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1.
Global North Atlantic Oscillation (NAO) oceanic precipitation features in the latter half of the twentieth century are documented based on the intercomparison of multiple state-of-the-art precipitation datasets and the analysis of the NAO atmospheric circulation and SST anomalies. Most prominent precipitation anomalies occur over the ocean in the North Atlantic, where in winter a “quadrupole-like” pattern is found with centers in the western tropical Atlantic, sub-tropical Atlantic, high-latitude eastern Atlantic and over the Labrador Sea. The extent of the sub-tropical and high-latitude center and the amount of explained variance (over 50%) are quite remarkable. However, the tropical Atlantic center is probably the most intriguing feature of this pattern apparently linking the NAO with ITCZ variability. In summer, the pattern is “tripole-like” with centers in the eastern Mediterranean Sea, the North Sea/Baltic Sea and in the sub-polar Atlantic. In the eastern Indian Ocean, the correlation is positive in winter and negative in summer, with some link to ENSO variability. The sensitivity of these patterns to the choice of the NAO index is minor in winter while quite important in summer. Interannual NAO precipitation anomalies have driven similar fresh water variations in these “key” regions. In the sub-tropical and high-latitude Atlantic in winter precipitation anomalies have been roughly 15 and 10% of climatology per unit change of the NAO, respectively. Decadal changes of the NAO during the last 50 years have also influenced precipitation and fresh water flux at these time-scales, with values lower (higher) than usual in the high-latitude eastern North Atlantic (Labrador Sea) in the 1960s and the late 1970s, and an opposite situation since the early 1980s; in summer the North Sea/Baltic region has been drier than usual during the period 1965–1975 when the NAO was generally positive. 相似文献
2.
H. Feidas Ch. Noulopoulou T. Makrogiannis E. Bora-Senta 《Theoretical and Applied Climatology》2007,87(1-4):155-177
Summary In this study, the trends of annual and seasonal precipitation time series were examined on the basis of measurements of 22
surface stations in Greece for the period 1955–2001, and satellite data during the period 1980–2001. For this purpose, two
statistical tests based on the least square method and one based on the Mann-Kendall test, which is also capable of detecting
the starting year of possible climatic discontinuities or changes, are applied. Greece, in general, presents a clear significant
downward trend in annual precipitation for the period 1955–2001, which is determined by the respective decreasing trend in
winter precipitation. Both winter and annual series exhibit a downward trend with a starting year being 1984. Satellite-derived
precipitation time series could be an alternative means for diagnosing the variability of precipitation in Greece and detecting
trends provided that they have been adjusted by surface measurements in the wider area of interest. The relationship between
precipitation variability in Greece and atmospheric circulation was also examined using correlation analysis with three circulation
indices: the well-known North Atlantic Oscillation Index (NAOI), a Mediterranean Oscillation Index (MOI) and a new Mediterranean
Circulation Index (MCI). NAOI is the index that presented the most interesting correlation with winter, summer and annual
precipitation in Greece, whereas the MOI and MCI were found to explain a significant proportion of annual and summer precipitation
variability, respectively. The observed downward trend in winter and annual precipitation in Greece is linked mainly to a
rising trend in the hemispheric circulation modes of the NAO, which are connected with the Mediterranean Oscillation Index. 相似文献
3.
R. Nieto L. Gimeno L. De la Torre P. Ribera D. Barriopedro R. García-Herrera A. Serrano A. Gordillo A. Redaño J. Lorente 《Meteorology and Atmospheric Physics》2007,96(1-2):85-101
Summary An earlier developed multidecadal database of Northern Hemisphere cut-off low systems (COLs), covering a 41 years period (from
1958 to 1998) is used to study COLs interannual variability in the European sector (25°–47.5° N, 50° W–40° E) and the major
factors controlling it. The study focus on the influence on COLs interannual variability, of larger scale phenomena such as
blocking events and other main circulation modes defined over the Euro-Atlantic region. It is shown that there is a very large
interannual variability in the COLs occurrence at the annual and seasonal scales, although without significant trends. The
influence of larger scale phenomena is seasonal dependent, with the positive phase of the NAO favoring autumn COL development,
while winter COL occurrence is mostly related to blocking events. During summer, the season when more COLs occur, no significant
influences were found. 相似文献
4.
Precipitation (P) and freshwater (E-P) fluxes at the air-sea interface are investigated in the Atlantic Ocean sector using the reanalyses of the European Centre
for Medium Range Weather Forecasts (ERA) and of the National Centers for Environmental Prediction (NCEP). A canonical correlation
analysis method between these fields and sea level pressure (SLP) is used to identify patterns. We also test whether precipitation
and freshwater fluxes can be reconstructed from SLP data. In the winter months, patterns associated with both the North Atlantic
Oscillation (NAO) and the East Atlantic (EA) mode are identified. The signals are strong enough to be reconstructed from the
reanalysis fields, and they correspond to a significant part of the variability. The NAO signal is more robust than the EA
one. The NAO-related variability mode is also present when the monthly precipitation rate is averaged for the winter season
and even for annual averages. However, in the later case, other variability of natural origin (for instance, ENSO variability)
or noise from the model and assimilation system prevents the reconstruction of E-P associated with NAO from SLP variability. Difficulties are identified in the tropical Atlantic with a different behaviour
of NCEP and ERA precipitation variability, especially near the Inter Tropical Convergence Zone (ITCZ). The ERA patterns suggest
a NAO signature in the tropical Atlantic which has clear monthly patterns and indicates a link between the phase of NAO and
changes in the position and intensity of ITCZ. However, the analysis of winter rainfall based on satellite and in situ data
does not support the monthly tropical pattern of ERA precipitation although it suggests a relation between convection near
15°S and NAO during northern winter.
Received: 10 February 2000 / Accepted: 7 May 2001 相似文献
5.
Summary Series of annual and seasonal temperature and precipitation representing respectively northern and southern Italy are compared
for trend, interannual variability and periodicity in the period 1866–1995. Temperature and precipitation trends are almost
always anticorrelated except in winter in the North where an anomalous behavior is evident till about 1980. The result is
that the Italian climate has become warmer and drier especially in the South since about 1930.The interannual variability
does not present significant maxima, but only minima that cannot be related tothe start of a trend either for temperature
or for precipitation. The power spectra of the series show broad significant peaks containing the quasi-biennial oscillation
and other well known periodicities probably due to solar cycles or to the North Atlantic ocean-atmosphere oscillation (NAO).
Received December 16, 1998 Revised October 21, 1999 相似文献
6.
This paper analyses the role played by the North Atlantic Oscillation (NAO) in the creation of drought conditions in a semi-arid
region of north-east Spain (the middle Ebro valley), from 1600 to the year 2000. The study used documents from ecclesiastical
archives for the seventeenth, eighteenth and nineteenth centuries. For the twentieth century, instrumental precipitation records
were used as well. A December–August drought index from 1600 to 1900 was compiled from the historical documentary sources
(rogation ceremonies). The index was validated by means of precipitation records between 1858 and 1900 and independent precipitation
data from 1600 reconstructed by means of dendrochronological records. Using instrumental data a drought index was also calculated
(Standardized Precipitation Index, SPI) for the 1958–2000 period. We found that the NAO was important in explaining the droughts
identified in the study area from documents and instrumental data. Positive values of the winter NAO index are prone to cause
droughts in the middle Ebro valley. This finding has been verified since 1600 by means of two independent reconstructions
of the winter NAO index. The same behaviour has been observed during the nineteenth and twentieth centuries by means of instrumental
records. The climatic and geographic factors that explain the high influence of North Atlantic Oscillation on droughts in
this region are discussed in depth. 相似文献
7.
M. Rebetez 《Theoretical and Applied Climatology》2001,69(1-2):13-21
Summary An analysis of day-to-day variability was performed on two century-long daily minimum and maximum temperature series from
Switzerland. Warmer temperatures during the 20th century have been accompanied by a reduction in day-to-day variability, particularly for minimum temperatures and for winter.
There is a significant negative correlation between day-to-day variability and the skewness of the temperature distribution,
particularly in winter and for minimum temperatures. Lower variability is linked to a reduced number of cold days and nights.
Higher NAO index values tend to be associated not only with warmer temperatures but also with lower day-to-day variability.
This paper confirms that the temperature warming during the 20th century has happened mainly through the loss of the coldest part of the series, not only in the 24-hour or yearly cycle,
but also through the loss of the coldest episodes in each month.
Received September 20, 2000 Revised January 8, 2001 相似文献
8.
This study investigates the response of wintertime North Atlantic Oscillation (NAO) to increasing concentrations of atmospheric carbon dioxide (CO2) as simulated by 18 global coupled general circulation models that participated in phase 2 of the Coupled Model Intercomparison Project (CMIP2). NAO has been assessed in control and transient 80-year simulations produced by each model under constant forcing, and 1% per year increasing concentrations of CO2, respectively. Although generally able to simulate the main features of NAO, the majority of models overestimate the observed mean wintertime NAO index of 8 hPa by 5–10 hPa. Furthermore, none of the models, in either the control or perturbed simulations, are able to reproduce decadal trends as strong as that seen in the observed NAO index from 1970–1995. Of the 15 models able to simulate the NAO pressure dipole, 13 predict a positive increase in NAO with increasing CO2 concentrations. The magnitude of the response is generally small and highly model-dependent, which leads to large uncertainty in multi-model estimates such as the median estimate of 0.0061±0.0036 hPa per %CO2. Although an increase of 0.61 hPa in NAO for a doubling in CO2 represents only a relatively small shift of 0.18 standard deviations in the probability distribution of winter mean NAO, this can cause large relative increases in the probabilities of extreme values of NAO associated with damaging impacts. Despite the large differences in NAO responses, the models robustly predict similar statistically significant changes in winter mean temperature (warmer over most of Europe) and precipitation (an increase over Northern Europe). Although these changes present a pattern similar to that expected due to an increase in the NAO index, linear regression is used to show that the response is much greater than can be attributed to small increases in NAO. NAO trends are not the key contributor to model-predicted climate change in wintertime mean temperature and precipitation over Europe and the Mediterranean region. However, the models’ inability to capture the observed decadal variability in NAO might also signify a major deficiency in their ability to simulate the NAO-related responses to climate change. 相似文献
9.
本文基于中国地面气温日值网格数据集(V2.0),采用滑动相关和相关分析等方法,揭示了冬季北大西洋涛动(NAO)对中国北方极端低温影响的事实,进一步证实了东北后冬(1、2月)冷日(夜)与同期NAO相关性的年代际变化。研究发现:在20世纪80年代中期前,东北后冬冷日(夜)频发,与NAO的相关性较好,而在80年代中期后东北后冬冷日(夜)少发,与NAO的相关性减弱。其中,1月在1969~1988阶段,东北冷日(夜)与NAO的相关性最好,相关区域显著,相关系数可达-0.68(-0.66),而在1989~2009阶段二者相关性最弱,相关区域不显著。进一步分析发现,在不同年代际背景下,NAO引起的大气环流异常是导致东北1月冷日(夜)与1月NAO相关性年代际变化的重要原因。相关性较好的年代,NAO引起的环流异常有利于冷涡等天气系统维持在贝加尔湖到东北一带,使东北地区气温偏低,冷日(夜)频发;相关性较弱的年代,不利于冷空气南下,使东北地区气温偏高,冷日(夜)少发。 相似文献
10.
Summary ?The variability of the winter mean precipitation observed at 40 rainfall stations in Emilia-Romagna (a region in northern
Italy) from 1960 to 1995 is examined. The results are compared with those obtained for the whole of Italy using records from
32 stations. Temporal variability of the time series is investigated by means of Mann-Kendall and Pettitt tests, in order
to estimate the presence of trends and “change points”. Before analysis the original precipitation data set have been tested
to detect the inhomogeneity points, using the Standard Normal Homogeneity Test (SNHT). Almost all stations situated in Emilia
Romagna exhibit a significant decreasing trend in winter precipitation during the 1960–1995 period. The same characteristics
are revealed, more significant in the northern and central part of the region, when the stations for all Italy are analysed.
A significant downward shift in the winter precipitation is detected through the Pettitt test in Emilia Romagna, around 1980 at some stations, while the rest of the stations reveal the shift point occurrence around 1985. A significant downward shift in the winter precipitation is detected around 1985, when analysing the time series for all Italy. Spatial variability of winter precipitation is studied using the Empirical
Orthogonal Function. The first pattern indicates that a common large-scale process could be responsible for the winter precipitation
variability. The second EOF pattern shows an opposite sign of climate variability, which highlights the influence of relief
on the precipitation regime. The time series associated with the first precipitation pattern (PC1) at both scales emphasises
a significant decreasing trend and a downward shift point around 1985. The internal structure analysis of the North Atlantic
Oscillation (NAO) index during the 1960–1995 period reveals a significant increasing trend and an upward shift around 1980.
Strong correlation is also detected between PC1 (Emilia Romagna and at the scale of all Italy) and the NAO index. Thus, the
changes detected in the winter precipitation around 1985 could be due to an intensification of the positive phase of the [NAO],
especially after 1980.
Received March 23, 2001; revised February 20, 2002; accepted March 3, 2002 相似文献
11.
Interannual and long-term variability in the North Atlantic Oscillation and Indian Summer monsoon rainfall 总被引:5,自引:0,他引:5
Summary The interannual and decadal scale variability in the North Atlantic Oscillation (NAO) and its relationship with Indian Summer monsoon rainfall has been investigated using 108 years (1881–1988) of data. The analysis is carried out for two homogeneous regions in India, (Peninsular India and Northwest India) and the whole of India. The analysis reveals that the NAO of the preceding year in January has a statistically significant inverse relationship with the summer monsoon rainfall for the whole of India and Peninsular India, but not with the rainfall of Northwest India. The decadal scale analysis reveals that the NAO during winter (December–January–February) and spring (March–April–May) has a statistically significant inverse relationship with the summer monsoon rainfall of Northwest India, Peninsular India and the whole of India. The highest correlation is observed with the winter NAO. The NAO and Northwest India rainfall relationship is stronger than that for the Peninsular and whole of India rainfall on climatological and sub-climatological scales.Trend analysis of summer monsoon rainfall over the three regions has also been carried out. From the early 1930s the Peninsular India and whole of India rainfall show a significant decreasing trend (1% level) whereas the Northwest India rainfall shows an increasing trend from 1896 onwards.Interestingly, the NAO on both climatological and subclimatological scales during winter, reveals periods of trends very similar to that of Northwest Indian summer monsoon rainfall but with opposite phases.The decadal scale variability in ridge position at 500 hPa over India in April at 75° E (an important parameter used for the long-range forecast of monsoon) and NAO is also investigated.With 4 Figures 相似文献
12.
An ensemble of twenty-three 14-year experiments conducted with the ECHAM-4 GCM has been examined to test the model's capability
to simulate the principal modes of interannual variability. The integrations were performed under specified monthly SST between
1979–1993. The analysis was focused on the Southern Hemisphere (SH) extratropics. Empirical orthogonal functions analysis
(EOF) using seasonal anomaly fields has been performed to isolate the principal modes that dominate the southern extratropical
variability at the interannual time scale. Leading patterns of 500 hPa geopotential height (z500) have been compared with
those estimated from the ECMWF re-analysis dataset. The model is able to adequately reproduce the spatial pattern of the annular
mode, but it represents the temporal variations of the oscillation less satisfactorily. The model simulation of the Pacific
South American (PSA) pattern is better, both in the shape of the pattern and in the temporal evolution. To verify if the capability
of the model to adequately simulate the temporal oscillation of the propagating patterns is related to the increased influence
of the tropical external forcing, covarying SST-atmospheric modes have been identified by singular value decomposition (SVD).
In winter (July-August-September, JAS) the tropical SST variability is highly correlated with the ENSO mode. In summer (January-February-March,
JFM) the strength of the teleconnections is related to strong westerly anomalies, disrupted by a meridional out of phase relation
near to South America. The large size of the ensemble was exploited by comparing the time-varying model spread and degrees
of freedom of the simulated extratropical circulation. Results show that when the extratropical circulation has a few degrees
of freedom, the reproducibility is relatively low and the ensemble is governed by a fairly robust zonally symmetric structure
of dispersion.
Received: 9 May 2000 / Accepted: 30 January 2001 相似文献
13.
The space–time structure of the daily atmospheric variability in the South American monsoon system has been studied using multichannel singular spectrum analysis of daily outgoing longwave radiation. The three leading eigenmodes are found to have low-frequency variability while four other modes form higher frequency oscillations. The first mode has the same time variability as that of El Nino-Southern Oscillation (ENSO) and exhibits strong correlation with the Pacific sea surface temperature (SST). The second mode varies on a decadal time scale with significant correlation with the Atlantic SST suggesting an association with the Atlantic multidecadal oscillation (AMO). The third mode also has decadal variability but shows an association with the SST of the Pacific decadal oscillation (PDO). The fourth and fifth modes describe an oscillation that has a period of about 165 days and is associated with the North Atlantic oscillation (NAO). The sixth and seventh modes describe an intraseasonal oscillation with a period of 52 days which shows strong relation with the Madden-Julian oscillation. There exists an important difference in the variability of convection between Amazon River Basin (ARB) and central-east South America (CESA). Both regions have similar variations due to ENSO though with higher magnitude in ARB. The AMO-related mode has almost identical variations in the two regions, whereas the PDO-related mode has opposite variations. The interseasonal NAO-related mode also has variations of opposite sign with comparable magnitudes in the two regions. The intraseasonal variability over the CESA is robust while it is very weak over the ARB region. The relative contributions from the low-frequency modes mainly determine the interannual variability of the seasonal mean monsoon although the interseasonal oscillation may contribute in a subtle way during certain years. The intraseasonal variability does not seem to influence the interannual variability in either region. 相似文献
14.
We describe an improved tree-ring reconstruction of mean warm-season (November–April) temperatures for Tasmania from Huon
pine. This record extends back to 1600 BC and is based on a tree-ring chronology that was processed to retain as much low-frequency
variance as possible. The resulting reconstruction explains 46.6% of the variance and verifies significantly when compared
to withheld instrumental data. Cross-spectral analysis of actual and estimated temperatures over the 1886–1991 common period
indicates that most of the unexplained variance is at periods < 12 years in length. At periods > 12 years, the squared coherency
ranges between 0.6–0.8, and the cross-spectral gain indicates that the amplitude of the reconstruction is a nearly unbiased
estimate of the true temperature amplitude. Therefore, this reconstruction should be especially useful for studying multi-decadal
temperature variability in the Tasmanian sector of the Southern Hemisphere over the past 3592 years. To this end, we examined
the time evolution of low-frequency temperature amplitude fluctuations and found evidence for a 35% amplitude reduction after
AD 100 that persisted until about AD 1900. Since that time, the low-frequency temperature amplitude has systematically increased.
We also show how this reconstruction is related to large-scale sea surface temperatures (SST) in the Indian Ocean and eastward
to the dateline. Pointwise correlations between the Tasmanian record and SSTs reveal a relationship that extends across the
southern Indian Ocean and towards the Arabian Sea. This pattern is largely determined by inter-decadal temperature variability,
with correlations in this > 10-year bandwidth commonly exceeding 0.6 over most of the southern Indian and southwestern Pacific
sectors. A rotated empirical orthogonal function analysis reveals that the pattern of pointwise correlations found between
the temperature reconstruction and SSTs is largely explained by the linear combination of three orthogonal modes of SST variability.
Received: 12 January 1999 / Accepted: 31 July 1999 相似文献
15.
近百年东亚冬季气温及其大气环流变化型态 总被引:7,自引:2,他引:5
利用最新20世纪近百年再分析气象资料,研究近百年东亚冬季气温变化型及其相关的大气环流型态.结果表明近百年内东亚冬季气温主要有两种变化型:第一是东亚西南与东北相反气温变化型,表现在40°N以南及105°E以西地区(西南地区)气温变化与40°N以北及105°E以东地区(东北地区)变化相反;第二是40°N以南气温一致变化型.与第一种气温变化型耦合的大气模态是500hPa欧亚型遥相关、西伯利亚高压及北大西洋涛动.当欧亚型遥相关负位相,北大西洋涛动正位相及西伯利亚高压减弱时,有利于蒙古和我国105° E以东的区域增温而我国西南地区和青藏高原降温,反之亦然.第二种气温变化型耦合大气模态是500hPa西太平洋型遥相关,北太平洋涛动.当西太平洋型遥相关及北太平洋涛动处于正位相时(北太平洋北负南正),东亚40°N以南地区增温,东亚40°N以北地区降温.耦合的大气模态的型态差异,影响各阶段气温的年际变化.近一百年中,欧亚型遥相关和北大西洋涛动在1984~2010期间的型态最显著,是20世纪80年代东亚显著增暖的原因之一.研究还发现20世纪中期后东亚气温的年际变化与极地环流的变化联系紧密,表现在西伯利亚高压范围东扩并与极地环流联系,也是近百年气温趋势上升的一个原因. 相似文献
16.
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
17.
The aim of this paper is to re-examine and quantify a hypothesis first put forward by J. Bjerknes concerning the anomalous
coldness during the AD 1790–1820 period in western Europe. Central to Bjerknes’ hypothesis is an anomalous interaction between
ocean and atmosphere studied here using an ocean-atmosphere coupled climate model of intermediate complexity. A reconstruction
of the sea-level pressure pattern over the North Atlantic sector averaged over the period 1790–1820 is assimilated in this
model, using a recently developed technique which has not been applied to paleoclimatic modelling before. This technique ensures
that averaged over the simulation the reconstructed pattern is retrieved whilst leaving atmospheric and climatic variability to develop
freely. In accordance with Bjerknes’ hypothesis, the model results show anomalous southward advection of polar waters into
the northeastern North Atlantic in the winter season, lowering the sea-surface temperatures (SSTs) there with 0.3–1.0°C. This
SST anomaly is persistent into the summer season. A decrease in western European winter surface air temperatures is found
which can be related almost completely to advection of cold polar air. The decrease in summer surface air temperatures is
related to a combination of low SSTs and anomalous atmospheric circulation. The modelled winter and summer temperatures in
Europe compare favourably with reconstructed temperatures. Enhanced baroclinicity at the Atlantic seaboard and over Baffin
Island is observed along with more variability in the position of the North Atlantic storm tracks. The zone of peak winter
storm frequency is drawn to the European mid-latitudes.
In the original article (Climate Dynamics (2005) 24: 355-371; ) figures 4, 7, 8, 10, 11 and 13 were unfortunately incorrect. The correct version is shown here. 相似文献
18.
The relationship between winter sea ice variability and the North Atlantic Oscillation (NAO) is examined for the time period
1860–2300. This study uses model output to extend recently reported observational results to multi-century time scales. Nine
ensemble members are used in two Global Climate Models with forcing evolving from pre-industrial conditions through the so-called
A1B scenario in which carbon dioxide stabilizes at 720 ppm by 2100. Throughout, the NAO generates an east-west dipole pattern
of sea ice concentration (SIC) anomalies with oppositely signed centers of action over the Labrador and Barents Seas. During
the positive polarity of the NAO, SIC increases over the Labrador Sea due to wind-driven equatorward advection of ice, and
SIC decreases over the Barents Sea due to wind-driven poleward transport of heat within the mixed layer of the ocean. Although
this NAO-driven SIC variability pattern can always be detected, it accounts for a markedly varying fraction of the total sea
ice variability depending on the strength of the forced sea ice extent trend. For the first half of the 20th century or 1990
control conditions, the NAO-driven SIC pattern accounts for almost a third of the total SIC variance. In the context of the
long term winter sea ice retreat from 1860 to 2300, the NAO-driven SIC pattern is robustly observable, but accounts for only
2% of the total SIC variance. The NAO-driven SIC dipole retreats poleward with the retreating marginal ice zone, and its Barents
Sea center of action weakens. Results presented here underscore the idea that the NAO’s influence on Arctic climate is robustly
observable, but time dependent in its form and statistical importance. 相似文献
19.
Summary Dubbed Ice Storm ’98, an extreme weather event characterized by two synoptic systems in succession dropped about 70–100 mm
(in terms of water equivalent) of freezing precipitation over southeastern Ontario, southwestern Quebec and northeastern New
York during a 6-day period from January 5 to 10 in 1998. Individually, the two synoptic systems were not dramatically more
extreme in freezing precipitation than other major freezing rain events (4 since 1961) which occurred in the past over the
affected area. Some regions in the target area, however, were impacted more by the second system. Based on an analysis of
the 500 hPa vorticity field during the ’98 event, we suggest that the 1997/98 El Ni?o had a role in creating a flow environment
conducive to the rapid formation of the second synoptic system. In contrast, other major freezing rain events in the last
30 years involved only one synoptic system per event lasting no more than 3 days, and producing 20–50 mm of precipitation.
We have also found that, 3 out of 4 past major freezing rain events since 1958 were associated with the positive phase of
the North Atlantic Oscillation (NAO). Consistent with this usual past association between the NAO and a major freezing rain
event, Ice Storm ’98 also occurred when the phase of the NAO was positive. Analysis of these 3 past and the ’98 events also
indicates an apparent connection between the positive phase of the NAO and the northern Quebec high pressure system, which
is an essential synoptic feature of a major freezing rain occurrence over the southcentral region of Canada. As measured by
their respective indices, the maximum positive NAO state leads the maximum northern Quebec high by about 2 days (5 days in
the ’98 event). There is some suggestive evidence to indicate that the persistence of the northern Quebec high pressure system
is connected to the persistence of the positive phase of the NAO.
Received January 17, 2000 相似文献
20.
We address the issue of whether the Arctic (AO), and North Atlantic oscillations (NAO) are inseparable, forming an annular mode in the Northern Hemisphere atmospheric circulation. This annular mode is the leading empirical orthogonal function of hemispheric sea level pressure (SLP) data, explaining the largest amount of its variability. We examine whether the NAO and AO are inseparable spatial modes of the atmospheric circulation using rotated principal component analysis (RPCA), a methodology that identifies simple and unique patterns of spatial dataset variability. RPCA of the spring, summer, and autumn SLP fields from 1946-1998 reveal NAO and AO-like patterns, occurring as separate regional teleconnections forming the first and second principal components respectively. The RPCA-based NAO dipole pattern is like that observed in many prior studies, while the AO-like pattern exhibits high SLP variability over the Kara and Laptev seas. During winter however, and in annual analyses, a distinct AO-like pattern is not obtained and the two patterns may be inseparable using commonly accepted RPCA methods. The RPCA-based AO-like mode is significantly linked to north-central Siberian seasonal air temperatures and to the prevailing direction of motion of the underlying Arctic Ocean in summer, suggesting that the non-winter AO-like pattern, as a stand alone teleconnection separate from the NAO, contributes significantly to high-latitude climate and ocean variability. The winter NAO/AO inseparability is discussed as a possible effect of a shared winter storm track between the northeastern Atlantic and the Arctic. 相似文献