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1.
Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region
This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes
of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical
files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the
tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System
on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable
water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress
data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies
conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall.
It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal
well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter
periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains
considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter
frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like
Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific.
This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the
relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes
at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the
north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical
Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or
delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through
three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West
Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic
modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental
rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create
meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy
and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and
hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near
the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked
to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel
rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region
but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations:
anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of
Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level
pressure over West Africa.
Received: 14 April 1998 / Accepted: 24 December 1998 相似文献
2.
The correlation between Sahel rainfall and El Niño–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200?hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Niño-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa. 相似文献
3.
This study presents the spatial-temporal structure of droughts in West Africa and evaluates the capability of CORDEX regional climate models in simulating the droughts. The study characterize droughts with the standardized evapo-transpiration index (SPEI) computed using the monthly rainfall and temperature data from the Climatic Research Unit (CRU) and CORDEX models simulation datasets. To obtain the spatial-temporal structure of the droughts, we applied the principal component analysis on the observed and simulated SPEIs and retained the first four principal factors as the leading drought modes over West Africa. The relationship between the drought modes and atmospheric teleconnections was studied using wavelet coherence analysis, while the ability of the CORDEX models to simulate the drought modes was quantified with correlation analysis. The analysis of the relationship between drought modes and atmospheric teleconnections is based on SPEI from observation dataset (CRU). The study shows that about 60 % of spatial-temporal variability in SPEI over West Africa can be grouped into four drought modes. The first drought mode features drought over east Sahel, the second over west Sahel, the third over the Savanna, and the fourth over the Guinea coast. Each drought mode is linked to sea surface temperature anomalies (SSTAs) over tropical areas of Pacific, Atlantic, and Indian Oceans. Most CORDEX models reproduce at least two of the drought modes, but only two models (REMO and CNRM) reproduce all the four drought modes. REMO and WRF give the best simulation of the seasonal variation of the drought mode over the Sahel in March-May and June-August seasons, while CNRM gives the best simulation of seasonal variation in the drought pattern over the Savanna. Results of this study may guide in selecting appropriate CORDEX models for seasonal prediction of droughts and for downscaling projected impacts of global warming on droughts in West Africa. 相似文献
4.
Summary Previous studies have highlighted the crucial role of sea surface temperature (SST) anomalies in the tropical Atlantic region
in forcing the summer monsoon rainfall over subsaharan West Africa. Understanding the physical processes, relating SST variations
to changes in the amount and distribution of African rainfall, is a key factor in improving weather and climate forecasts
in this highly vulnerable region.
Here, we present sensitivity experiments from a regional climate model with prescribed warmer tropical SSTs, according to
enhanced greenhouse conditions at the end of the 21st century. This dynamical downscaling approach provides information about
the nonlinear response of the atmosphere to oceanic heating. It has been suggested that the response is at least partly accounted
for by the linear theory of tropical dynamics, involving a Kelvin and Rossby wave response to a tropical heat source. We compute
the major modes of the linear Matsuno-Gill model for geopotential height and horizontal wind components and project the simulated
response patterns onto these linear modes, in order to evaluate to which extent the simple linear theory may explain the SST-induced
climate anomalies over Africa. A multivariate Hotelling T2 test is used to evaluate whether these anomalies are statistically significant.
Forcing the regional climate model by warmer SSTs leads to substantial climate anomalies over tropical Africa: Rainfall is
increases over the Guinea Coast region (GCR) and tropical East Africa, but decreases over the Congo Basin and the Sahel Zone
(SHZ). At the 850 hPa level, a trough develops over southern West Africa and the Gulf of Guinea, and is associated with stronger
surface wind convergence over the GCR. These changes in the atmospheric dynamics strongly project onto the leading modes of
the linear Matsuno-Gill model at various zonal wave numbers. The corresponding atmospheric heating pattern is highly reminiscent
of the simulated nonlinear model reponse. The T2 test statistics reveal that the SST forcing induces a statistically significant climate anomaly over tropical Africa if the
climate state vector is reduced by projecting the simulated data onto the leading 10 linear modes. It is also shown that the
linear response prevails in a long-term simulation with more realistic lower and lateral boundary conditions. Thus, linear
tropical dynamics are assumed to be a major physical process on the ground of the prominent SST-African rainfall relationship. 相似文献
5.
Monthly sea surface temperature anomalies (SSTA) at near-global scale (60 °N–40 °S) and May to October rainfall amounts in
West Africa (16 °N–5 °N; 16 °W–16 °E) are first used to investigate the seasonal and interannual evolutions of their relationship.
It is shown that West African rainfall variability is associated with two types of oceanic changes: (1) a large-scale evolution
involving the two largest SSTA leading eigenmodes (16% of the total variance with stronger loadings in the equatorial and
southern oceans) related to the long-term (multiannual) component of rainfall variability mainly expressed in the Sudan–Sahel
region; and (2) a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical Atlantic due
to the linear combination of the two largest SSTA modes in the Atlantic (11% with strong inverse loadings over the northern
and southern tropics) which is associated with the interannual and quasi-decadal components of regional rainfall in West Africa.
Linear regression and discriminant analyses provide evidence that the main July–September rainfall anomalies in Sudan–Sahel
can be detected with rather good skills using the leading (April–June) or synchronous (July–September) values of the four
main oceanic modes. In particular, the driest conditions over Sahel, more marked since the beginning of the 1970s, are specifically
linked to the warm phases of the two global modes and to cold/warm anomalies in the northern/southern tropical Atlantic. Idealized
but realistic SSTA patterns, obtained from some basic linear combinations of the four main oceanic modes appear sufficient
to generate quickly (from mid-July to the end of August) significant West African rainfall anomalies in model experiments,
consistent with the statistical results. The recent negative impact on West African rainfall exerted by the global oceanic
forcing is primarily due to the generation of subsidence anomalies in the mid-troposphere over West Africa. When an idealized
north to south SSTA gradient is added in the tropical Atlantic, strong north to south height gradients in the middle levels
appear. These limit the northward excursion of the rainbelt in West Africa: the Sahelian area experiences drier conditions
due to the additive effect (subsidence anomalies+latitudinal blocking) while over the Guinea regions wet conditions do not
significantly increase, since the subsidence anomalies and the blocking effect act here in opposite ways.
Received: 26 June 1997 / Accepted: 3 October 1997 相似文献
6.
Mark R. Jury 《Theoretical and Applied Climatology》2010,101(1-2):29-40
Ethiopian decadal climate variability is characterized by application of singular value decomposition to gridded rainfall data over the period 1901–2007. Two distinct modes are revealed with different annual cycles and opposing responses to regional and global forcing. The northern zone that impacts the Nile River and underlies the tropical easterly jet has a unimodal rainy season that is enhanced by Atlantic Multidecadal Oscillation warm phase. This rainfall mode is linked with the Atlantic zonal overturning circulation and exhibits 10–12-year cycles through much of the twentieth century. The southern zone has a bimodal rainy season that is enhanced by Pacific Decadal Oscillation cool phase and the southern meridional overturning circulation. Multiyear wet and dry spells are characterized by sympathetic responses in the near-equatorial trough extending from Central America across the African Sahel to Southeast Asia. The interaction of Walker and Hadley cells over Africa appears to be a key feature that modulates Ethiopian climate at decadal frequency through anomalous north–south displacement of the near-equatorial trough. 相似文献
7.
In this study, statistical techniques are employed to decompose climate signals around southern Africa into the dominant temporal frequencies, with the aim of modelling and predicting area-averaged rainfall. In the rainfall time series over the period 1900–1999, the annual cycle accounts for 83% of variance. Residual spectral energy cascades from biennial (42%) to interannual (20%) to decadal bands (3%). Regional climate signals are revealed through a multi-taper singular value decomposition analysis of sea surface temperature and sea level pressure fields over the Atlantic and Indian Oceans, in conjunction with southern Africa rainfall. Rossby wave action in the South Indian Ocean dominates the biennial scale variability. El Niño-Southern Oscillation (ENSO) and related Indian Ocean dipole patterns are important for interannual variability. Significant sea temperature and pressure fluctuations occurring 6–12 months prior to rainfall contribute biennial and interannual indices to a multi-variate model that demonstrates useful predictive skill. 相似文献
8.
The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981?C1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally. 相似文献
9.
Summary Climatic patterns associated with extreme modes of summer rainfall over southern Africa are investigated using composite techniques. Differences between the wet summers of the mid-1970s and the dry summers of the early 1980s are highlighted. In dry summers both the Southern Oscillation Index (SOI) and Quasi-Biennial Oscillation (QBO) are negatively biased. Composite difference fields of outgoing longwave radiation (OLR), sea surface temperature (SST), and upper and lower tropospheric wind are analysed. The OLR difference field indicates the widespread nature of convective variations with a consistent sign in the domain 15–33° S, 0–40° E. An area of opposing sign is conspicuous over the southwest Indian Ocean and represents a dipole, whereby wet summers over southern Africa coincide with dry summers over the adjacent ocean. This dipole behaviour is an expression of the primary mode of interannual climatic variability in the region. SST composite differences are negative over a wide portion of the central equatorial Indian Ocean and SE Atlantic, and positive to the south of Africa where the Agulhas Current flows. Wind composites reveal distinctive circulation differences in the extreme summers considered. In the tropical zone off the east coast of Africa difference vectors indicate upper westerly and lower easterly circulation anomalies, and distinguish a pathway for moist Indian Ocean air. A deep anticyclonic gyre is located over the region of positive SST differences in the sub-tropics to the SE of Africa. The identification of climatic patterns in extreme summers offers some useful guidelines in seasonal forecasts.With 6 Figures 相似文献
10.
Sharon E. Nicholson 《Climate Dynamics》2009,32(7-8):1155-1171
This article presents an overview of the land ITCZ (Intertropical Convergence Zone) over West Africa, based on analysis of NCAR–NCEP Reanalysis data. The picture that emerges is much different than the classic one. The most important feature is that the ITCZ is effectively independent of the system that produces most of the rainfall. Rainfall linked directly to this zone of surface convergence generally affects only the southern Sahara and the northern-most Sahel, and only in abnormally wet years in the region. A second feature is that the rainbelt normally assumed to represent the ITCZ is instead produced by a large core of ascent lying between the African Easterly Jet and the Tropical Easterly Jet. This region corresponds to the southern track of African Easterly Waves, which distribute the rainfall. This finding underscores the need to distinguish between the ITCZ and the feature better termed the “tropical rainbelt”. The latter is conventionally but improperly used in remote sensing studies to denote the surface ITCZ over West Africa. The new picture also suggests that the moisture available for convection is strongly coupled to the strength of the uplift, which in turn is controlled by the characteristics of the African Easterly Jet and Tropical Easterly Jet, rather than by moisture convergence. This new picture also includes a circulation feature not generally considered in most analyses of the region. This feature, a low-level westerly jet termed the African Westerly Jet, plays a significant role in interannual and multidecadal variability in the Sahel region of West Africa. Included are discussions of the how this new view relates to other aspects of West Africa meteorology, such as moisture sources, rainfall production and forecasting, desertification, climate monitoring, hurricanes and interannual variability. The West African monsoon is also related to a new paradigm for examining the interannual variability of rainfall over West Africa, one that relates changes in annual rainfall to changes in either the intensity of the rainbelt or north–south displacements of this feature. The new view presented here is consistent with a plethora of research on the synoptic and dynamic aspects of the African Easterly Waves, the disturbances that are linked to rainfall over West Africa and spawn hurricanes over the Atlantic, and with our knowledge of the prevailing synoptic and dynamic features. This article demonstrate a new aspect of the West Africa monsoon, a bimodal state, with one mode linked to dry conditions in the Sahel and the other linked to wet conditions. The switch between modes appears to be linked to an inertial instability mechanism, with the cross-equatorial pressure gradient being a critical factor. The biomodal state has been shown for the month of August only, but this month contributes most of the interannual variability. This new picture of the monsoon and interannual variability shown here appears to be relevant not only to interannual variability, but also to the multidecadal variability evidenced in the region between the 1950s and 1980s. 相似文献
11.
Decadal Sahelian rainfall variability was mainly driven by sea surface temperatures (SSTs) during the twentieth century. At the same time SSTs showed a marked long-term global warming (GW) trend. Superimposed on this long-term trend decadal and multi-decadal variability patterns are observed like the Atlantic Multidecadal Oscillation (AMO) and the inter-decadal Pacific Oscillation (IPO). Using an atmospheric general circulation model we investigate the relative contribution of each component to the Sahelian precipitation variability. To take into account the uncertainty related to the use of different SST data sets, we perform the experiments using HadISST1 and ERSSTv3 reconstructed sets. The simulations show that all three SST signals have a significant impact over West Africa: the positive phases of the GW and the IPO lead to drought over the Sahel, while a positive AMO enhances Sahel rainfall. The tropical SST warming is the main cause for the GW impact on Sahel rainfall. Regarding the AMO, the pattern of anomalous precipitation is established by the SSTs in the Atlantic and Mediterranean basins. In turn, the tropical SST anomalies control the impact of the IPO component on West Africa. Our results suggest that the low-frequency evolution of Sahel rainfall can be interpreted as the competition of three factors: the effect of the GW, the AMO and the IPO. Following this interpretation, our results show that 50% of the SST-driven Sahel drought in the 1980s is explained by the change to a negative phase of the AMO, and that the GW contribution was 10%. In addition, the partial recovery of Sahel rainfall in recent years was mainly driven by the AMO. 相似文献
12.
Previous studies have highlighted the crucial role of land degradation in tropical African climate. This effect urgently has
to be taken into account when predicting future African climate under enhanced greenhouse conditions. Here, we present time
slice experiments of African climate until 2025, using a high-resolution regional climate model. A supposable scenario of
future land use changes, involving vegetation loss and soil degradation, is prescribed simultaneously with increasing greenhouse-gas
concentrations in order to detect, where the different forcings counterbalance or reinforce each other. This proceeding allows
us to define the regions of highest vulnerability with respect to future freshwater availability and food security in tropical
and subtropical Africa and may provide a decision basis for political measures. The model simulates a considerable reduction
in precipitation amount until 2025 over most of tropical Africa, amounting to partly more than 500 mm (20–40% of the annual
sum), particularly in the Congo Basin and the Sahel Zone. The change is strongest in boreal summer and basically reflects
the pattern of maximum vegetation cover during the seasonal cycle. The related change in the surface energy fluxes induces
a substantial near-surface warming by up to 7°C. According to the modified temperature gradients over tropical Africa, the
summer monsoon circulation intensifies and transports more humid air masses into the southern part of West Africa. This humidifying
effect is overcompensated by a remarkable decrease in surface evaporation, leading to the overall drying tendency over most
of Africa. Extreme daily rainfall events become stronger in autumn but less intense in spring. Summer and autumn appear to
be characterized by more severe heat waves over Subsaharan West Africa. In addition, the Tropical Easterly Jet is weakening,
leading to enhanced drought conditions in the Sahel Zone. All these results suggest that the local impact of land degradation
and reduction of vegetation cover may be more important in tropical Africa than the global radiative heating, at least until
2025. This implies that vegetation protection measures at a national scale may directly lead to a mitigation of the expected
negative implications of future climate change in tropical Africa. 相似文献
13.
A 15 member ensemble of 20th century simulations using the ECHAM4–T42 atmospheric GCM is utilized to investigate the potential predictability of interannual variations of seasonal rainfall over Africa. Common boundary conditions are the global sea surface temperatures (SST) and sea ice extent. A canonical correlation analysis (CCA) between observed and ensemble mean ECHAM4 precipitation over Africa is applied in order to identify the most predictable anomaly patterns of precipitation and the related SST anomalies. The CCA is then used to formulate a re-calibration approach similar to model output statistics (MOS) and to derive precipitation forecasts over Africa. Predictand is the climate research unit (CRU) gridded precipitation over Africa. As predictor we use observed SST anomalies, ensemble mean precipitation over Africa and a combined vector of mean sea level pressure, streamfunction and velocity potential at 850 hPa. The different forecast approaches are compared. Most skill for African precipitation forecasts is provided by tropical Atlantic (Gulf of Guinea) SST anomalies which mainly affect rainfall over the Guinean coast and Sahel. The El Niño/Southern Oscillation (ENSO) influences southern and East Africa, however with a lower skill. Indian Ocean SST anomalies, partly independent from ENSO, have an impact particularly on East Africa. As suggested by the large agreement between the simulated and observed precipitation, the ECHAM4 rainfall provides a skillful predictor for CRU precipitation over Africa. However, MOS re-calibration is needed in order to provide skillful forecasts. Forecasts using MOS re-calibrated model precipitation are at least as skillful as forecast using dynamical variables from the model or instantaneous SST. In many cases, MOS re-calibrated precipitation forecasts provide more skill. However, differences are not systematic for all regions and seasons, and often small. 相似文献
14.
West African monsoon response to greenhouse gas and sulphate aerosol forcing under two emission scenarios 总被引：1，自引：0，他引：1
The impact of increased greenhouse gases (GHG) and aerosols concentrations upon the West African monsoon (WAM) is investigated
for the late twenty-first century period using the Météo-France ARPEGE-IFS high-resolution atmospheric model. Perturbed (2070–2100)
and current (1961–2000) climates are compared using the model in time-slice mode. The model is forced by global sea surface
temperatures provided by two transient scenarios performed with low-resolution coupled models and by two GHG evolution scenarios,
SRES-A2 and SRES-B2. Comparing to reanalysis and observed data sets, the model is able to reproduce a realistic seasonal cycle
of WAM despite a clear underestimation of the African Easterly Jet (AEJ) during the boreal summer. Mean temperature change
indicates a global warming over the continent (stronger over North and South Africa). Simulated precipitation change at the
end of the twenty-first century shows an increase in precipitation over Sudan-Sahel linked to a strong positive feedback with
surface evaporation. Along Guinea Gulf coast, rainfall regimes are driven by large-scale moisture advection. Moreover, results
show a mean precipitation decrease (increase) in the most (less) enhanced GHG atmosphere over this region. Modification of
the seasonal hydrological cycle consists in a rain increase during the monsoon onset. There is a significant increase in rainfall
variance over the Sahel, which extends over the Guinea coast region in the moderate emission scenario. Enhanced precipitation
over Sahel is linked to large-scale circulation changes, namely a weakening of the AEJ and an intensification of the Tropical
Easterly Jet. 相似文献
15.
Raphael Neukom David J. Nash Georgina H. Endfield Stefan W. Grab Craig A. Grove Clare Kelso Coleen H. Vogel Jens Zinke 《Climate Dynamics》2014,42(9-10):2713-2726
This study presents the first consolidation of palaeoclimate proxy records from multiple archives to develop statistical rainfall reconstructions for southern Africa covering the last two centuries. State-of-the-art ensemble reconstructions reveal multi-decadal rainfall variability in the summer and winter rainfall zones. A decrease in precipitation amount over time is identified in the summer rainfall zone. No significant change in precipitation amount occurred in the winter rainfall zone, but rainfall variability has increased over time. Generally synchronous rainfall fluctuations between the two zones are identified on decadal scales, with common wet (dry) periods reconstructed around 1890 (1930). A strong relationship between seasonal rainfall and sea surface temperatures (SSTs) in the surrounding oceans is confirmed. Coherence among decadal-scale fluctuations of southern African rainfall, regional SST, SSTs in the Pacific Ocean and rainfall in south-eastern Australia suggest SST-rainfall teleconnections across the southern hemisphere. Temporal breakdowns of the SST-rainfall relationship in the southern African regions and the connection between the two rainfall zones are observed, for example during the 1950s. Our results confirm the complex interplay between large-scale teleconnections, regional SSTs and local effects in modulating multi-decadal southern African rainfall variability over long timescales. 相似文献
16.
利用代表南海夏季风季节内振荡特征的850 hPa纬向风EOF分解的前两个主成分,定义南海夏季风季节内振荡指数,并利用美国国家环境预测中心第2代气候预报系统 (NCEP Climate Forecast System Version 2, NCEP/CFSv2) 提供的1982—2009年逐日回算预报场计算了南海夏季风季节内振荡指数的预报值,用于我国南方地区持续性强降水的预报试验。试验结果表明:利用南海夏季风季节内振荡实时监测指数与模式直接预报降水量相结合的统计动力延伸预报方法,能够有效提高季节内降水分量的预报效果。同时,该方法能够避免末端数据损失,修正了对模式预报降水直接进行带通滤波而导致的负相关现象,并起到消除模式系统误差的作用。 相似文献
17.
针对1970年代末及1990年代初中国东部夏季降水（ECP）的年代际变化格局,采用EOF分解、相关分析、回归分析等统计方法诊断了全球海表面温度对ECP年代际变化前两个模态（EOF1、EOF2）的影响。发现大西洋多年代际振荡（AMO）序列,印度洋偶极子（DMI）序列,太平洋年代际振荡（PDO）序列与ECP前两个模态时间系数（PC1、PC2）相关性较好,结合各海温指数的年代际变化特征,发现ECP在1970年代末受PDO及DMI的影响在低纬及中纬度地区分别呈现EOF1、EOF2的正位相分布特征;而在1990年代初受AMO及PDO的影响主要呈现EOF1的特征。由各海温指数及PC1、PC2重建的ECP分布特征可知,AMO及DMI与PC1重建的ECP型相近,对ECP的影响范围集中在低纬地区。除去变暖影响的DMI及PC1回归的高度场中发现一个源起大西洋的波列,黄河以北为异常反气旋中心,以南为异常气旋中心,低层南风异常,水汽被输送到北方,导致中国北方降水增加,南方降水减少。PDO与PC2重建的ECP型相近,对ECP的影响集中在中纬度地区。二者回归得到中国东部低层北风异常,水汽在长江流域辐合,长江流域降水增加。 相似文献
18.
A near-global grid-point nudging of the Arpege-Climat atmospheric General Circulation Model towards ECMWF reanalyses is used to diagnose the regional versus remote origin of the summer model biases and variability over West Africa. First part of this study revealed a limited impact on the monsoon climatology compared to a control experiment without nudging, but a significant improvement of interannual variability, although the amplitude of the seasonal anomalies remained underestimated. Focus is given here on intraseasonal variability of monsoon rainfall and dynamics. The reproducible part of these signals is investigated through 30-member ensemble experiments computed for the 1994 rainy season, a year abnormally wet over the Sahel but representative of the model systematic biases. In the control experiment, Arpege-Climat simulates too few rainy days that are associated with too low rainfall amounts over the central and western Sahel, in line with the seasonal dry biases. Nudging the model outside Africa tends to slightly increase the number of rainy days over the Sahel, but has little effect on associated rainfall amounts. However, results do indicate that a significant part of the monsoon intraseasonal variability simulated by Arpege-Climat is controlled by lateral boundary conditions. Parts of the wet/dry spells over the Sahel occur in phase in the 30 members of the nudging experiment, and are therefore embedded in larger-scale variability patterns. Inter-member spread is however not constant across the selected summer season. It is partly controlled by African Easterly Waves, which show dissimilar amplitude from one member to another, but a coherent phasing in all members. A lowpass filtering of the nudging fields suggests that low frequency variations in the lateral boundary conditions can lead to eastward extensions of the African Easterly Jet, creating a favorable environment for easterly waves, while high frequency perturbations seem to control their phasing. 相似文献
19.
Twentieth century Sahel rainfall variability as simulated by the ARPEGE AGCM, and future changes 总被引：2，自引：2，他引：0
The ability of the ARPEGE AGCM in reproducing the twentieth century Sahelian drought when only forced by observed SST time
evolution has been characterized. Atmospheric internal variability is shown to have a strong contribution in driving the simulated
precipitation variability over the Sahel at decadal to multi-decadal time scales. The simulated drought is associated with
a southward shift of the continental rainbelt over central and eastern Sahel, associated with an inter-hemispheric SST mode
(the southern hemisphere oceans warming faster than the northern ones after 1970). The analysis of idealized experiments further
highlights the importance of the Pacific basin. The related increase of the tropospheric temperature (TT) over the tropics
is then suggested to dry the margin of convection zones over Africa, in agreement with the so-called “upped-ante” mechanism.
A simple metric is then defined to determine the ability of the CMIP3 coupled models in reproducing both the observed Sahel
drying and these mechanisms, in order to determine the reliability of the twenty-first century scenarios. Only one model reproduces
both the observed drought over the Sahel and consistent SST/TT relationships over the second half of the twentieth century.
This model predicts enhanced dry conditions over the Sahel at the end of the twenty-first century. However, as the mechanisms
highlighted here for the recent period are not stationary during the twenty-first century when considering the trends, similarities
between observed and simulated features of the West African monsoon for the twentieth century are a necessary but insufficient
condition for a trustworthy prediction of the future. 相似文献
20.
Summary Tropical North African climate variability is investigated using a Sahel rainfall index and streamflow of the Nile River in
the 20th century. The mechanisms that govern tropical North Africa climate are diagnosed from NCEP reanalysis data in the period 1958–1998:
spatially – using composite and correlation analysis, and temporally – using wavelet co-spectral analysis. The Sahelian climate
is characterised by a decadal rhythm, whilst the mountainous eastern and equatorial regions exhibit interannual cycles. ENSO-modulated
zonal circulations over the Atlantic/Pacific sector are important for decadal variations, and create a climatic polarity between
South America and tropical North Africa as revealed through upper-level velocity potential and convection patterns. A more
localised N–S shift in convection between the Sahel and Guinea coast is associated with the African Easterly Jet. 相似文献