors generated by coupled ocean-atmosphere models, this suggests that these more complete models will likely still have problems in simulating the regional climate change at the LGM. Received: 11 October 1999 / Accepted: 9 June 2000