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1.
Daniel Steiner Andreas Pauling Samuel U. Nussbaumer Atle Nesje Jürg Luterbacher Heinz Wanner Heinz J. Zumbühl 《Climatic change》2008,90(4):413-441
A nonlinear backpropagation network (BPN) has been trained with high-resolution multiproxy reconstructions of temperature
and precipitation (input data) and glacier length variations of the Alpine Lower Grindelwald Glacier, Switzerland (output
data). The model was then forced with two regional climate scenarios of temperature and precipitation derived from a probabilistic
approach: The first scenario (“no change”) assumes no changes in temperature and precipitation for the 2000–2050 period compared
to the 1970–2000 mean. In the second scenario (“combined forcing”) linear warming rates of 0.036–0.054°C per year and changing
precipitation rates between −17% and +8% compared to the 1970–2000 mean have been used for the 2000–2050 period. In the first
case the Lower Grindelwald Glacier shows a continuous retreat until the 2020s when it reaches an equilibrium followed by a
minor advance. For the second scenario a strong and continuous retreat of approximately −30 m/year since the 1990s has been
modelled. By processing the used climate parameters with a sensitivity analysis based on neural networks we investigate the
relative importance of different climate configurations for the Lower Grindelwald Glacier during four well-documented historical
advance (1590–1610, 1690–1720, 1760–1780, 1810–1820) and retreat periods (1640–1665, 1780–1810, 1860–1880, 1945–1970). It
is shown that different combinations of seasonal temperature and precipitation have led to glacier variations. In a similar
manner, we establish the significance of precipitation and temperature for the well-known early eighteenth century advance
and the twentieth century retreat of Nigardsbreen, a glacier in western Norway. We show that the maritime Nigardsbreen Glacier
is more influenced by winter and/or spring precipitation than the Lower Grindelwald Glacier. 相似文献
2.
D. Narayana Rao H. R. Singh J. R. Kulkarni A. Y. Chandrika S. Vijaya Bhaskara Rao 《Meteorology and Atmospheric Physics》2000,73(1-2):55-59
Summary Mesosphere-Stratosphere-Troposphere (MST) Radar wind data for the period June through September 1996 have been examined to
study vertical variation of Madden-Jullian Oscillations in wind and eddy kinetic energy (eke) in the normal monsoon season.
The domain of analysis in the vertical is from 6 to 20 km with a height resolution of 150 m. Fast-Fourier-Transformation (FFT)
has been applied to zonal (u), meridional(v) components of wind to extract the Madden-Jullian oscillations and eke. There
are three dominant modes viz., 50–70, 30–40 and 10–20 day periodicity, which contain considerable fraction of energy and show
high degree of vertical variability. The peak amplitude of 50–70 day mode in u, 30–40 mode in v and eke were observed at 16–17 km
just below the tropopause level. The peak amplitudes of 30–40 day mode in u and 50–70 day mode in v were found in the height
region of 13–16 km. To understand the origin and propagation of these waves, wave energy is calculated. The wave energy is
higher at tropospheric heights than at lower stratospheric heights indicating that the origin of these waves is in the troposphere,
and a part of the energy leaks into the stratosphere.
Received September 17, 1998/Revised September 26, 1999 相似文献
3.
May–July Standardized Precipitation Index (SPI) for the land area of most of Turkey and some adjoining regions are reconstructed
from tree rings for the period 1251–1998. The reconstruction was developed from principal components analysis (PCA) of four
Juniperus excelsa chronologies from southwestern and south-central Turkey and is based on reliable and replicable statistical relationships
between climate and tree ring growth. The SPI reconstruction shows climate variability on both interannual and interdecadal
time scales. The longest period of consecutive drought years in the reconstruction (SPI threshold ≤−1) is 2 yr. These occur
in 1607–1608, 1675–1676, and 1907–1908. There are five wet events (SPI threshold ≥+1) of two consecutive years each (1330–1331,
1428–1429, 1503–1504, 1629–1630, and 1913–1914). A 5-yr moving average of the reconstructed SPI shows that two sustained drought
periods occurred from the mid to late 1300s and the early to mid 1900s. Both episodes are characterized by low variability. 相似文献
4.
The global mean surface temperature (GMST) response of HadCM3 to a 1,000 year 4×CO2 forcing is analysed using a transfer function methodology. We identify a third order transfer function as being an appropriate
characterisation of the dynamic relationship between the radiative forcing input and GMST output of this Atmosphere-Ocean
General Circulation Model (A-OGCM). From this transfer function the equilibrium climate sensitivity is estimated as 4.62 (3.92–11.88) K
which is significantly higher than previously estimated for HadCM3. The response is also characterised by time constants of
4.5 (3.2–6.4), 140 (78–191) and 1,476 (564–11,737) years. The fact that the longest time constant element is significantly
longer than the 1,000 year simulation run makes estimation of this element of the response problematic, highlighting the need
for significantly longer model runs to express A-OGCM behaviour fully. The transfer function is interpreted in relation to
a three box global energy balance model. It was found that this interpretation gave rise to three fractions of ocean heat
capacity with effective depths of 63.0 (46.7–85.4), 1291.7 (787.3–2,955.3) and 2,358.0 (661.3–17,283.8) meters of seawater,
associated with three discrete time constants of 4.6 (3.2–6.5), 107.7 (68.9–144.3) and 537.1 (196.2–1,243.1) years. Given
this accounts for approximately 94% of the ocean heat capacity in HadCM3, it appears HadCM3 could be significantly more well
mixed than previously thought when viewed on the millennial timescale. 相似文献
5.
Climate Dynamics - The second empirical orthogonal function mode (EOF2) of winter surface air temperature (SAT) over 0°–180° E, 40°–90° N during 1979–2005 is... 相似文献
6.
The variations noticed in the atmospheric electric field recorded at Pune (18o32’N, 73o51’E, 559 m ASL), a tropi-cal inland station located in Deccan Plateau, India, during the period 1930-1987, have been examined in relation to the variations observed in the Angstrom turbidity coefficient (β) and selected meteorological parameters. The month-ly and annual mean values of the atmospheric electric field, Angstrom turbidity coefficient (β), rainfall, temperature and relative humidity for the years 1930-1938, 1957-1958, 1964-1965, 1973-1974 and 1987 were considered in the study.The results of the above study indicated gradual increases in the atmospheric electric field over the period of study (1930-1987) which is statistically significant at less than 5% level. The increases noticed during different periods varied from 30 to 109 %. The increase noticed during the period (1930-1938) and (1973-1974) was maximum (109%). The Angstrom turbidity coefficient also showed systematic increases during the period of study, which is consistent. The diurnal curve of the atmospheric electric field at the station by and large, showed a double oscillation, which is generally observed in the continental environments. 相似文献
7.
The horizontal and vertical structure of the 3–5-day and 6–9-day easterly waves over West Africa and tropical Atlantic are
investigated. NCEP/NCAR reanalyses are used for the period 1979–1995 to produce a 17-year climatology of both 3–5-day and
6–9-day easterly waves. Composite patterns of convection, wind, temperature and vertical velocity are analysed with respect
to the following: the modulation by 3–5-day and 6–9-day wave regimes; the contrasts between the ITCZ (5°N–10°N) and the Sahelo-Saharan
band (15°N–20°N); the difference between land and ocean, and seasonal variations. Similarities and differences in the characteristics
of the two wave regimes are identified.
Received: 18 August 1999 / Accepted: 14 March 2001 相似文献
8.
Sensitivity studies of the RegCM3 simulation of summer precipitation, temperature and local wind field in the Caribbean Region 总被引:6,自引:1,他引:6
D. Martínez-Castro R. Porfirio da Rocha A. Bezanilla-Morlot L. Alvarez-Escudero J. P. Reyes-Fernández Y. Silva-Vidal R. W. Arritt 《Theoretical and Applied Climatology》2006,86(1-4):5-22
Summary We present a preliminary evaluation of the performance of three different cumulus parameterization schemes in the ICTP Regional
Climate Model RegCM3 for two overlapping domains (termed “big” and “small”) and horizontal resolutions (50 and 25 km) in the
Caribbean area during the summer (July–August–September). The cumulus parameterizations were the Grell scheme with two closure
assumptions (Arakawa–Schubert and Fritsch–Chappell) and the Anthes-Kuo scheme. An additional sensitivity test was performed
by comparing two different flux parameterization schemes over the ocean (Zeng and BATS).
There is a systematic underestimation of air temperature and precipitation when compared with analyzed data over the big domain
area. Greater (∼2 °C) and smaller (∼0.9 °C) negative temperature biases are obtained in Grell–FC and Kuo convective scheme,
respectively, and intermediate values are obtained in Grell–AS. The small domain simulation produces results substantially
different, both for air temperature and precipitation. Temperature estimations are better for the small domain, while the
precipitation estimations are better for the big domain.
An additional experiment showed that by using BATS to calculate the ocean fluxes in the big domain instead of the Zeng scheme,
precipitation increases by 25% and the share of convective precipitation rose from 18% to 45% of the total, which implies
a better simulation of precipitation. These changes were attributed to an increase of near surface latent heating when using
BATS over the ocean. The use of BATS also reduces the cold bias by about 0.3–0.4 °C, associated with an increase of minimum
temperature.
The behavior of the precipitation diurnal cycle and its relation with sea breeze was investigated in the small domain experiments.
Results showed that the Grell–Arakawa–Schubert closure describes better this circulation as compared with Grell–Fritsch–Chappell
closure. 相似文献
9.
Blanca Mendoza Virginia García-Acosta Victor Velasco Ernesto Jáuregui Rosa Díaz-Sandoval 《Climatic change》2007,83(1-2):151-168
Using unprecedented catalogues of past severe drought data for the Yucatan Peninsula between 1502 and 1900 coming from historical
written documentation, we identified five conspicuous time lapses with no droughts between 1577–1647, 1662–1724, 1728–1764,
1774–1799 and 1855–1880, as well as time epochs with most frequent droughts between 1800 and 1850. Moreover, the most prominent
periodicity of the historical drought time series was that of ∼40 years. Using the Palmer Drought Severity Index for the Yucatan
Peninsula for the period 1921–1987 we found prominent negative phases between ∼1942–1946 and 1949–1952, 1923–1924, 1928–1929,
1935–1936, 1962–1963, 1971–1972 and 1986–1987. Two prominent periodicities clearly appear at ∼5 and 10 years. Most modern
and historical severe droughts lasted 1 year, and share a quasi-decadal frequency. Also, in the first 66 years of the twentieth
century the frequency of occurrence of severe drought has been lower compared with the nineteenth century. Some of the major
effects and impacts of the most severe droughts in the Yucatan region are examined. We also studied the relation between historical
and modern droughts and several large scale climate phenomena represented by the Atlantic Multidecadal Oscillation (AMO) and
the Southern Oscillation Index (SOI). Our results indicate that historical droughts and the cold phase of the AMO coincide,
while the influence of the SOI is less clear. The strongest coherence between historical droughts and AMO occurred at periodicities
of ∼40 years. For modern droughts the coherence of a drought indicator (the Palmer Drought Severity Index) is similar with
AMO and SOI, although it seems more sustained with the AMO. They are strongest at ∼10 years and very clearly with the AMO
cold phase. Concerning the solar activity proxies and historical droughts, the coherence with a record of beryllium isotope
Be10, which is a good proxy of cosmic rays, is higher than with Total Solar Irradiance. We notice that the strongest coherence
between historical droughts and Be10 occurs at periods ∼60–64 years. When studying modern droughts and solar activity, frequencies of ∼8 years appear, and the
coherences are similar for both sunspots and cosmic rays. Comparing natural terrestrial and solar phenomena, we found that
the most sustained and strongest modulation of historical drought occurrence is at ∼60–64 years and is between the historical
drought series and the solar proxy Be10. For modern droughts we notice that the coherence is similar among AMO, SOI and the solar indices. We can conclude that the
sea surface temperatures (AMO) and solar activity leave their signal in terms of severe droughts in the Maya lands, however
in the long term, the influence of the SOI on this type of phenomenon is less clear. 相似文献
10.
Y. N. Samsonov 《Journal of Atmospheric Chemistry》2007,56(2):127-147
A chemical substance being in a high-disperse state (fine aerosol particles and very thin films) in the environment reveals
specific chemical and physicochemical features which differ from the processes in a relatively coarse disperse object and,
even more, in ordinary liquid and solid “test-tube” assays. The kinetics and the mechanism of the direct and sensitized photochemical
destruction of pesticide compound fipronil C12H4Cl2F6N4OS have been experimented as applied to the aerosol particles ≈0.12–1.3 μm in diameter and thin films ≈0.02–0.6 μm thick on
the glass plates. A non-photochemical (“dark”) reaction of fipronil molecules with the OH radicals which spontaneously proceeds
in the ambient air was also observed. Quantitative estimations based on experimental results show that the fipronil pollutant,
observed in the atmosphere in the form of levitated aerosols, can convert chemically in the above reaction with the OH radicals
for a very short time (from several minutes for a particle 2 μm in diameter to 12–24 h for a particle of 20–30 μm). The fipronil
residues presented on foliage either in the form of 1–20 μm films or as a group of deposited 2–30 μm aerosols react under
sunlight by two photochemical pathways (photooxidation and photodecay). The lifetime of these residues in the ambient conditions
is expected to be 11–25 days. Besides, adding a small amount of the Shirvanol 2 sensitizer to the fipronil formulation, one
can increase the overall decomposition rate to 8–12 days. 相似文献
11.
A time series of microwave radiometric profiles over Arctic Canada’s Cape Bathurst (70°N, 124.5°W) flaw lead polynya region
from 1 January to 30 June, 2008 was examined to determine the general characteristics of the atmospheric boundary layer in
winter and spring. A surface based or elevated inversion was present on 97% of winter (January–March) days, and on 77% of
spring (April–June) days. The inversion was the deepest in the first week of March (≈1100 m), and the shallowest in June (≈250 m).
The mean temperature and absolute humidity from the surface to the top of the inversion averaged 250.1 K (−23.1°C), and 0.56 × 10−3 kg m−3 in winter, and in spring averaged 267.5 K (−5.6°C), and 2.77 × 10−3 kg m−3. The median winter atmospheric boundary-layer (ABL) potential temperature profile provided evidence of a shallow, weakly
stable internal boundary layer (surface to 350 m) topped by an inversion (350–1,000 m). The median spring profile showed a
shallow, near-neutral internal boundary layer (surface to 350 m) under an elevated inversion (600–800 m). The median ABL absolute
humidity profiles were weakly positive in winter and negative in spring. Estimates of the convergence of sensible heat and
water vapour from the surface that could have produced the turbulent internal boundary layers of the median profiles were
0.67 MJ m−2 and 13.1 × 10−3 kg m−2 for the winter season, and 0.66 MJ m−2 and 33.4 × 10−3 kg m−2 for the spring season. With fetches of 10–100 km, these accumulations may have resulted from a surface sensible heat flux
of 15–185 W m−2, plus a surface moisture flux of 0.001–0.013 mm h−1 (or a latent heat flux of 0.7–8.8 W m−2) in winter, and 0.003–0.033 mm h−1 (or a latent heat flux of 2–22 W m−2) in spring. 相似文献
12.
Tree rings and climate for the last 680 years in Wulan area of northeastern Qinghai-Tibetan Plateau 总被引:3,自引:0,他引:3
A 680-year ring-width chronology of Sabina przewalskii Kom. was developed for Wulan area of northeastern Qinghai-Tibetan Plateau, China. Response function and correlation analyses
showed that spring precipitation (May–June) is the critical limiting factor for tree-ring growth, and temperature in prior
November may also play a role in affecting tree-ring growth. Excessive spring precipitation occurred during AD 1380s–1390s,
1410s–1420s, 1520s–1560s and 1938 to present. Dry springs occurred during AD 1430s–1510s, 1640s–1730s and 1780s–1890s most
of which generally coincided with cold intervals of the Little Ice Age (LIA) on the plateau, suggesting that the LIA climate
on the northeastern Qinghai-Tibetan Plateau might be characterized by three episodes of dry spring and cold autumn. The relatively
driest spring and probably coldest autumn occurred in AD 1710s–1720s, 1787–1797, 1815–1824, 1869–1879 and 1891–1895. The extreme
drought in AD 1787–1797 might result from little monsoon precipitation due to the failure of Asian monsoon in this period.
The tree-ring data produced in this study contribute to the spatial expansion of proxy climate records for the Qinghai-Tibetan
Plateau. 相似文献
13.
Sergej Zilitinkevich Hannu Savijärvi Alexander Baklanov Branko Grisogono Kai Myrberg 《Boundary-Layer Meteorology》2006,119(3):591-593
In this short communication we highlight the NATO Advanced Research Workshop (ARW) “Atmospheric Boundary Layers: Modelling and Applications for Environmental Security”, to be held in Dubrovnik, Croatia, 18–22 April 2006 (http:// pbl-nato-arw.dmi.dk) and the “Summer School on Air-Sea Interaction” to be held in Helsinki, Finland, 28 August–1 September 2006 (http://www.scasi.fi). These two events are connected to the ongoing Ev Marie Curie Chair Project “Planetary boundary layers – Theory, modelling and role in earth systems” (PBL – TMRES, Contract MEXC-CT-2003-509742, www.atm.helsinki.fi/PBL/). 相似文献
14.
Variations in Vegetation Net Primary Production in the Qinghai-Xizang Plateau, China, from 1982 to 1999 总被引:11,自引:1,他引:11
Vegetation net primary production (NPP) derived from a carbon model (Carnegie–Ames–Stanford Approach, CASA) and its interannual change in the Qinghai-Xizang (Tibetan) Plateau were investigated in this study using 1982–1999 time series data sets of normalized difference vegetation index (NDVI) and paired ground-based information on vegetation, climate, soil, and solar radiation. The 18-year averaged annual NPP over the plateau was 125 g C m−2 yr−1, decreasing from the southeast to the northwest, consistent with precipitation and temperature patterns. Total annual NPP was estimated between 0.183 and 0.244 Pg C over the 18 years, with an average of 0.212 Pg C (1 Pg = 1015 g). Two distinct periods (1982–1990 and 1991–1999) of NPP variation were observed, separated by a sharp reduction during 1990–1991. From 1982 to 1990, annual NPP did not show a significant trend, while from 1991 to 1999 a marked increase of 0.007 Pg C yr−2 was observed. NPP trends for most vegetation types resembled that of the whole plateau. The largest annual NPP increase during 1991–1999 appeared in alpine meadows, accounting for 32.3% of the increment of the whole region. Changes in solar radiation and temperature significantly influenced NPP variation, suggesting that solar radiation may be one of the major factors associated with changes in NPP. 相似文献
15.
Summary Average SST anomalies of OCT-DEC months for Nino-3 region are predicted using the following parameters – (i) April rain over
Himachal Pradesh, (ii) Darwin pressure change (January–April), (iii) Southern Oscillation Index (Tahiti–Darwin) and (iv) SST
anomalies of Nino-3 region in the month of May. Principal component analysis is used to orthogonalise the predictors before
using them in the regression equation. The first two principal components, which explain nearly 73% of the variance, are used
to fit a regression line. The period 1951–1985 is used as the calibration period for the model and the period 1986–1997 as
the verification period for the forecast.
The Brier score with respect to a reference forecast (persistence) for the independent period is found to be 0.82 which is
indicative of good forecast skill.
Received April 1, 1999 Revised January 17, 2000 相似文献
16.
Jan Esper Lara Klippel Paul J. Krusic Oliver Konter Christoph C. Raible Elena Xoplaki Jrg Luterbacher Ulf Bntgen 《Climate Dynamics》2020,54(3):1367-1382
The Mediterranean has been identified as particularly vulnerable to climate change, yet a high-resolution temperature reconstruction extending back into the Medieval Warm Period is still lacking. Here we present such a record from a high-elevation site on Mt. Smolikas in northern Greece, where some of Europe’s oldest trees provide evidence of warm season temperature variability back to 730 CE. The reconstruction is derived from 192 annually resolved, latewood density series from ancient living and relict Pinus heldreichii trees calibrating at r1911–2015 = 0.73 against regional July–September (JAS) temperatures. Although the recent 1985–2014 period was the warmest 30-year interval (JAS Twrt.1961–1990 = + 0.71 °C) since the eleventh century, temperatures during the ninth to tenth centuries were even warmer, including the warmest reconstructed 30-year period from 876–905 (+ 0.78 °C). These differences between warm periods are statistically insignificant though. Several distinct cold episodes punctuate the Little Ice Age, albeit the coldest 30-year period is centered during high medieval times from 997–1026 (− 1.63 °C). Comparison with reconstructions from the Alps and Scandinavia shows that a similar cold episode occurred in central Europe but was absent at northern latitudes. The reconstructions also reveal different millennial-scale temperature trends (NEur = − 0.73 °C/1000 years, CEur = − 0.13 °C, SEur = + 0.23 °C) potentially triggered by latitudinal changes in summer insolation due to orbital forcing. These features, the opposing millennial-scale temperature trends and the medieval multi-decadal cooling recorded in Central Europe and the Mediterranean, are not well captured in state-of-the-art climate model simulations. 相似文献
17.
The spatio-temporal variability in summer rainfall within eastern China is identified based on empirical orthogonal function
(EOF) analysis of daily rain-gauge precipitation data for the period 1979–2003. Spatial coherence of rainfall is found in
the Yangtze Basin, and a wavelet transform is applied to the corresponding principal component to capture the intraseasonal
oscillation (ISO) of Yangtze rainfall. The ensemble mean wavelet spectrum, representing statistically significant intraseasonal
variability, shows a predominant oscillation in summer Yangtze rainfall with a period of 20–50 days; a 10–20-day oscillation
is pronounced during June and July. This finding suggests that the 20–50-day oscillation is a major agent in regulating summer
Yangtze rainfall. Composite analyses reveal that the 20–50-day oscillation of summer Yangtze rainfall arises in response to
intraseasonal variations in the western North Pacific subtropical high (WNPSH), which in turn is modulated by a Rossby wave-like
coupled circulation–convection system that propagates northward and northwestward from the equatorial western Pacific. When
an anomalous cyclone associated with this Rossby wave-like system reaches the South China Sea (SCS) and Philippine Sea, the
WNPSH retreats northeastward due to a reduction in local pressure. Under these conditions, strong monsoonal southwesterlies
blow mainly toward the SCS–Philippine Sea, while dry conditions form in the Yangtze Basin, with a pronounced divergent flow
pattern. In contrast, the movement of an anomalous anticyclone over the SCS–Philippine Sea results in the southwestward extension
of the WNPSH; consequently, the tropical monsoonal southwesterlies veer to the northeast over the SCS and then converge toward
the Yangtze Basin, producing wet conditions. Therefore, the 20–50-day oscillation of Yangtze rainfall is also manifest as
a seesaw pattern in convective anomalies between the Yangtze Basin and the SCS–Philippine Sea. A considerable zonal shift
in the WNPSH is associated with extreme dry (wet) episodes in the Yangtze Basin, with an abrupt eastward (westward) shift
in the WNPSH generally leading the extreme negative (positive) Yangtze rainfall anomaly by a 3/8-period of the 20–50-day oscillation.
This finding may have implications for improving extended-range weather forecasting in the Yangtze Basin. 相似文献
18.
Ramzi Touchan Elena Xoplaki Gary Funkhouser Jürg Luterbacher Malcolm K. Hughes Nesat Erkan Ünal Akkemik Jean Stephan 《Climate Dynamics》2005,25(1):75-98
This study represents the first large-scale systematic dendroclimatic sampling focused on developing chronologies from different
species in the eastern Mediterranean region. Six reconstructions were developed from chronologies ranging in length from 115 years
to 600 years. The first reconstruction (1885–2000) was derived from principal components (PCs) of 36 combined chronologies.
The remaining five, 1800–2000, 1700–2000, 1600–2000, 1500–2000 and 1400–2000 were developed from PCs of 32, 18, 14, 9, and
7 chronologies, respectively. Calibration and verification statistics for the period 1931–2000 show good levels of skill for
all reconstructions. The longest period of consecutive dry years, defined as those with less than 90% of the mean of the observed
May–August precipitation, was 5 years (1591–1595) and occurred only once during the last 600 years. The longest reconstructed
wet period was 5 years (1601–1605 and 1751–1755). No long term trends were found in May–August precipitation during the last
few centuries. Regression maps are used to identify the influence of large-scale atmospheric circulation on regional precipitation.
In general, tree-ring indices are influenced by May–August precipitation, which is driven by anomalous below (above) normal
pressure at all atmospheric levels and by convection (subsidence) and small pressure gradients at sea level. These atmospheric
conditions also control the anomaly surface air temperature distribution which indicates below (above) normal values in the
southern regions and warmer (cooler) conditions north of around 40°N. A compositing technique is used to extract information
on large-scale climate signals from extreme wet and dry summers for the second half of the twentieth century and an independent
reconstruction over the last 237 years. Similar main modes of atmospheric patterns and surface air temperature distribution
related to extreme dry and wet summers were identified both for the most recent 50 years and the last 237 years. Except for
the last few decades, running correlation analyses between the major European-scale circulation patterns and eastern Mediteranean
spring/summer precipitation over the last 237 years are non-stationary and insignificant, suggesting that local and/or sub-regional
geographic factors and processes are important influences on tree-ring variability over the last few centuries. 相似文献
19.
Here we present a multi-proxy paleolimnological record from a closed-basin lake (Ebinur Lake) in northwestern China to investigate
climate change in this arid region during the last 1,500 years. The 120-cm long sediment core was dated by AMS radiocarbon
and 210Pb methods. The fine-grained clay sediments contain 3–17% organic matter (OM) and 9–31% carbonate, and are interrupted by
multiple sand and silt layers. These sand/silt layers, having consistently low OM, were found at 700–800, 1000–1100, 1300–1400,
and 1700–1750 a.d., with a time spacing of 300–400 years. We interpret that the low OM sand/silt layers were deposited during higher lake levels
caused by increased river inflow from the surrounding mountains during wet climate intervals. This interpretation is supported
by concurrent decreases in δ
18O and δ
13C of bulk carbonate and in carbonate content. Wet climate intervals at 700–800 a.d. and at 1700–1750 a.d. also correlate with elevated snow accumulation and low δ
18O from Guliya ice core on the NW Tibetan Plateau, both regions strongly influenced by the westerlies. This approximate 400-year
periodicity of wet–dry climate oscillations appear to correlate with solar activity as shown by atmosphere 14C concentration and with paleo-moisture records in interior North America. Our results suggest that solar activities might
have played a significant role in driving wet–dry climate oscillations at centennial scales in the interior of Eurasian continent. 相似文献
20.
Easterly wave regimes and associated convection over West Africa and tropical Atlantic: results from the NCEP/NCAR and ECMWF reanalyses 总被引:1,自引:0,他引:1
NCEP/NCAR and ECMWF daily reanalyses are used to investigate the synoptic variability of easterly waves over West Africa
and tropical Atlantic at 700 hPa in northern summer between 1979–1995 (1979–1993 for ECMWF). Spectral analysis of the meridional
wind component at 700 hPa highlighted two main periodicity bands, between 3 and 5 days, and 6 and 9 days. The 3–5-day easterly
wave regime has already been widely investigated, but only on shorter datasets. These waves grow both north and south of the
African Easterly Jet (AEJ). The two main tracks, noted over West Africa at 5 °N and 15 °N, converge over the Atlantic on latitude
17.5 °N. These waves are more active in August–September than in June–July. Their average wavelength/phase speed varies from
about 3000 km/8 m s-1 north of the jet to 5000 km/12 m s-1 south of the jet. Rainfall, convection and monsoon flux are significantly modulated by these waves, convection in the Inter-Tropical
Convergence Zone (ITCZ) being enhanced in the trough and ahead of it, with a wide meridional extension. Compared to the 3–5-day
waves, the 6–9-day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics.
The only main track is located north of the AEJ along 17.5 °N both over West Africa and the Atlantic. The mean wavelength
is higher, about 5000 km long, and the average phase speed is about 7 m s-1. Then the wind field perturbation is mostly evident at the AEJ latitude and north of it. The perturbation structure is similar
to that of 3–5-days in the north except that the more developed circulation centers, moving more to the north, lead to a large
modulation of the jet zonal wind component. South of the AEJ, the wind field perturbation is weaker and quite different. The
zonal wind core of the jet appears to be an almost symmetric axis in the 6–9-day wind field pattern, a clockwise circulation
north of the AEJ being associated with a counter-clockwise circulation south of the jet, and vice versa. These 6–9-day easterly
waves also affect significantly rainfall, convection and monsoon flux but in a different way, inducing large zonal convective
bands in the ITCZ, mostly in the trough and behind it. As opposed to the 3–5-day wave regime, these rainfall anomalies are
associated with anomalies of opposite sign over the Guinea coast and the Sahelian regions. Over the continent, these waves
are more active in June–July, and in August–September over the ocean. GATE phase I gave an example of such an active 6–9-day
wave pattern. Considered as a sequence of weak easterly wave activity, this phase was also a sequence of high 6–9-day easterly
wave activity. We suggest that the 6–9-day regime results from an interaction between the 3–5-day easterly wave regime (maintained
by the barotropic/baroclinic instability of the AEJ), and the development of strong anticyclonic circulations, north of the
jet over West Africa, and both north and south of the jet over the Atlantic, significantly affecting the jet zonal wind component.
The permanent subtropical anticyclones (Azores, Libya, St Helena) could help initiation and maintenance of such regime over
West Africa and tropical Atlantic. Based on an a priori period-band criterion, our synoptic classification has enabled us
to point out two statistical and meteorological easterly wave regimes over West Africa and tropical Atlantic. NCEP/NCAR and
ECMWF reanalyses are in good agreement, the main difference being a more developed easterly wave activity in the NCEP/NCAR
reanalyses, especially for the 3–5-day regime over the Atlantic.
Received: 28 May 1998 / Accepted: 2 May 1999 相似文献