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1.
B. Alijani 《Theoretical and Applied Climatology》2002,72(1-2):41-54
Summary In order to explore the spatial and temporal variations of 500 hPa flow patterns and their relationship with the climate
of Iran, monthly mean geopotential heights for the region 0° E to 70° E and 20° N to 50° N, at 5 degree resolution, were analysed.
The study period covered the winter months October to March during the period 1961–90. The monthly height of the 500 hPa level
was averaged along each meridian from 25° N to 45° N. The height of the mean monthly pressure pattern was mapped against the
study years. The results showed that the characteristics of the 500 hPa flow pattern varied over monthly and annual time scales.
Principal Component Analysis, with S-mode and Varimax rotation, was also used to reduce the gridded data to 5 (6 in October)
significant factors. The factor scores for each month were then correlated with monthly Z-scores of precipitation and temperature
anomalies over Iran. The results showed that troughs and ridges located close to Iran had more influence on the climate of
Iran. Two troughs were identified and named the Caspian and Syrian troughs.
Received April 12, 2001 Revised July 24, 2001 相似文献
2.
Summary Reanalyzed 200 hPa velocity potential (χ) fields for the 1958–2005 period are used to examine the influences of the Pacific
Decadal Oscillation (PDO) in the average Hadley, Walker and monsoon circulations. These circulations are quantified and separated
from each other by computing their components contained in the 200 hPa χ fields for the 1958–1976 and 1977–1995 periods. The
main results found here are that the Hadley and monsoon circulations show small differences between the two periods, while
the Walker circulation shows weaker action centers during the second period, with its center in the western Pacific being
located further west. The differences in intensity and locations of the main action centers of the Walker circulation between
the first and second periods are analyzed here in terms of the PDO influences.
Authors’ address: Mary T. Kayano, Samia R. Garcia, Instituto Nacional de Pesquisas Espaciais/CPTEC, Avenida dos Astronautas
1758, 12227-010 S?o José dos Campos, SP, Brazil. 相似文献
3.
T. Ben-Gai A. Bitan A. Manes P. Alpert Y. Kushnir 《Theoretical and Applied Climatology》2001,69(3-4):171-177
Summary
Teleconnections associated with changing patterns of temperature and pressure anomalies over Israel during the second half
of the 20th century are investigated. Relatively high, statistically significant, correlation coefficients of −0.8 and +0.9
were found between the North Atlantic Oscillation (NAO) Index anomalies and smoothed (5 year running mean) cool season temperature
and surface pressure anomalies in Israel, respectively.
A relatively high positive correlation, (r = 0.8) was also found between the NAO Index anomalies and smoothed geopotential height of the 1000 hPa pressure level, during
the cool season at Bet Dagan radiosonde station located on the Israel Mediterranean coastal plain. Correlation coefficients
between NAO Index anomalies and the higher standard pressure levels, 850 and 700 hPa, decrease gradually and become negative
(not statistically significant) for the 500 hPa level.
Received January 25, 2000/Revised March 6, 2001 相似文献
4.
E. Bednorz 《Theoretical and Applied Climatology》2008,92(3-4):133-140
Summary Daily circulation patterns responsible for heavy snowfalls in the Polish – German lowlands were analysed. Composite maps of
sea level pressure (SLP) and 500 hPa geopotential height means and anomalies were constructed for the days with an increase
in snow cover depth by ≥5 cm. Contour maps show negative anomalies of SLP and 500 hPa level over central Europe, indicating
a low pressure system. Strong positive anomalies of SLP appear over Scandinavia and the northern Atlantic with the centre
of positive anomalies located over Iceland. Weaker negative anomalies are observed in the Azores region. This confirms the
strong negative correlation between snow cover appearance and the North Atlantic Oscillation index in Europe. The days with
heavy snowfalls were clustered using the Ward’s method. Three types of circulation patterns were distinguished, each of them
characterised by a low pressure system over central Europe. Type 3 represents the northern position of the low with its centre
over the Baltic Sea, Type 2 shows the southern position of the low with its centre over the Adriatic and the Ionic Sea and
Type 1 represents the low location between the two previous patterns with a wide meridional trough over the Atlantic.
Author’s address: Ewa Bednorz, Institute of Physical Geography and Environmental Planning, Adam Mickiewicz University, ul.
Dzięgielowa 27, 61-680 Poznań, Poland. 相似文献
5.
W. R. Skinner M. D. Flannigan B. J. Stocks D. L. Martell B. M. Wotton J. B. Todd J. A. Mason K. A. Logan E. M. Bosch 《Theoretical and Applied Climatology》2002,71(3-4):157-169
Summary
In Canada, the average annual area of burned forest has increased from around 1 million ha in the 1970’s to over 2.5 million
ha in the 1990’s. A previous study has identified the link between anomalous mid-tropospheric circulation at 500 hPa over
northern North America and wildland fire severity activity in various large regions of Canada over the entire May to August
fire season. In this study, a northern North American study region of the hemispheric gridded 5° latitude by 10° longitude
500 hPa dataset is identified and analysed from 1959 to 1996 for a sequence of six monthly periods through the fire season,
beginning in April and ending in September. Synoptic types, or modes of upper air behavior, are determined objectively by
the eigenvector method employing K-means cluster analysis. Monthly burned areas from the Canadian Large Fire Database (LFDB)
for the same period, 1959 to 1996, are analysed in conjunction with the classified monthly 500 hPa synoptic types. Relationships
between common monthly patterns of anomalous upper flow and spatial patterns of large fire occurrence are examined at the
ecozone level. Average occurrence of a monthly synoptic type associated with very large area burned is approximately 18% of
the years from 1959 to 1996. The largest areas burned during the main fire (May to August) season occur in the western Boreal
and Taiga ecozones – the Taiga Plains, Taiga Shield, Boreal West Shield and Boreal Plains. Monthly burned areas are also analysed
temporally in conjunction with a calculated monthly zonal index (Zim) for two separate areas defined to cover western and eastern Canada. In both western and eastern Canada, high area burned
is associated with synoptic types with mid-tropospheric ridging in the proximity of the affected region and low Zim with weak westerlies and strong meridional flow over western Canada.
Received April 3, 2001 Revised July 13, 2001 相似文献
6.
Raphael E. Okoola 《Meteorology and Atmospheric Physics》2000,73(3-4):177-187
Summary Climatological statistics of extreme temperature events over Kenya are established from the analysis of daily and monthly
maximum temperatures for a representative station (Nairobi Dagoretti Corner) over the period 1956–1997.
The months of June to August were shown to be the coldest with a mean monthly maximum temperature of less than 22 °C. Seasonal
(June to August) mean maximum temperature was 21.5 °C. Using this seasonal mean temperature for the period 1967–1997 delineated
1968 as the coldest year in this series and 1983 as the warmest year.
Spectral analysis of the seasonal data, for both the coldest and the warmest years, revealed that the major periods were the
quasi-biweekly (10 days) and the Intraseasonal Oscillations (23 days). Secondary peaks occurred at periods of 4–6 and 2.5–3.5
days.
A temperature threshold of 16.7 °C during July was used to define cold air outbreaks over Nairobi. This threshold temperature
of 16.7 °C was obtained from the mean July maximum temperature (20.9 °C) minus two standard deviations. Notable trends include
a decrease in the frequency of station-days, between 1956 and 1997, with temperatures less than 16.7 °C during July.
Surface pressure patterns indicate that the origin of the cold air is near latitude 25° S and to the east of mainland South
Africa. The cold air near 25° S is advected northwards ahead of the surface pressure ridge.
Received July 19, 1999 Revised January 11, 2000 相似文献
7.
利用中国气象局提供的1960—2019年江南区站点观测逐日降水数据,分析了江南春雨不同持续时长雨日的变化及其与欧亚大陆积雪的联系。结果表明,江南春雨以持续5 d及以上的长持续降水为主,但降水日数下降趋势明显,导致长持续降水减少。利用奇异值分解法(Singular Value Decomposition,SVD)发现,欧亚大陆3—5月积雪覆盖率与江南春雨雨日数有显著正相关关系。将(48°~59°N,90°~110°E)区域平均积雪覆盖率定义为积雪覆盖指数,通过指数与同期大气环流的回归分析发现,当积雪偏少时,我国中北部及西伯利亚地区500 hPa位势高度正异常,在江南区850 hPa风场和水汽通量场西南向负异常,导致江南春雨雨日数减少。合成分析进一步验证了积雪偏少会在江南区形成异常东北风抑制水汽输送至江南地区,不利于降水发生。 相似文献
8.
Based on January 1962–October 1993 mean value series of monthly mean temperature anomalies of 16 Antarctic stations on 10
standard isobaric surfaces from the surface to the 30 hPa, long term trends and periodic features of climate changes from
the troposphere to the lower stratosphere over the Antarctic region are investigated by maximum entropy power spectrum analysis,
and the relation between climate change of the stratosphere (troposphere) and total ozone (southern 500 hPa circulation) is
discussed.
This work is supported by the research item on the Theory and Method of Long Range Weather Forecasts. 相似文献
9.
W. R. Skinner B. J. Stocks D. L. Martell B. Bonsal A. Shabbar 《Theoretical and Applied Climatology》1999,63(1-2):89-105
Summary There is evidence that the area burned by wildland fire has increased in certain regions of Canada in recent decades. One
cause for this increase is changes in the mid-tropospheric circulation at 500 hPa over northern North America. This study
examines the physical links between anomalous mid-tropospheric circulation over various regions of Canada and wildland fire
severity. Analysis of monthly and seasonal burned areas for the period 1953 to 1995 reveals a bimodal distribution with distinct
low and extreme high burned area years. The high/low burned area years coincide with positive/negative 500 hPa height anomalies
over north-western, western, west-central and east-central Canada. Total area burned and the 500 hPa height anomaly data are
analyzed for statistical relationships using the Spearman rank correlation non-parametric measure. Results for the May to
August fire season indicate statistically significant correlations between regional total area burned and clusters of anomalous
500 hPa geopotential height values immediately over, and immediately upstream of the affected region. For the north-western
and west-central regions, significantly correlated clusters are found in the central Pacific as well, providing evidence of
the influence of a teleconnection structure on the summer climate of western and north-western North America. Two sample comparison
tests show statistically significant differences in both the means and variances of the fire data populations during negative
and positive phases of mid-tropospheric flow, and the means of the height anomaly populations during extremely high and extremely
low area burned seasons. Increases in regional total area burned are related to increases in mean 500 hPa heights, taken from
the significantly correlated clusters of height values, between two successive periods 1953–74 and 1975–95. For Canada as
a whole, the five lowest area burned seasons all occurred during the early period, while the five highest seasons occurred
during the later period. The difference in the geopotential height fields between the two periods identifies an increase in
500 hPa heights over most of Canada with an amplification of the western Canada ridge and an eastward shifted Canadian Polar
Trough (CPT).
Received October 19, 1998 相似文献
10.
The approach of getting useful information of monthly dynamical prediction from ensemble forecasts is studied. The extended
range ensemble forecasts (8 members, the initial perturbations of the lagged average forecast (LAF)(0000, 0600, 1200 and 1800
GMT in two consecutive days) of the 500 hPa height field with the global spectral model (T63L16) from January to May 1997
are provided by the National Climate Center of China. The relationship between the spread of ensemble measured by root–mean–square
deviation of ensemble member from ensemble mean and forecast skill (the anomaly correlation or the root–mean–square distance
between the ensemble mean forecast and the observation) is significant. The spread of ensemble can evaluate the useful forecast
days N for the best estimate of 30 days mean. Thus, a weighted mean approach based on ensemble spread is put forward for monthly
dynamical prediction. The anomaly correlation of the weighted monthly mean by the ensemble spread is higher than that of both
the arithmetic mean and the linear weighted mean. Better results of the monthly mean circulation and anomaly are obtained
from the ensemble spread weighted mean.
Supported by the Excellent National State Key Laboratory Project (49823002), the National Key Project ‘Study on Chinese Short-Term
Climate Forecast System’ (96-908-02) and IAP Innovation Foundation (8-1308).
The data were provided through the National Climate Center of China. The authors wish to thank Ms. Chen Lijuan for her assistance. 相似文献
11.
Summary Anomalously wet and dry months in the Mediterranean basin were identified during the period 1860–1990 from observations at
five stations located along the west-east axis of the Mediterranean basin (Barcelona, Florence, Malta, Athens and Jerusalem),
supplemented by data from Madrid and Lisbon. Wet and dry months were characterized by hydric indices (HI) based on values
of the standardized precipitation anomalies. Different patterns of anomalously wet and dry months were qualitatively identified
on the basis of the spatial distributions of the hydric indices. The standardized sea level pressure values at 56 grid points
in the domain 35° N–65° N, 30° W–40° E, for each of the anomalously wet and dry months, were subjected to T-mode Principal
Component Analysis.
The mean hydric indices associated with each principal component in each season are arranged in four distinct different spatial
distributions for wet months and in three for dry months as following: (a) Mediterranean wide distribution of positive/negative
anomalies; (b1) Strong positive anomalies to the west, but weaker to eastern Mediterranean; (b2) Strong negative anomalies
to the west, but weaker or normal to the east; (c1) Strong positive anomalies to the west and to the east and weaker ones
to the central Mediterranean; (c2) Negative anomalies to the west and east, but weaker, or normal, or positive to the central
Mediterranean; (d) Relatively strong positive anomalies to the east and weaker ones to the western Mediterranean.
Finally, monthly mean charts of standardized anomaly and mean sea level pressure are presented for each principalcomponent
in each season. These charts are used to interpret the spatial distribution of the positive and negative precipitation anomalies
in terms of mean circulation over the domain.
Received December 10, 1998 Revised June 14, 1999 相似文献
12.
A. P. Dimri 《Theoretical and Applied Climatology》2007,90(1-2):49-63
Summary The western Himalayas receive higher precipitation than the eastern Himalayas during the winter season (December–March). This
differential pattern of winter precipitation over the Himalayas can be attributed to topography and to a higher frequency
of disturbances over the western Himalayas, which result in variations in the circulation features. These circulation features,
in turn, result in variations in the meridional transport of heat, momentum, potential energy, and moisture across the Himalayas
due to mean and eddy motion.
Significant meridional transport due to mean motion takes place in the upper troposphere at 300 hPa and 200 hPa. Transport
east of 100° E dominates the transport over the western Himalayas. The eddy transport of heat, momentum, and potential energy
is considerably smaller than that due to mean motion. Eddy transport magnitudes are smaller up to 500 hPa and increase rapidly
aloft to 300 hPa and 200 hPa. Eddy transport over the western Himalayas is greater than over the eastern Himalayas. 相似文献
13.
Interannual variability in the onset of the summer monsoon over the Eastern Bay of Bengal 总被引:3,自引:0,他引:3
Summary Climatological characteristics associated with summer monsoon onset over the eastern Bay of Bengal (BOB) are examined in terms
of the westerly-easterly boundary surface (WEB). The vertical tilt of the WEB depends on the horizontal meridional temperature
gradient (MTG) near the WEB, under the constraint of the thermal wind balance. The switch in the WEB tilt firstly occurs between
90 and 100°E during the first pentad of May. At this time the 850 hPa ridgeline splits over the BOB and heavy rainfall commences
over the eastern BOB, indicating the onset of the BOB summer monsoon (BOBSM). The area-averaged MTG (200–500 hPa) is proposed
as an index to define the BOBSM onset. A comparison of the onset determined by the MTG, 850 hPa zonal wind, and outgoing longwave
radiation (OLR) shows that the MTG index is the most effective in characterizing the interannual variability of the BOBSM
onset.
Strong precursor signals are found prior to an anomalous BOBSM onset. Composite results show that early (late) BOBSM onset
follows excessive (deficient) rainfall over the western Pacific and anomalous lower tropospheric cyclonic circulation which
extends zonally from the northern Indian Ocean into the western Pacific, and strong (weak) equatorial westerly anomalies in
the preceding winter and spring. Prior to an early (late) BOBSM onset, significant positive (negative) thickness anomalies
exist around the Tibetan Plateau, accompanied by anomalous upper tropospheric anticyclonic (cyclonic) circulation. The interannual
variations of the BOBSM onset are significantly correlated with anomalous sea surface temperature related to ENSO. These occurs
through changes in the Walker circulation and local Hadley circulation, leading to middle and upper tropospheric temperature
anomalies over the Asian sector. The strong precursor signals around the Tibetan Plateau may be partly caused by local snow
cover anomalies, and an early (late) BOBSM onset is preceded by less (more) snow accumulation over the Tibetan Plateau during
the preceding winter. 相似文献
14.
The predictability of atmospheric responses to global sea surface temperature (SST) anomalies is evaluated using ensemble
simulations of two general circulation models (GCMs): the GENESIS version 1.5 (GEN) and the ECMWF cycle 36 (ECM). The integrations
incorporate observed SST variations but start from different initial land and atmospheric states. Five GEN 1980–1992 and six
ECM 1980–1988 realizations are compared with observations to distinguish predictable SST forced climate signals from internal
variability. To facilitate the study, correlation analysis and significance evaluation techniques are developed on the basis
of time series permutations. It is found that the annual mean global area with realistic signals is variable dependent and
ranges from 3 to 20% in GEN and 6 to 28% in ECM. More than 95% of these signal areas occur between 35 °S–35 °N. Due to the
existence of model biases, robust responses, which are independent of initial condition, are identified over broader areas.
Both GCMs demonstrate that the sensitivity to initial conditions decreases and the predictability of SST forced responses
increases, in order, from 850 hPa zonal wind, outgoing longwave radiation, 200 hPa zonal wind, sea-level pressure to 500 hPa
height. The predictable signals are concentrated in the tropical and subtropical Pacific Ocean and are identified with typical
El Ni?o/ Southern Oscillation phenomena that occur in response to SST and diabatic heating anomalies over the equatorial central
Pacific. ECM is less sensitive to initial conditions and better predicts SST forced climate changes. This results from (1)
a more realistic basic climatology, especially of the upper-level wind circulation, that produces more realistic interactions
between the mean flow, stationary waves and tropical forcing; (2) a more vigorous hydrologic cycle that amplifies the tropical
forcing signals, which can exceed internal variability and be more efficiently transported from the forcing region. Differences
between the models and observations are identified. For GEN during El Ni?o, the convection does not carry energy to a sufficiently
high altitude, while the spread of the tropospheric warming along the equator is slower and the anomaly magnitude smaller
than observed. This impacts model ability to simulate realistic responses over Eurasia and the Indian Ocean. Similar biases
exist in the ECM responses. In addition, the relationships between upper and lower tropospheric wind responses to SST forcing
are not well reproduced by either model. The identification of these model biases leads to the conclusion that improvements
in convective heat and momentum transport parametrizations and basic climate simulations could substantially increase predictive
skill.
Received: 25 April 1996 / Accepted: 9 December 1996 相似文献
15.
利用1958—2001年NCEP/NCAR再分析资料,探讨了热带太平洋(100°E~60°W,10°S~10°N)10 m风场的时空变化特征及其与东亚大气环流的可能联系。结果表明:1)热带太平洋风场异常存在两种主模态,第一模态对应中西太平洋一致的西(东)风异常,关于赤道呈准对称分布,与ENSO(El Nio-Southern Oscillation)暖(冷)位相时风场的分布对应;第二模态则关于赤道呈反对称分布,西北太平洋存在显著的反气旋(气旋)式环流,中太平洋异常西风不再位于赤道上,而是南移到了10°S左右,对应ENSO暖(冷)位相向相反位相转换时的风场分布特征。2)两模态时间系数的主振荡周期不同,与ENSO循环的位相关系也不同。研究发现,当两模态呈正(负)位相分布时,贝加尔湖南侧(South to Lake Baikal,SLB)容易发生持续的高压(低压)异常环流。3)两模态与SLB异常环流的联系途径不同。第一模态正位相对应热带中东太平洋大范围暖海温引起的二极型Walker环流异常,SLB异常高压不仅能通过东亚沿岸北风和南海低槽的作用促进第一模态的前期发展,还对其后期维持起重要作用。负位相时,情况相反。该环流系统既与热带中东太平洋大范围垂直运动有关,还与邻近的中国东南沿海低层异常辐合有关;第二模态则对应热带西太平洋及东印度洋为主、大西洋为辅的暖海温引起的热带四极型Walker环流异常。此时热带西太平洋到东印度洋局地偏强的经圈Hadley环流可能是SLB异常环流维持的主要原因。 相似文献
16.
A multi-model ensemble approach for assessment of climate change impact on surface winds in France 总被引:2,自引:1,他引:1
Statistical downscaling of 14 coupled atmosphere-ocean general circulation models (AOGCM) is presented to assess potential
changes of the 10 m wind speeds in France. First, a statistical downscaling method is introduced to estimate daily mean 10 m
wind speed at specific sites using general circulation model output. Daily 850 hPa wind field has been selected as the large
scale circulation predictor. The method is based on a classification of the daily wind fields into a few synoptic weather
types and multiple linear regressions. Years are divided into an extended winter season from October to March and an extended
summer season from April to September, and the procedure is conducted separately for each season. ERA40 reanalysis and observed
station data have been used to build and validate the downscaling algorithm over France for the period 1974–2002. The method
is then applied to 14 AOGCMs of the coupled model intercomparison project phase 3 (CMIP3) multi-model dataset. Three time
periods are focused on: a historical period (1971–2000) from the climate of the twentieth century experiment and two climate
projection periods (2046–2065 and 2081–2100) from the IPCC SRES A1B experiment. Evolution of the 10 m wind speed in France
and associated uncertainties are discussed. Significant changes are depicted, in particular a decrease of the wind speed in
the Mediterranean area. Sources of those changes are investigated by quantifying the effects of changes in the weather type
occurrences, and modifications of the distribution of the days within the weather types. 相似文献
17.
P. Domonkos 《Theoretical and Applied Climatology》1998,59(3-4):165-179
Summary Investigation of diurnal temperature data series in Hungary indicates that extreme anomalies often occur in groups. Periods
which include at least 3–6 extreme anomalous days with 3–5 times higher frequency relative to the climatological average are
referred to as periods of extremity (PE). Although PEs cover only 5–12% of the whole time-series (1901–1993), they include
40–90% of all days with extreme temperatures. This refers both to the positive and negative anomalies. This paper shows a
method for delimitation technique of PEs which is more suitable for the revelation of time-sequence structure of extreme temperature
value occurrences, than traditional investigations of daily or monthly values. As it was checked by Monte-Carlo simulations,
a one step autoregressive model, exhibiting nearly normal distribution gives a fairly good approach of extreme temperature
occurrences. However, significant differences between the statistical characteristics of real and simulated PEs are also experienced.
Statistical connections between PEs and macrocirculation are also investigated.
Received November 12, 1996 Revised July 6, 1997 相似文献
18.
Atmospheric circulation anomalies over the Ural Mountains are crucial indicators of the anomalous downstream weather and climate over East Asia. Here, we provide a new perspective on the mechanism of Ural circulation anomalies. We use a simple theoretical model to determine that the relationship between the solar forcing and three Ural circulation patterns, namely, neutral type, trough anomaly and ridge anomaly, is a nonlinear relationship following the supercritical pitchfork bifurcation theory. The theory predicts that when the total solar irradiance (TSI) is below a critical value, trough and ridge anomalies represent duplex equilibria and are equally likely to occur at the same TSI. Based on 180 winter months record, we have estimated the bidimensional probability density of TSI and the monthly mean geopotential height at 500 hPa or zonal wind at 850 hPa over the Ural Mountains. Results show that Sc = 1360.9 W m−2 is a critical value of TSI, the neutral type pattern is the single circulation regime when TSI > Sc, whereas trough and ridge anomaly patterns are duplex circulation regimes when TSI < Sc. Besides, when TSI < Sc, during the same TSI range, trough and ridge anomaly events occur at nearly the same frequencies. These results generally agree with the theoretical model. We demonstrate that trough and ridge anomalies, as duplex equilibria, result from the large-scale zonal flow interacting with the Ural Mountains. Low TSI tends to strengthen the large-scale zonal flow over the Ural Mountains, hence inducing either a trough anomaly or ridge anomaly. 相似文献
19.
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
20.
R. Villalba E. R. Cook R. D. D’Arrigo G. C. Jacoby P. D. Jones M. J. Salinger J. Palmer 《Climate Dynamics》1997,13(6):375-390
A tree-ring chronology network recently developed from the subantarctic forests provides an opportunity to study long-term
climatic variability at higher latitudes in the Southern Hemisphere. Fifty long (1911–1985), homogeneous records of monthly
mean sea-level pressure (MSLP) from the southern latitudes (15–65 °S) were intercorrelated on a seasonal basis to establish the most consistent, long-term Trans-Polar teleconnections during
this century. Variations in summer MSLP between the South America-Antarctic Peninsula and the New Zealand sectors of the Southern
Ocean are significantly correlated in a negative sense (r=−0.53, P<0.001). Climatically sensitive chronologies from Tierra del Fuego (54–55°) and New Zealand (39–47°) were used to develop verifiable reconstructions of summer (November to February) MSLP for both sectors of the Southern Ocean.
These reconstructions, which explain between 37 and 43% of the instrumentally recorded pressure variance, indicate that inverse
trends in MSLP from diametrically opposite sides of Antarctica have prevailed during the past two centuries. However, the
strength of this relationship varies over time. Differences in normalized MSLP between the New Zealand and the South America-Antarctic
Peninsula sectors were used to develop a Summer Trans-Polar Index (STPI), which represents an index of sea-level pressure
wavenumber one in the Southern Hemisphere higher latitudes. Tree-ring based reconstructions of STPI show significant differences
in large-scale atmospheric circulation between the nineteenth and the twentieth centuries. Predominantly-negative STPI values
during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the New Zealand
sector during the 1800s. In contrast, cyclonic activity appears to have been stronger in the mid-twentieth than previously
for the South American sector of the Southern Ocean. Recent variations in MSLP in both regions are seen as part of the long-term
dynamics of the atmosphere connecting opposite sides of Antarctica. A detailed analysis of the MSLP and STPI reconstructions
in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency
bands with a period around 3.3–3.6 y. Cross spectral analysis between the STPI reconstruction and the Southern Oscillation
Index suggests that teleconnections between the tropical ocean and extra-tropical MSLP variations may be influencing climate
fluctuations at southern latitudes.
Received: 18 December 1996/Accepted: 10 January 1997 相似文献