Comparative influence of land and sea surfaces on the Sahelian drought: a numerical study

The aim of this work is to compare the relative impact of land and sea surface anomalies on Sahel rainfall and to describe the associated anomalies in the atmospheric general circulation. This sensitivity study was done with the Météo-France climate model: ARPEGE. The sensitivity to land surface conditions consists of changes in the management of water and heat exchanges by vegetation cover and bare soil. The sensitivity to ocean surfaces consists in forcing the lower boundary of the model with worldwide composite sea surface temperature (SST) anomalies obtained from the difference between 4 dry Sahel years and 4 wet Sahel years observed since 1970. For each case, the spatiotemporal variability of the simulated rainfall anomaly and changes in the modelled tropical easterly jet (TEJ) and African easterly jet (AEJ) are discussed. The global changes in land surface evaporation have caused a rainfall deficit over the Sahel and over the Guinea Coast. No significant changes in the simulated TEJ and an enhancement of the AEJ are found; at the surface, the energy budget and the hydrological cycle are substantially modified. On the other hand, SST anomalies induce a negative rainfall anomaly over the Sahel and a positive rainfall anomaly to the south of this area. The rainfall deficit due to those anomalies is consistent with previous diagnostic and sensitivity studies. The TEJ is weaker and the AEJ is stronger than in the reference. The composite impact of SST and land surfaces anomalies is also analyzed: the simulated rainfall anomaly is similar to the observed mean African drought patterns. This work suggests that large-scale variations of surface conditions may have a substantial influence on Sahel rainfall and shows the importance of land surface parameterization in climate change modelling. In addition, it points out the interest in accurately considering the land and sea surfaces conditions in sensitivity studies on Sahel rainfall.     

Uncertainties of the relationship between the equatorial quasi-biennial oscillation, the Arctic stratosphere and solar forcing

Two different equatorial quasi-biennial oscillation (QBO) indices, two reanalyses and radiosonde observations are used to analyze the Arctic stratospheric temperature and height. This analysis was used to assess the uncertainties in the connection of solar forcing, QBO and the Arctic variability. The results show that (1) the frequency of the westerly/easterly phases of the QBO over the stratospheric equator has a significant multiple peak seasonal variation. The primary seasonal peaks occur in February, March and April for the westerly phase of the QBO and the easterly phase peaks in June, July and August. (2) The correlation of stratospheric Arctic temperature and height with the solar radio flux shows statistical significance in February or July/August even if there is no stratified phase of QBO (easterly and westerly phases) involved. However, when the correlation was computed according to the stratified phase of QBO, the solar signals in both temperature and height fields are remarkably amplified in February and November under the westerly phase, but the signal in the height field is most significant only in August under the easterly phase. (3) The impact of the QBO and solar forcing on the stratospheric temperature and heights in the Arctic varies depending on the season. The impacts are also sensitive to the specific height of the QBO-defined level that is used, the specific period of the analysis and the dataset used.     

Dynamical diagnosis of the breakup of the stratospheric polar vortex in the Northern Hemisphere

The research on climate change in polar regions, especially on the role of polar in the global climate system, has gain unprecedented level of interest. It has been the key scientific issue of the International Polar Year program (IPY, 2007―2008). In this paper, we dealt with the debate upon the breakup time of the stratospheric polar vortex in boreal spring. An observational study of the relation between strato- spheric polar vortex breakup and the extra-tropical circulation was performed. The mean breakup date―when the winter westerly at the core of polar jet turns to summer easterly―is about April 10. The breakup time has large interannual variation with a time span of about 2 months. It also has a long-term trend with the 1990s and 2000s witnessing more and more late breakups of polar vortex. Composite of wind speed at the core of polar jet for the extremely early and late breakup years shows that late years have two periods of westerly weakening while early breakup years have only one. The first weakening in the late years happens in middle January with wind speed dropping sharply from more than 40 m s?1 to about 15 m s?1. This is accompanied with anomalous activities of planetary waves in both strato- sphere and troposphere; while the second weakening in the late breaking years is mainly the results of diabatic heating with very weak wave activities. In early breakup years, the transition from westerly to easterly is rapid with wind speed dropping from more than 30 m s?1 to less than ?10 m s?1 within a month. This evolution is associated with a strong bidirectional dynamical coupling of the stratosphere and troposphere. The circulation anomalies at low troposphere are also analyzed in the extremely early and late breakup years. It shows that there are significant differences between the two kinds of extreme years in the geopotential height and temperature composite analysis, indicating the dynamical cou- pling of stratosphere and troposphere with the evolution of stratospheric polar vortex.     

Global atmospheric response to specific linear combinations of the main SST modes. Part I: numerical experiments and preliminary results

This article investigates through numerical experiments the controversial question of the impact of El NinCo-Southern Oscillation (ENSO) phenomena on climate according to large-scale and regional-scale interhemispheric thermal contrast. Eight experiments (two considering only inversed Atlantic thermal anomalies and six combining ENSO warm phase with large-scale interhemispheric contrast and Atlantic anomaly patterns) were performed with the Météo-France atmospheric general circulation model. The definition of boundary conditions from observed composites and principal components is presented and preliminary results concerning the month of August, especially over West Africa and the equatorial Atlantic are discussed. Results are coherent with observations and show that interhemispheric and regional scale sea-surface-temperature anomaly (SST) patterns could significantly modulate the impact of ENSO phenomena: the impact of warm-phase ENSO, relative to the atmospheric model intercomparison project (AMIP) climatology, seems stronger when embedded in global and regional SSTA patterns representative of the post-1970 conditions [i.e. with temperatures warmer (colder) than the long-term mean in the southern hemisphere (northern hemisphere)]. Atlantic SSTAs may also play a significant role.     

Stratosphere–troposphere interaction associated with biennial oscillation of Indian summer monsoon

Quasi-biennial oscillation (QBO) is a predominant phenomenon in the tropical stratosphere and troposphere. The possible interactions between the stratospheric QBO and tropospheric biennial oscillation (TBO) over the Indian monsoon region as well as the equatorial region is investigated using the zonal wind data of 23 vertical levels (1000–1 hpa) from 1960–2002. The structure of lower stratosphere and troposphere are entirely different over the equator and India. In biennial scales, both the stratosphere and troposphere over the Indian region are closely related and winter season QBO is a good predictor of Indian summer monsoon rainfall.     

Dynamical effect of the zonal wind anomalies over the tropical western Pacific on ENSO cycles

The circulation and zonal wind anomalies in the lower troposphere over the equatorial western Pacific and their roles in the developing and decaying processes of the 1982–1983, 1986 –1987, 1991–1992 and 1997–1998 El Ni?o events and the occurrence of La Ni?a events are analyzed by using the observed data in this paper. The results show that before the developing stage of these El Ni?o events, there were cyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies brought the westerly anomalies over the Indonesia and the tropical western Pacific. However, when the El Ni?o events developed to their mature phase, there were anticyclonic circulation anomalies in the lower troposphere over the tropical western Pacific, and the anomalies made the easterly anomalies appear over the tropical western Pacific. A simple, dynamical model of tropical ocean is used to calculate the response of the equatorial oceanic waves to the observed anomalies of wind stress near the sea surface of the equatorial Pacific during the 1997/98 ENSO cycle, which was the strongest one in the 20th century. It is shown that the zonal wind stress anomalies have an important dynamical effect on the devel-opment and decay of this El Ni?o event and the occurrence of the following La Ni?a event.     

Planetary Boundary Layer Flows in the Atmosphere and the Ocean at the Equator

—The boundary layer flows created by the frictional dissipation of the wind speed at the surface in the atmosphere and by surface wind stress in the ocean at the equator and in the equatorial region, are obtained by taking the influence of the surface friction on the zonal velocity as being balanced by vertical transport for the long-term mean flow and by a corresponding time variation for time-dependent flow fields. Solutions are expressed in terms of the velocities in zonal and vertical directions and the divergence of the horizontal current in the two media. It is found that under the ever present easterly flow in the lower atmosphere, the boundary layer flow in the atmosphere is convergence and ascending motion in the lower troposphere, and divergence at the surface and uplift in ocean, and in reverse directions for the westerly flow. Similar results are obtained for time-dependent wind fields and they give way to the steady asymptotic solutions when the period of the variation exceeds 10 months.     

Studies of variations in the vertical ozone profiles over India

The latitudinal and temporal variations in the vertical profiles of ozone over the Indian subcontinent are discussed. In the equatorial atmosphere represented by Trivandrum (8°N) and Poona (18°N), while tropospheric ozone shows marked seasonal variations, the basic pattern of the vertical distribution of ozone in the stratosphere remains practically unchanged throughout the year, with a maximum at about 28 to 26 km and a minimum just below the tropopause. The maximum total ozone occurs over Trivandrum in the summer monsoon season and the latitudinal anomaly observed over the Indian monsoon area at this time is explained as arising from the horizontal transport of ozone-rich stratospheric air from over the thermal equator to the southern regions.In the higher latitudes represented by New Delhi (28°N), the maximum occurs at 23 km. Delhi, which lies in the temperate regime in winter, shows marked day-to-day variations in association with western disturbances and the strong westerly jet stream that lies over north and central India at this time.Although the basic pattern of the vertical distribution of ozone in the equatorial atmosphere is generally the same in all seasons, significant though small changes occur in the lower stratosphere and in the troposphere. There are small perturbations in the ozone and temperature structures, distinct ozone maxima being always associated with temperature inversions. There are also large perturbances not related to temperature, ozone-depleted regions normally reflecting a stratification of either destructive processes or materials such as dust layers or clouds at these levels. Particularly interesting are the upper tropospheric levels just below the tropopause where the ozone concentration is consistently the smallest, in all seasons and at all places where soundings have been made in India.     

The response in the Pacific to the sun's decadal peaks and contrasts to cold events in the Southern Oscillation

van Loon et al. [2007. Coupled air–sea response to solar forcing in the Pacific region during northern winter. Journal of Geophysical Research 112, D02108, doi:10.1029/2006JD007378] showed that the Pacific Ocean in northern winter is sensitive to the influence of the sun in its decadal peaks. We extend this study by three solar peaks to a total of 14, examine the response in the stratosphere, and contrast the response to solar forcing to that of cold events (CEs) in the Southern Oscillation. The addition of three solar peak years confirms the earlier results. That is, in solar peak years the sea level pressure (SLP) is, on average, above normal in the Gulf of Alaska and south of the equator, stronger southeast trades blow across the Pacific equator and cause increased upwelling and thus anomalously lower sea surface temperatures (SSTs). Since the effect on the Pacific climate system of solar forcing resembles CEs in the Southern Oscillation, we compare the two and note that, even though their patterns appear similar in some ways, they are particularly different in the stratosphere and are thus due to separate processes. That is, in July–August (JA) of the year leading into January–February (JF) of the solar peak years, the Walker cell expands in the Pacific troposphere, and the stratospheric wind anomalies are westerly below 25 hPa and easterly above, whereas this signal in the stratosphere is absent in CEs. Thus the large-scale east–west tropical atmospheric (Walker) circulation is enhanced, though not to the extent that it is in CEs in the Southern Oscillation, and the solar influence thus appears as a strengthening of the climatological mean regional precipitation maxima in the tropical Pacific. Additionally, CEs have a 1-year evolution, while the response to solar peaks extends across 3 years such that the signal in the Pacific SLP of the solar peaks is similar but weaker in the year leading into the peak and in the year after the peak. The concurrent negative SST anomalies develop during the year before the solar peak, and after the peak the anomalies are still present but are waning. In the stratosphere in solar peaks, the equatorial quasi-biennial oscillation (QBO) is amplified when it is in its westerly phase in the lower stratosphere and easterly phase above; and the QBO is suppressed when in its easterly phase below–westerly phase above. Such an association is not evident in CEs.     

On the Integrability of Limited-Area Numerical Weather Prediction Model ALADIN over Extended Time Periods

This note summarizes results of the first integration of regional numerical weather prediction model ALADIN in a climate mode. The ALADIN model, developed in an international cooperation led by Météo France, is operationally used for weather prediction. The grid step of the model is 12 km; the integration domain covers a major part of Europe. A one-month-long run has been performed with this model on observed boundary conditions (represented by assimilations by the global model ARPEGE). It is demonstrated that no excessive error is generated and accumulated in the model during the integration; hence the model is integrable for extended time periods and may serve a basis for a development towards a regional climate model.     

副热带急流强度和赤道QBO对平流层突然增温的影响

使用一个全球原始方程半谱模式模拟了副热带急流强度和赤道准两年振荡（ＱＢＯ）对平流层突然增温（ＳＳＷ）的影响．结果指出：副热带急流强度对ＳＳＷ有明显影响，副热带急流越强，ＳＳＷ发展越快，极区最大增温区的高度越低；赤道ＱＢＯ不同相位零风线的南北位移对ＳＳＷ没有明显影响；ＱＢＯ东风相位时副热带急流更强，而ＱＢＯ西风相位时副热带急流较弱的观测事实，可能是大的中冬ＳＳＷ更容易发生在ＱＢＯ东风相位的主要原因．     

Trends in total ozone and the effect of the equatorial zonal wind QBO

Trends in total column ozone have been analyzed in terms of the equatorial zonal wind. We used zonal monthly mean total ozone from Total Ozone Mapping Spectrometer (TOMS) and monthly mean zonal wind in the equatorial stratosphere at 30 hPa to define the phases of the quasi-biennial oscillation (QBO). Total column ozone trends have been assessed during the period 1979–2004, for both Hemispheres, and for each month, under three conditions considering, all the ozone dataset, ozone values during easterly phase and ozone values during westerly phase of the QBO. When the whole dataset is considered, negative trends are observed. From low to midlatitudes a zonal pattern is noticed with increasing negative values toward higher latitudes. When the data is filtered according to the QBO phase, statistically significant positive trends appear in the westerly case during January to May at low latitudes .The trend pattern in the case of the easterly phase presents more negative values.     

BCC_CSM1.1(m)模式对于夏季亚洲-太平洋涛动的模拟

亚洲-太平洋涛动是北半球夏季亚洲大陆和北太平洋副热带地区对流层中高层扰动温度场上大尺度的东西反相的遥相关现象,其异常变化与亚洲-太平洋地区夏季风气候有着密切的联系.基于欧洲中心的ERA-40再分析资料和国家气候中心BCC_CSM1.1（m）气候系统模式多年的数值模拟结果,本文主要评估了BCC_CSM1.1（m）模式对于夏季亚洲-太平洋涛动的空间分布、指数的时间演变及与其变化所对应的亚洲地区夏季环流异常等方面的模拟能力,结果表明：BCC_CSM1.1（m）模式能够较好地模拟出北半球夏季对流层中高层扰动温度在亚-太地区中纬度存在的西高东低"跷跷板"现象;模式能够模拟出夏季亚洲-太平洋涛动指数的年际变率,但是不能模拟出该指数在20世纪60-70年代明显下降的年代际趋势;模式还能较好地模拟出亚洲-太平洋涛动高低指数年亚洲-太平洋地区夏季环流的异常：指数偏高年份,南亚高压增强,高空西风急流带和热带东风急流均加强,索马里越赤道气流增强,南亚热带季风和东亚副热带季风均增强,东亚季风低压槽加强,西北太平洋副热带高压增强,南亚和东亚北部降水增加,菲律宾地区、中国长江流域-朝鲜半岛-日本一带地区降水减少,反之亦然.     

Sudden stratospheric warmings: the role of normal atmospheric modes

The role of normal atmospheric modes in the beginning and development of sudden stratospheric warming (SSW) events is studied on the basis of calculations with the use of the general circulation model of the middle and upper atmosphere. The analysis of the effect of a phase of quasi-biennial oscillations on the dynamics of the extratropical stratosphere has shown that the conditions for SSW commencement are more favorable and the SSW events are more intense during the easterly phase of these oscillations as compared to the westerly phase. The conclusion has been drawn that fundamental normal atmospheric modes can be recorded in the temperature field at mesopause altitudes during ground-based optical measurements.     

A modified ISBA surface scheme for modeling the hydrology of Athabasca River Basin with GCM-scale data

A soil–vegetation–atmosphere transfer model (SVAT), interactions between the soil–biosphere–atmosphere (ISBA) of Météo France, is modified and applied to the Athabasca River Basin (ARB) to model its water and energy fluxes. Two meteorological datasets are used: the archived forecasts from the Meteorological Survey of Canada’s Global Environmental Multiscale Model (GEM) and the European Centre for Mid-range Weather Forecasts global re-analysis (ERA-40), representing spatial scales typical of a weather forecasting model and a global circulation model (GCM), respectively. The original treatment of soil moisture and rainfall in ISBA (OISBA) is modified to statistically account for sub-grid heterogeneity of soil moisture and rainfall to produce new, highly non-linear formulations for surface and sub-surface runoff (MISBA). These new formulations can be readily applied to most existing SVATs. Stand alone mode simulations using the GEM data demonstrate that MISBA significantly improves streamflow predictions despite requiring two fewer parameters than OISBA. Simulations using the ERA-40 data show that it is possible to reproduce the annual variation in monthly, mean annual, and annual minimum flows at GCM scales without using downscaling techniques. Finally, simulations using a simple downscaling scheme show that the better performance of higher resolution datasets can be primarily attributed to improved representation of local variation of land cover, topography, and climate.     

Observational evidence of the semiannual oscillation in the tropical middle atmosphere—A review

Observational studies on the semiannual oscillation in the tropical stratosphere and mesosphere are reviewed. Results of many statistics based on rocket and satellite observations reveal that the long-term behavior of the mean zonal wind exhibits two semiannual cycles which have their maximum amplitudes centered at the stratopause level and the mesopause level, each one being associated with the semiannual temperature variations predominating at levels about 10 km lower.Observational evidence obtained from recent studies of the dynamical properties of upper stratospheric waves strongly supports the theoretical consideration that the stratospheric semiannual oscillation is the manifestation of the wave-zonal flow interaction with alternating accelerations of the westerly flow by Kelvin waves and the easterly flow by planetary Rossby waves.Regarding the semiannual variation in the upper mesosphere, however, very little is known about the possible momentum source. Therefore, emphasis is placed on the need for further observations of the structure and behavior of the tropical middle atmosphere.     

A distribution law for relative humidity in the upper troposphere and lower stratosphere derived from three years of MOZAIC measurements

Data from three years of MOZAIC measurements made it possible to determine a distribution law for the relative humidity in the upper troposphere and lower stratosphere. Data amounting to 13.5% of the total were obtained in regions with ice supersaturation. Troposphere and stratosphere are distinguished by an ozone concentration of 130 ppbv as threshold. The probability of measuring a certain amount of ice supersaturation in the troposphere decreases exponentially with the degree of ice supersaturation. The probability of measuring a certain relative humidity in the stratosphere (both with respect to water and ice) decreases exponentially with the relative humidity. A stochastic model that naturally leads to the exponential distribution is provided. Mean supersaturation in the troposphere is about 15%, whereas ice nucleation requires 30% supersaturation on the average. This explains the frequency of regions in which aircraft induce persistent contrails but which are otherwise free of clouds. Ice supersaturated regions are 3-4 K colder and contain more than 50% more vapour than other regions in the upper troposphere. The stratospheric air masses sampled are dry, as expected, having mean relative humidity over water of 12% and over ice of 23%, respectively. However, 2% of the stratospheric data indicate ice supersaturation. As the MOZAIC measurements have been obtained on commercial flights mainly between Europe and North America, the data do not provide a complete global picture, but the exponential character of the distribution laws found is probably valid globally. Since water vapour is the most important greenhouse gas and since it might enhance the anthropogenic greenhouse effects via positive feedback mechanisms, it is important to represent its distribution correctly in climate models. The discovery of the distribution law of the relative humidity makes possible simple tests to show whether the hydrological cycle in climate models is represented in an adequate way or not.     

气候系统模式对北极涛动的模拟

本文系统分析了参与IPCC AR4 “20世纪气候模拟"（20C3M）的23个气候系统模式模拟的1950～1999年冬季（JFM）北极涛动（AO）.结果表明,在22个模式中,AO模态都表现为北半球中高纬大气年际变率的第一模态.不过,18个模式模拟的AO模态在北太平洋地区表现偏强,有两组模式结果均表明,提高模式的水平分辨率能够克服此偏差.所有模式模拟的AO指数均未出现与观测相当的增强趋势,只有两个模式（ECHO-G和UKMO-HadGEM1）模拟的AO指数与观测存在显著正相关.多数模式能够模拟出纬向风伴随AO位相变化在中高纬出现的偶极子异常特征,部分模式对平流层AO特征的模拟能力仍需要提高.对AO垂直结构模拟较好的模式,例如CCSM3、MRI CGCM2-32和UKMO-HadGEM1,能够较为合理地再现伴随AO指数增强出现的极区平流层变冷和中纬度对流层增暖现象.AO与欧亚大陆地表气温和降水的相关分布,在多数模式中有较好的体现,个别模式还对AO与东亚气候的相关关系具有一定的模拟能力.对AO各指标均有较好模拟能力的模式是UKMO-HadGEM1.本文结果为改进气候系统模式对北极涛动的模拟能力提供了依据,亦为学术界利用IPCC AR4的耦合模式数据、开展与AO相关的气候变率研究提供了基础参考.     

The relationship between significant wave height and Indian Ocean Dipole in the equatorial North Indian Ocean

Based on reanalysis data, we find that the Indian Ocean Dipole (IOD) plays an important role in the variability of wave climate in the equatorial Northern Indian Ocean (NIO). Significant wave height (SWH) in the equatorial NIO, especially over the waters southeast to Sri Lanka, exhibits strong interannual variations. SWH anomalies in the waters southeast to Sri Lanka correlate well with dipole mode index (DMI) during both summer and autumn. Negative SWH anomalies occur over the oceanic area southeast to Sri Lanka during positive IOD events and vary with different types of IOD. During positive prolonged (unseasonable) IOD, the SWH anomalies are the strongest in autumn (summer); while during positive normal IOD, the SWH anomalies are weak in both summer and autumn. Strong easterly wind anomalies over the southeast oceanic area of Sri Lanka during positive IOD events weaken the original equatorial westerly wind stress, which leads to the decrease in wind-sea waves. The longer wave period during positive IOD events further confirms less wind-sea waves. The SWH anomaly pattern during negative IOD events is nearly opposite to that during positive IOD events.     

Climate hypersensitivity to solar forcing?

We compare the equilibrium climate responses of a quasi-dynamical energy balance model to radiative forcing by equivalent changes in CO₂, solar total irradiance (S_tot) and solar UV (S_UV). The response is largest in the S_UV case, in which the imposed UV radiative forcing is preferentially absorbed in the layer above 250 mb, in contrast to the weak response from global-columnar radiative loading by increases in CO₂ or S_tot. The hypersensitive response of the climate system to solar UV forcing is caused by strongly coupled feedback involving vertical static stability, tropical thick cirrus ice clouds and stratospheric ozone. This mechanism offers a plausible explanation of the apparent hypersensitivity of climate to solar forcing, as suggested by analyses of recent climatic records. The model hypersensitivity strongly depends on climate parameters, especially cloud radiative properties, but is effective for arguably realistic values of these parameters. The proposed solar forcing mechanism should be further confirmed using other models (e.g., general circulation models) that may better capture radiative and dynamical couplings of the troposphere and stratosphere.