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1.
本文探究了不同海表温度（SST）模态对6—8月和12月—次年2月全球陆地降水的趋势以及年代际变化的相对贡献。首先对热带地区陆地降水和SST进行SVD分析,得到影响陆地降水的趋势和年代际变化主要的海洋模态为：海洋中的全球变暖（Global Warming,GW）、大西洋多年代际振荡（Atlantic Multidecadal Oscillation,AMO）和太平洋多年代际振荡（Interdecadal Pacific Oscillation,IPO）。其次利用多元线性回归模型进一步定量评估了全球变暖、AMO和IPO对不同地区陆地降水的相对贡献大小。结果表明,全球变暖对陆地降水变化的贡献在冬夏季都是最大的,AMO在6—8月的贡献次之。IPO在12月—次年2月的贡献次之。不同纬度带,三者的贡献不同。GW的贡献在6—8月期间对10°N以北地区较大,南半球受GW的贡献相对较小,GW在12月—次年2月对40°N以北降水贡献异常显著;AMO主要在6—8月对10°～40°S和50°～60°S纬度带上的降水变化的贡献比较大;而IPO主要在12月—次年2月对北半球中纬度降水变化的贡献比较大。GW对许多地区降水变化的方差贡献都是最大的,例如6—8月期间,对北美洲东北部和亚洲降水变化贡献最大,12月—次年2月期间,对欧洲降水变化贡献最大。AMO对6—8月降水变化的方差贡献最大的区域为非洲萨赫勒、西伯利亚和南美洲。12月—次年2月期间,IPO对美国西南部的降水变化贡献最大,此外,北美洲东北部、南美洲西北部、非洲南部、澳大利亚东部、南亚季风区和我国北部的降水在12月—次年2月期间同样受IPO影响显著。进一步利用信息流的方法,探究了GW、AMO和IPO与陆地降水变化之间的因果关系,验证了上述结论。 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.
虽然中国降水以年际变化为主,但可利用奇异谱分析辨析出10—20 a、20—50 a 年代际变化的显著性区域以及>50 a 的长期趋势的显著性区域。本研究通过奇异值分解、多元线性回归等方法探究了1934—2018年不同海洋模态对6—8月（夏季）和12月—翌年2月（冬季）中国陆地降水趋势以及年代际振荡的相对贡献。通过对中国降水及中低纬度地区海温进行奇异值分解发现,不论冬夏,影响中国降水的主要模态是全球变暖,其次是太平洋年代际振荡。利用多元线性回归模型定量评估全球变暖、太平洋年代际振荡、大西洋多年代际振荡对中国不同区域降水的方差贡献及各因子的相对贡献,结果表明:夏季,三者可以解释西北和华北大约30%的年代际降水,其中全球变暖的相对贡献最大、太平洋年代际振荡次之;冬季,三者可以解释东北42%、西北和华北30%左右的年代际降水,东北和西北以全球变暖的相对贡献为主、大西洋多年代际振荡为辅,华北仍以全球变暖的影响为主、太平洋年代际振荡为辅。 相似文献
4.
Elsa Mohino Bel��n Rodr��guez-Fonseca Teresa Losada S��bastien Gervois Serge Janicot Juergen Bader Paolo Ruti Fabrice Chauvin 《Climate Dynamics》2011,37(9-10):1707-1725
Rainfall over West Africa shows strong interannual variability related to changes in Sea Surface Temperature (SST). Nevertheless, this relationship seem to be non-stationary. A particular turning point is the decade of the 1970s, which witnessed a number of changes in the climatic system, including the climate shift of the late 1970s. The first aim of this study is to explore the change in the interannual variability of West African rainfall after this shift. The analysis indicates that the dipolar features of the rainfall variability over this region, related to changes in the Atlantic SST, disappear after this period. Also, the Pacific SST variability has a higher correlation with Guinean rainfall in the recent period. The results suggest that the current relationship between the Atlantic and Pacific El Ni?o phenomena is the principal responsible for these changes. A fundamental goal of climate research is the development of models simulating a realistic current climate. For this reason, the second aim of this work is to test the performance of Atmospheric General Circulation models in simulating rainfall variability over West Africa. The models have been run with observed SSTs for the common period 1957?C1998 as part of an intercomparison exercise. The results show that the models are able to reproduce Guinean interannual variability, which is strongly related to SST variability in the Equatorial Atlantic. Nevertheless, problems in the simulation of the Sahelian interannual variability appear: not all models are able to reproduce the observed negative link between rainfall over the Sahel and El Ni?o-like anomalies in the Pacific, neither the positive correlation between Mediterranean SSTs and Sahelian rainfall. 相似文献
5.
A multi-model approach to the Atlantic Equatorial mode: impact on the West African monsoon 总被引：2，自引：0，他引：2
T. Losada B. Rodríguez-Fonseca S. Janicot S. Gervois F. Chauvin P. Ruti 《Climate Dynamics》2010,35(1):29-43
This paper is focused on the West African anomalous precipitation response to an Atlantic Equatorial mode whose origin, development and damping resembles the observed one during the last decades of the XXth century. In the framework of the AMMA-EU project, this paper analyses the atmospheric response to the Equatorial mode using a multimodel approach with an ensemble of integrations from 4 AGCMs under a time varying Equatorial SST mode. The Guinean Gulf precipitation, which together with the Sahelian mode accounts for most of the summer West African rainfall variability, is highly coupled to this Equatorial Atlantic SST mode or Atlantic Niño. In a previous study, done with the same models under 1958–1997 observed prescribed SSTs, most of the models identify the Equatorial Atlantic SST mode as the one most related to the Guinean Gulf precipitation. The models response to the positive phase of equatorial Atlantic mode (warm SSTs) depicts a direct impact in the equatorial Atlantic, leading to a decrease of the local surface temperature gradient, weakening the West African Monsoon flow and the surface convergence over the Sahel. 相似文献
6.
Results from nine coupled ocean-atmosphere simulations have been used to investigate changes in the relationship between the
variability of monsoon precipitation over western Africa and tropical sea surface temperatures (SSTs) between the mid-Holocene
and the present day. Although the influence of tropical SSTs on the African monsoon is generally overestimated in the control
simulations, the models reproduce aspects of the observed modes of variability. Thus, most models reproduce the observed negative
correlation between western Sahelian precipitation and SST anomalies in the eastern tropical Pacific, and many of them capture
the positive correlation between SST anomalies in the eastern tropical Atlantic and precipitation over the Guinea coastal
region. Although the response of individual model to the change in orbital forcing between 6 ka and present differs somewhat,
eight of the models show that the strength of the teleconnection between SSTs in the eastern tropical Pacific and Sahelian
precipitation is weaker in the mid-Holocene. Some of the models imply that this weakening was associated with a shift towards
longer time periods (from 3–5 years in the control simulations toward 4–10 years in the mid-Holocene simulations). The simulated
reduction in the teleconnection between eastern tropical Pacific SSTs and Sahelian precipitation appears to be primarily related
to a reduction in the atmospheric circulation bridge between the Pacific and West Africa but, depending on the model, other
mechanisms such as increased importance of other modes of tropical ocean variability or increased local recycling of monsoonal
precipitation can also play a role. 相似文献
7.
Seasonality and time scales in the relationship between global SST and African rainfall 总被引：1，自引：0，他引：1
The main goal of this study is to determine the oceanic regions corresponding to variability in African rainfall and seasonal differences in the atmospheric teleconnections. Canonical correlation analysis (CCA) has been applied in order to extract the dominant patterns of linear covariability. An ensemble of six simulations with the global atmospheric general circulation model ECHAM4, forced with observed sea surface temperatures (SSTs) and sea ice boundary variability, is used in order to focus on the SST-related part of African rainfall variability. Our main finding is that the boreal summer rainfall (June–September mean) over Africa is more affected by SST changes than in boreal winter (December–March mean). In winter, there is a highly significant link between tropical African rainfall and Indian Ocean and eastern tropical Pacific SST anomalies, which is closely related to El Niño-Southern Oscillation (ENSO). However, long-term changes are found to be associated with SST changes in the Indian and tropical Atlantic Oceans, thus, showing that the tropical Atlantic plays a critical role in determining the position of the intertropical convergence zone (ITCZ). Since ENSO is less in summer, the tropical Pacific and the Indian Oceans are less important for African rainfall. The African summer monsoon is strongly influenced by SST variations in the Gulf of Guinea, with a response of opposite sign over the Sahelian zone and the Guinean coast region. SST changes in the subtropical and extratropical oceans mostly take place on decadal time scales and are responsible for low-frequency rainfall fluctuations over West Africa. The modelled teleconnections are highly consistent with the observations. The agreement for most of the teleconnection patterns is remarkable and suggests that the modelled rainfall anomalies serve as suitable predictors for the observed changes. 相似文献
8.
Some drought years over sub-Saharan west Africa (1972, 1977, 1984) have been previously related to a cross-equatorial Atlantic gradient pattern with anomalously warm sea surface temperatures (SSTs) south of 10°N and anomalously cold SSTs north of 10°N. This SST dipole-like pattern was not characteristic of 1983, the third driest summer of the twentieth century in the Sahel. This study presents evidence that the dry conditions that persisted over the west Sahel in 1983 were mainly forced by high Indian Ocean SSTs that were probably remanent from the strong 1982/1983 El Ni?o event. The synchronous Pacific impact of the 1982/1983 El Ni?o event on west African rainfall was however, quite weak. Prior studies have mainly suggested that the Indian Ocean SSTs impact the decadal-scale rainfall variability over the west Sahel. This study demonstrates that the Indian Ocean also significantly affects inter-annual rainfall variability over the west Sahel and that it was the main forcing for the drought over the west Sahel in 1983. 相似文献
9.
本文研究了1934～2018年期间太平洋年代际振荡（Interdecadal Pacific Oscillation,IPO）、大西洋年代际振荡（Atlantic Multidecadal Oscillation,AMO）以及全球变暖（Global Warming,GW）对北美地区陆地降水年代际变化的相对贡献。首先通过对冬（12至次年2月）、夏季（6～8月）北美地区的陆地降水与中低纬地区的海表面温度进行奇异值分解分析,得到对北美陆地冬季降水相对贡献较大的主要海温模态为IPO（42.33%）和AMO（23.21%）,夏季则为AMO（32.66%）和IPO（21.60%）。其次利用线性回归模型,分析三种信号分别对北美冬、夏季陆地降水的相对贡献及对北美陆地不同区域降水的相对重要性,结果表明AMO对夏季北美陆地降水变化的贡献最大,IPO次之,冬季则相反,GW对冬夏季北美陆地降水都有一定的贡献。夏季期间阿拉斯加地区AMO的贡献最大,约占65.8%,加拿大地区GW的贡献最大,约占44.5%,美国本土及墨西哥地区三者贡献基本一致;冬季期间阿拉斯加和加拿大地区GW的贡献最大,分别为62.3%和44.7%,美国本土和墨西哥地区IPO的贡献最大,分别为47.9%和71.5%。进一步利用信息流方法,验证了IPO、AMO、GW对降水的敏感性区域。最后运用全球大气环流模式ECHAM 4.6进一步确定了太平洋和大西洋海温异常对北美地区陆地降水变化的影响途径,结果表明印度洋海表面温度异常在AMO和IPO对北美陆地降水变化的作用中至关重要。 相似文献
10.
Previous works suggest decadal predictions of Sahel rainfall could be skillful. However, the sources of such skill are still under debate. In addition, previous results are based on short validation periods (i.e. less than 50 years). In this work we propose a framework based on multi-linear regression analysis to study the potential sources of skill for predicting Sahel trends several years ahead. We apply it to an extended decadal hindcast performed with the MPI-ESM-LR model that span from 1901 to 2010 with 1 year sampling interval. Our results show that the skill mainly depends on how well we can predict the timing of the global warming (GW), the Atlantic multidecadal variability (AMV) and, to a lesser extent, the inter-decadal Pacific oscillation signals, and on how well the system simulates the associated SST and West African rainfall response patterns. In the case of the MPI-ESM-LR decadal extended hindcast, the observed timing is well reproduced only for the GW and AMV signals. However, only the West African rainfall response to the AMV is correctly reproduced. Thus, for most of the lead times the main source of skill in the decadal hindcast of West African rainfall is from the AMV. The GW signal degrades skill because the response of West African rainfall to GW is incorrectly captured. Our results also suggest that initialized decadal predictions of West African rainfall can be further improved by better simulating the response of global SST to GW and AMV. Furthermore, our approach may be applied to understand and attribute prediction skill for other variables and regions.
相似文献11.
Holocene climate modes are identified by the statistical analysis of reconstructed sea surface temperatures (SSTs) from the tropical and North Atlantic regions. The leading mode of Holocene SST variability in the tropical region indicates a rapid warming from the early to mid Holocene followed by a relatively weak warming during the late Holocene. The dominant mode of the North Atlantic region SST captures the transition from relatively warm (cold) conditions in the eastern North Atlantic and the western Mediterranean Sea (the northern Red Sea) to relatively cold (warm) conditions in these regions from the early to late Holocene. This pattern of Holocene SST variability resembles the signature of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO). The second mode of both tropical and North Atlantic regions captures a warming towards the mid Holocene and a subsequent cooling. The dominant modes of Holocene SST variability emphasize enhanced variability around 2300 and 1000 years. The leading mode of the coupled tropical-North Atlantic Holocene SST variability shows that an increase of tropical SST is accompanied by a decrease of SST in the eastern North Atlantic. An analogy with the instrumental period as well as the analysis of a long-term integration of a coupled ocean-atmosphere general circulation model suggest that the AO/NAO is one dominant mode of climate variability at millennial time scales. 相似文献
12.
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
13.
J. A. Adedoyin 《Meteorology and Atmospheric Physics》2000,75(3-4):135-147
Summary The growth rates of amplifying mid-tropospheric perturbations in tropical North Africa is known to reduce with increased
vertical shear in the troposphere. This phenomenon leads to a reduction in the frequency of generation of squall lines – the
main rain-producing mechanism in tropical North Africa – because squalls are initiated by amplifying modes of African Easterly
Waves (AEW). Ultimately, therefore, tropical North Africa experiences a shortfall, with respect to long-term averages, in
annual rainfall. Weakening of AEW intensity is shown to be linked with the warming up to the sea-surface temperatures (SST)
of the South Atlantic, Pacific and Indian Oceans. These findings are consistent with the observed reduction in the incidence
of intense hurricanes along the entire westem Atlantic in Sahelian dry years. It is shown that the frequency of occurrence
of Atlantic tropical storms and hurricanes is unaffected by the dryness or otherwise in the Sahel, but the paths of the storms
are determined by the zonal exit point, from the African continental land mass to the Atlantic, of West African disturbance
lines. These results have applications, and implications, in the level of preparedness for the economic impacts of Atlantic
storms and hurricanes.
Received June 8, 1996 Revised June 8, 2000 相似文献
14.
A regional climate model, the Weather Research and Forecasting (WRF) Model, is forced with increased atmospheric CO2 and anomalous SSTs and lateral boundary conditions derived from nine coupled atmosphere–ocean general circulation models to produce an ensemble set of nine future climate simulations for northern Africa at the end of the twenty-first century. A well validated control simulation, agreement among ensemble members, and a physical understanding of the future climate change enhance confidence in the predictions. The regional model ensembles produce consistent precipitation projections over much of northern tropical Africa. A moisture budget analysis is used to identify the circulation changes that support future precipitation anomalies. The projected midsummer drought over the Guinean Coast region is related partly to weakened monsoon flow. Since the rainfall maximum demonstrates a southward bias in the control simulation in July–August, this may be indicative of future summer drying over the Sahel. Wetter conditions in late summer over the Sahel are associated with enhanced moisture transport by the West African westerly jet, a strengthening of the jet itself, and moisture transport from the Mediterranean. Severe drought in East Africa during August and September is accompanied by a weakened Indian monsoon and Somali jet. Simulations with projected and idealized SST forcing suggest that overall SST warming in part supports this regional model ensemble agreement, although changes in SST gradients are important over West Africa in spring and fall. Simulations which isolate the role of individual climate forcings suggest that the spatial distribution of the rainfall predictions is controlled by the anomalous SST and lateral boundary conditions, while CO2 forcing within the regional model domain plays an important secondary role and generally produces wetter conditions. 相似文献
15.
Rainfall over eastern Africa (10°S–10°N; 35°E–50°E) is bimodal, with seasonal maxima during the "long rains" of March–April–May (MAM) and the "short rains" of October–November–December (OND). Below average precipitation during consecutive long and short rains seasons over eastern Africa can have devastating long-term impacts on water availability and agriculture. Here, we examine the forcing of drought during consecutive long and short rains seasons over eastern Africa by Indo-Pacific sea surface temperatures (SSTs). The forcing of eastern Africa precipitation and circulation by SSTs is tested using ten ensemble simulations of a global weather forecast model forced by 1950–2010 observed global SSTs. Since the 1980s, Indo-Pacific SSTs have forced more frequent droughts spanning consecutive long and short rains seasons over eastern Africa. The increased frequency of dry conditions is linked to warming SSTs over the Indo-west Pacific and to a lesser degree to Pacific Decadal Variability. During MAM, long-term warming of tropical west Pacific SSTs from 1950–2010 has forced statistically significant precipitation reductions over eastern Africa. The warming west Pacific SSTs have forced changes in the regional lower tropospheric circulation by weakening the Somali Jet, which has reduced moisture and rainfall over the Horn of Africa. During OND, reductions in precipitation over recent decades are oftentimes overshadowed by strong year-to-year precipitation variability forced by the Indian Ocean Dipole and the El Niño–Southern Oscillation. 相似文献
16.
Although previous studies reported upward trends in the basin-wide average lifetime, annual frequency, proportion of intense hurricanes and annual accumulated power dissipation index of Atlantic tropical cyclones (TCs) over the past 30?years, the basin-wide intensity did not increase significantly with the rising sea surface temperature (SST). Observational analysis and numerical simulation conducted in this study suggest that Sahel rainfall is the key to understanding of the long-term change of Atlantic TC intensity. The long-term changes of the basin-wide TC intensity are generally associated with variations in Sahara air layer (SAL) activity and vertical wind shear in the main development region (MDR), both of which are highly correlated with Sahel rainfall. The drying Sahel corresponds to an equatorward shift in the African easterly jet and African easterly wave activity, introducing the SAL to lower latitudes and increasing the MDR vertical wind shear. As a result, Atlantic TCs are more vulnerable to the suppressing effects of the SAL and vertical wind shear. Since the SST warming, especially in the tropical Indian Ocean, is a dominant factor for the Sahel drying that occurred over the past 30?years, it is suggested that the remote effect of SST warming is important for the long-term change of Atlantic TC intensity. Although influence of the AMO warm phase that started in the early 1990s alone can provide a favorable condition for TC intensification, its influence may have been offset by the influence of the ongoing SST warming, particularly in the Indian Ocean. As a result, there was no significant trend observed in the basin-wide average and peak intensity of Atlantic TCs. 相似文献
17.
The twentieth century record of the annual count of Atlantic tropical cyclones (TCs) is analyzed to develop consistent estimates of its natural variability and secular change components. The analysis scheme permits development of multidecadal trends from natural variability alone, reducing aliasing of the variability and change components. The scheme is rooted in recurrent variability modes of the influential SST field and cognizant of Pacific-Atlantic links. The origin of increased cyclone counts in the early 1930s, suppressed counts in 1950?C1960s, and the recent increase (since 1990s) is investigated using the count data set developed by Landsea et al. (J Clim 23: 2508?C2519, 2010). We show that annual TC counts can be more closely reconstructed from Pacific and Atlantic SSTs than SST of the main development region (MDR) of Atlantic TCs; the former accounting for ~60% of the decadal count variance as opposed to ~30% for MDR SST. Atlantic Multidecadal Oscillation (AMO) dominates the reconstruction, accounting for ~55% of the natural decadal count variance, followed by the ENSO Non-Canonical and Pan-Pacific decadal variability contributions. We argue for an expansive view of the domain of influential SSTs??extending much beyond the MDR. The additional accounting of count variance by SSTs outside the MDR suggests a role for remotely-forced influences over the tropical Atlantic: the Pan-Pacific decadal mode is linked with decreased westerly wind shear (200?C850?hPa) in its warm phase, much as the AMO impact itself. Non-canonical ENSO variability, in contrast, exerts little influence on decadal timescales. Interestingly, the secular but non-uniform warming of the oceans is linked with increased westerly shear, leading to off-setting dynamical and thermodynamical impacts on TC activity! The early-1930s increase in smoothed counts can be partially (~50%) reconstructed from SST natural variability. The 1950?C1960s decrease, in contrast, could not be reconstructed at all, leading, deductively, to the hypothesis that it results from increased aerosols in this period. The early-1990s increase is shown to arise both from the abatement of count suppression maintained by SST natural variability and the increasing SST secular trend contribution; the abatement is related to the AMO phase-change in early-1990s. Were it not for this suppression, TC counts would have risen since the early 1970s itself, tracking the secular change contribution. The analysis suggests that when SST natural variability begins to significantly augment counts in the post-1990 period??some evidence for which is present in the preceding decade??Atlantic TC counts could increase rapidly on decadal timescales unless offset by SST-unrelated effects which apparently account for a non-trivial amount (~40%) of the decadal count variance. 相似文献
18.
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
19.
Besides sea surface temperature (SST), soil moisture (SM) exhibits a significant memory and is likely to contribute to atmospheric
predictability at the seasonal timescale. In this respect, West Africa was recently highlighted as a “hot spot” where the
land–atmosphere coupling could play an important role, through the recycling of precipitation and the modulation of the meridional
gradient of moist static energy. Particularly intriguing is the observed relationship between summer monsoon rainfall over
Sahel and the previous second rainy season over the Guinean Coast, suggesting the possibility of a soil moisture memory beyond
the seasonal timescale. The present study is aimed at revisiting this question through a detailed analysis of the instrumental
record and a set of numerical sensitivity experiments. Three ensembles of global atmospheric simulations have been designed
to assess the relative influence of SST and SM boundary conditions on the West African monsoon predictability over the 1986–1995
period. On the one hand, the results indicate that SM contributes to rainfall predictability at the end and just after the
rainy season over the Sahel, through a positive soil-precipitation feedback that is consistent with the “hot spot” hypothesis.
On the other hand, SM memory decreases very rapidly during the dry season and does not contribute to the predictability of
the all-summer monsoon rainfall. Though possibly model dependent, this conclusion is reinforced by the statistical analysis
of the summer monsoon rainfall variability over the Sahel and its link with tropical SSTs. Our results indeed suggest that
the apparent relationship with the previous second rainy season over the Guinean Coast is mainly an artefact of rainfall teleconnections
with tropical modes of SST variability both at interannual and multi-decadal timescales. 相似文献
20.
A multi-model analysis of the role of the ocean on the African and Indian monsoon during the mid-Holocene 总被引：1，自引：6，他引：1
Y. Zhao P. Braconnot O. Marti S.P. Harrison C. Hewitt A. Kitoh Z. Liu U. Mikolajewicz B. Otto-Bliesner S.L. Weber 《Climate Dynamics》2005,25(7-8):777-800
We investigate the role of the ocean feedback on the climate in response to insolation forcing during the mid-Holocene (6,000 year
BP) using results from seven coupled ocean–atmosphere general circulation models. We examine how the dipole in late summer
sea-surface temperature (SST) anomalies in the tropical Atlantic increases the length of the African monsoon, how this dipole
structure is created and maintained, and how the late summer SST warming in the northwest Indian Ocean affects the monsoon
retreat in this sector. Similar mechanisms are found in all of the models, including a strong wind evaporation feedback and
changes in the mixed layer depth that enhance the insolation forcing, as well as increased Ekman transport in the Atlantic
that sharpens the Atlantic dipole pattern. We also consider changes in interannual variability over West Africa and the Indian
Ocean. The teleconnection between variations in SST and Sahelian precipitation favor a larger impact of the Atlantic dipole
mode in this region. In the Indian Ocean, the strengthening of the Indian dipole structure in autumn has a damping effect
on the Indian dipole mode at the interannual time scale. 相似文献