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利用美国NOAA卫星观测的SOI(Southern Oscillation Index,南方涛动指数)资料以及NCEP/NCAR、CMAP月平均资料,采用相关分析等方法,研究了南方涛动年际变化与夏季亚澳季风环流及海洋性大陆区域气候异常的联系。结果表明:南方涛动具有显著的年际变化特征,这种年际变化对夏季亚澳季风区及海洋性大陆区域的环流、降水及温度异常有重要影响。当SOI正位相时,赤道以南的澳大利亚东部地区以及西北太平洋海域高层为气旋,低层为反气旋,赤道地区的东部太平洋低层为辐散中心,高层为辐合中心,有利于下沉运动维持;加里曼丹岛附近低层辐合,高层辐散,有利于上升运动维持;海洋性大陆地区降水为显著的正异常,东亚地区降水存在较弱的正异常;海洋性大陆地区以及我国青藏高原到东海一带温度为正异常,孟加拉湾及印度半岛区域温度为负异常。 相似文献
3.
不同太平洋年代际振荡和ENSO位相下大气水分收支变化对北半球冬季太平洋蒸发量的影响 总被引：1，自引：1，他引：0
本文利用OAFlux资料研究了1958~2015年北半球冬季太平洋蒸发量在不同厄尔尼诺—南方涛动（ENSO）和太平洋年代际振荡（PDO）位相下的分布特征,并从水汽收支的角度分析了蒸发量异常的成因,结果表明:ENSO主要影响热带东太平洋、副热带西北太平洋和中纬度北太平洋中部的蒸发量。El Ni?o（La Ni?a）时水汽在北太平洋中部异常辐散（辐合）,有利于当地大气水汽含量减小（增大）,造成蒸发量增大（减小）;副热带西北太平洋异常的水汽辐合（辐散）有利于蒸发量减小（增大）;除此以外,蒸发量在热带东太平洋蒸发量增大（减小）则主要是降水量增大（减小）导致。与此同时,ENSO对上述海区蒸发量的影响还受到PDO的调控,当PDO处于暖（冷）位相时,El Ni?o（La Ni?a）造成蒸发量异常程度在中纬度北太平洋中部显著增大,这主要是由降水量增大（减小）引起的大气水汽含量减小（增大）所致,此时对应着风暴轴异常增大（减小）;当PDO处于冷（暖）位相时,El Ni?o（La Ni?a）造成的蒸发量异常程度在副热带西北太平洋和热带东太平洋显著增大,而这与湿度变化引起的水汽平流异常程度增大紧密相关。 相似文献
4.
D. M. Sonechkin N. M. Astafyeva N. M. Datsenko N. N. Ivachtchenko B. Jakubiak 《Theoretical and Applied Climatology》1999,64(1-2):131-142
Summary The surface air temperature time series of both hemispheres and the North Atlantic European area as well as the Southern
Oscillation (SO) index time series were analysed using a wavelet transform technique. The values of the so-called singularity
exponents of these series were estimated and compared with such estimations for some surrogate time series artificially created
from the observed temperature series. It was concluded that the climate dynamics on interannual and interdecadal scales may
be considered as a kind of classical Brownian motion although its consideration as a flicker-noise is also possible. The extracted
temperature variations were shown to be closely coupled with the SO process. The wavelet-transformed SO series reveals itself
as a whole self-similar “tree” the main branches of which are the appearances of the strongest El-Ninos of 1898 – 1899, 1941 – 1942,
and 1982 – 1983. Similar “trees” can be seen in the wavelet-transformed temperature series. Thus, the extracted temperature
variations were shown to be closely coupled with the SO process, and a decomposition of the current global climate dynamics
into three climatic epochs (of about 40-year long) seems to be appropriate.
Received May 4, 1998 Revised April 25, 1999 相似文献
5.
This work concerns an analysis of inter-basin and inter-layer exchanges in the component ocean part of the coupled ECHAM4/OPYC3
general circulation model, aimed at documenting the simulation of North Atlantic Deep Water (NADW) and related thermohaline
circulations in the Indian and Pacific Oceans. The modeled NADW is formed mainly in the Greenland– Iceland–Norwegian Seas
through a composite effect of deep convection and downward cross-isopycnal transport. The modeled deep-layer outflow of NADW
can reach 16 Sv near 30 °S in the South Atlantic, with the corresponding upper-layer return flow mainly coming from the “cold
water path” through Drake Passage. Less than 4 Sv of the Agulhas “leakage” water contributes to the replacement of NADW along
the “warm water path”. In the South Atlantic Ocean, the model shows that some intermediate isopycnal layers with potential
densities ranging between 27.0 and 27.5 are the major water source that compensate the NADW return flow and enhance the Circumpolar
Deep Water (CDW) flowing from the Atlantic into Indian Ocean. The modeled thermohaline circulations in the Indian and Pacific
Oceans indicate that the Indian Ocean may play the major role in converting deep water into intermediate water. About 16 Sv
of the CDW-originating deep water enters the Indian Ocean northward of 31 °S, of which more than 13 Sv “upwell” mainly near
the continental boundaries of Africa, South Asia and Australia through inter-layer exchanges and return to the Antarctic Circumpolar
Current (ACC) as intermediate-layer water. As a contrast, only 4 Sv of Pacific intermediate water is connected to “upwelling”
flow southward across 31 °S while the magnitude of northward deep flow across 31 °S in the Pacific Ocean is significantly
greater than that in the Indian Ocean. The model suggests that a large portion of the deep waters entering the Pacific Ocean
(about 14 Sv) “upwells” continually into some upper layers through the thermocline, and becomes the source of the Indonesian
throughflow. Uncertainties in these results may be related to the incomplete adjustment of the model’s isopycnal layers and
the sensitivity of the Indonesian throughflow to the model’s geography and topography.
Received: 12 August 1997/Accepted: 12 March 1998 相似文献
6.
Summary This paper is to promote a further understanding of the interdecadal mode of the South Pacific. With this focus, we will specifically
aim at better understanding the difference between interannual and interdecadal SSTA modes over South Pacific. We define the
difference of the normalization area-averaged SSTA in the southern extratropical Pacific (160° W–110° W, 40° S–25° S) and
the south subpolar Pacific (150° W–110° W, 60° S–45° S) as the South Pacific interdecadal index (I
spd). It is found that the interannual mode is more coherent than the interdecadal mode in the central and eastern tropical Pacific,
and the interdecadal mode is significant only during boreal winter (DJF). The interdecadal variation of SSTA firstly occurring
in the extratropic South Pacific propagates to the western boundary of the South Pacific, then moves northeast to cross the
equator, and finally reaches the central tropic Pacific. It takes about 8 years to propagate from southeast subtropical Pacific
to the north hemisphere. The previous studies have suggested the mechanism of waves in the subsurface in the South Pacific.
Our study also highlights the Rossby waves play important roles in linkage between the extratropics-tropics South Pacific
SSTA on interdecadal time scales. Moreover, the paper shows that the interdecadal variability originated in the extrotropic
southeast Pacific is mainly induced by interannual variability in the tropic Pacific. 相似文献
7.
The mid-Atlantic trough (MAT) is one of the most prominent circulation systems over the subtropical North Atlantic during the boreal summer, and it can be viewed as a bridge linking the climate in the American-Atlantic-Eurasian region. The upper-tropospheric MAT attains its maximum intensity of 200–150?hPa in June and July. An index measuring the variability of MAT intensity is defined, which reveals significant interannual and interdecadal variations of the trough.On interannual time scales, the variation of MAT is significantly associated with the North Atlantic Oscillation, a southeastward propagating stationary wave that possibly originates from the northeastern Pacific, and the Atlantic Meridional Mode. A stronger trough is associated with warmer surface temperatures and higher pressure over central-northern North America and the extratropical North Atlantic and with colder surface temperatures and lower pressure over the Arctic, the subtropical North Atlantic, and the northeastern Pacific. In the meantime, significant negative precipitation anomalies occur over the north of the Mediterranean Sea and the Black Sea, as well as the northeastern Atlantic because of the anomalous low-level northeasterly winds over these areas. On an interdecadal time scale, the variation of MAT seems to be related to the Atlantic Multi-decadal Oscillation. Warmer surface temperatures appear over almost the entire North Atlantic, southern Europe, East Asia, and the North Pacific during the weak phase of the trough. A weak trough is also associated with the dipole pattern of anomalous precipitation over the extratropical North Atlantic, Greenland, and northeastern North America, corresponding to a dipole of low-level atmospheric circulation over these regions. 相似文献
8.
S. Das A. K. Mitra G. R. Iyengar S. Mohandas 《Meteorology and Atmospheric Physics》2001,78(3-4):227-244
Summary
The global spectral model of NCMRWF at T80 horizontal resolution and 18 vertical levels has been integrated for the summer
season (July) using different cumulus parameterization schemes namely, the Simplified Arakawa-Schubert scheme (SAS), the Relaxed
Arakawa-Schubert Scheme (RAS), and the Kuo-type cumulus parameterization scheme (KUO). The results have been compared with
mean analysis of the operational NCMRWF model (ANA) and other available observations.
Results indicate that, while the global distributions of basic fields such as the wind, temperature and moisture are fairly
well simulated by all the three schemes, there are many differences seen in the simulation of the typical features of the
Indian summer monsoon. The strength of the Low Level Westerly Jet (LLWJ), the Cross Equatorial Flow (CEF), and the Tropical
Easterly Jet (TEJ) are better simulated by RAS and SAS as compared to ANA than the KUO scheme. RAS and SAS produce strong
rising motion owing to strong intensity of convection produced by these two schemes. This in turn produces stronger Hadley
cell by RAS and SAS than compared to the KUO scheme. Simulation of the 200 mb velocity potential and divergent wind by RAS
and SAS produced two prominent centers, one in the Bay of Bengal and another in the Western Pacific, which correspond to the
intense latent heating by cumulus convection during the active monsoon phase. The velocity potential and divergent winds were
weaker in KUO, than compared to RAS and SAS.
The simulation of OLR is improved by RAS as compared to observations. The cold bias produced by KUO at 200 mb is reduced by
RAS and is substantially improved by SAS. Study of observed and simulated rainfall indicated that RAS and SAS produced better
distribution of precipitation over the Western Ghat Mountains and the Arakan coast, where deep cumulus convection is produced
due to orographic forcing of the warm moist air. The KUO scheme underestimated the rainfall over these two regions, but produced
slightly better distribution of rainfall over the northwest and central India, where the intensity of convection is relatively
weaker.
Evaluation of overall dynamics, thermal structure and rainfall indicates that in general, SAS is able to provide relatively
better results compared to other two schemes.
Received October 3, 2000/Revised December 5, 2000 相似文献
9.
Demarcating the worldwide monsoon 总被引：10，自引：1，他引：10
Summary The monsoon is a global climate phenomenon. This paper addresses the seasonal reversal of atmospheric circulation and the
transition of dry/wet spells in the monsoon regions worldwide. The NCEP/NCAR 850 hPa wind reanalysis data for 1950–1999 and
the upper-tropospheric water vapour (UTWV) band brightness temperature (BT) data observed by NOAA polar orbiting satellites
for 1980–1995 are used. In the tropics, there are three large wet-UTWV centres. The summer monsoon in the subtropics can be
defined as the expansion of these centres associated with the influence of cross-equatorial flows. Specifically, the dry/wet
spell transition is determined by the BT values that are smaller than 244 K. The regions of the South and North African monsoons,
the Asian-northwest Pacific and Australian-Southwest Pacific monsoons, and the North and South American monsoons are examined
with a focus on the dry/wet spell transition and stream field features.
Findings suggest that the summer monsoons over many subtropical regions can be defined by both cross-equatorial flows and
dry/wet spell transitions. In the mid- and low-latitudes, there exist six major dry/wet spell transition regions with a dry
or wet period lasting more than one month. The region of most significant change is located over the subtropical North Africa–Asia–northwest
Pacific. The others appear over subtropical South Africa, Indonesia–Australia, southwest Pacific, the Mexico-Caribbean Sea,
and subtropical South America. In addition, three regions exist where only one of the two indicators (cross-equatorial flow
and dry/wet transition) is satisfied. The first is near the Equator where the directions of cross-equatorial flows alternate
but a dry/wet spell transition is never experienced. The second is over North Africa where only the dry/wet spell transition
can be found but not the cross-equatorial flows. The third is over the mid-latitude regions in North China, South Africa,
and northern North America. These regions are influenced by cross-equatorial flows but the upper-tropospheric water vapour
content is not as high as that in tropical regions.
Received June 29, 2000 Revised May 15, 2001 相似文献
10.
Many coupled ocean–atmosphere general circulation models (GCMs) suffer serious biases in the tropical Atlantic including a
southward shift of the intertropical convergence zone (ITCZ) in the annual mean, a westerly bias in equatorial surface winds,
and a failure to reproduce the eastern equatorial cold tongue in boreal summer. The present study examines an ensemble of
coupled GCMs and their uncoupled atmospheric component to identify common sources of error. It is found that the westerly
wind bias also exists in the atmospheric GCMs forced with observed sea surface temperature, but only in boreal spring. During
this time sea-level pressure is anomalously high (low) in the western (eastern) equatorial Atlantic, which appears to be related
to deficient (excessive) precipitation over tropical South America (Africa). In coupled simulations, this westerly bias leads
to a deepening of the thermocline in the east, which prevents the equatorial cold tongue from developing in boreal summer.
Thus reducing atmospheric model errors during boreal spring may lead to improved coupled simulations of tropical Atlantic
climate. 相似文献
11.
J. Räisänen 《Climate Dynamics》1997,13(3):197-211
Four transient GCM experiments simulating the climatic response to gradually increasing CO2, and two equilibrium doubled CO2 experiments are compared. The zonally symmetric and asymmetric features of climate are both examined. Surface air temperature,
sea level pressure, the 500 mb height and the relative topography between 500 and 1000 mb are analyzed. In the control simulations,
the broad aspects of the present climate are in most cases well reproduced, although the stationary eddies tend to be less
reliably simulated than the zonal means. However, the agreement between the four transient experiments on the geographical
patterns of climate change is less impressive. While some zonally symmetric features, in particular the meridional distribution
of surface air warming in the boreal winter, are rather similar in all models, the intermodel cross correlations for the zonally
asymmetric changes are low. The agreement is largely restricted to some very general features such as more warming over the
continents than over the oceans. The largest discrepancies between the two equilibrium-doubled CO2 experiments and the transient experiments are found at the high southern latitudes, in particular in the austral winter.
To identify the most robust geographical patterns of change in the transient experiments, the standard t test is used to determine if the four-model mean change is significantly above or below the global mean.
Received: 18 January 1996 / Accepted: 5 July 1996 相似文献
12.
Summary ?This paper presents an objective analysis of the structure of daily rainfall variability over the South American/South Atlantic
region (15°–60° W and 0°–40° S) during individual austral summer months of November to March. From EOF analysis of satellite
derived daily rainfall we find that the leading mode of variability is represented by a highly coherent meridional dipole
structure, organised into 2 extensive bands, oriented northwest to southeast across the continent and Atlantic Ocean. We argue
that this dipole structure represents variability in the meridional position of the South Atlantic Convergence Zone (SACZ).
During early and later summer, in the positive (negative) phase of the dipole, enhanced (suppressed) rainfall over eastern
tropical Brazil links with that over the subtropical and extra-tropical Atlantic and is associated with suppressed (enhanced)
rainfall over the sub-tropical plains and adjacent Atlantic Ocean. This structure is indicative of interaction between the
tropical, subtropical and temperate zones. Composite fields from NCEP reanalysis products (associated with the major positive
and negative events) show that in early and late summer the position of the SACZ is associated with variability in: (a) the
midlatitude wave structure, (b) the position of the continental low, and (c) the zonal position of the South Atlantic Subtropical
High. Harmonic analysis of the 200 hPa geopotential anomaly structure in the midlatitudes indicates that reversals in the
rainfall dipole structure are associated primarily with variability in zonal wave 4. There is evidence of a wave train extending
throughout the midlatitudes from the western Pacific into the SACZ region. During positive (negative) events the largest anomalous
moisture advection occurs within westerlies (easterlies) primarily from Amazonia (the South Atlantic). In both phases a convergent
poleward flow results along the leading edge of the low-level trough extending from the tropics into temperate latitudes.
High summer events differ from those in early and late summer in that the rainfall dipole is primarily associated with variability
in the phase of zonal wave 3, and that tropical-temperate link is not clearly evident in positive events.
Received May 31, 2001; revised October 17, 2001; accepted June 13, 2002 相似文献
13.
Summary The National Hurricane Center and Joint Typhoon Warning Center operational tropical cyclone intensity forecasts for the three
major northern hemisphere tropical cyclone basins (Atlantic, eastern North Pacific, and western North Pacific) for the past
two decades are examined for long-term trends. Results show that there has been some marginal improvement in the mean absolute
error at 24 and 48 h for the Atlantic and at 72 h for the east and west Pacific. A new metric that measures the percent variance
of the observed intensity changes that is reduced by the forecast (variance reduction, VR) is defined to help account for
inter-annual variability in forecast difficulty. Results show that there have been significant improvements in the VR of the
official forecasts in the Atlantic, and some marginal improvement in the other two basins. The VR of the intensity guidance
models was also examined. The improvement in the VR is due to the implementation of advanced statistical intensity prediction
models and the operational version of the GFDL hurricane model in the mid-1990s. The skill of the operational intensity forecasts
for the 5-year period ending in 2005 was determined by comparing the errors to those from simple statistical models with input
from climatology and persistence. The intensity forecasts had significant skill out to 96 h in the Atlantic and out to 72 h
in the east and west Pacific. The intensity forecasts are also compared to the operational track forecasts. The skill was
comparable at 12 h, but the track forecasts were 2 to 5 times more skillful by 72 h. The track and intensity forecast error
trends for the two-decade period were also compared. Results showed that the percentage track forecast improvement was almost
an order of magnitude larger than that for intensity, indicating that intensity forecasting still has much room for improvement. 相似文献
14.
2007年,Ashok等揭示了赤道太平洋区域存在一种三极型分布海表温度异常并称之为厄尔尼诺-Modoki,同时定义了相应的海表温度异常指数EMI（记为IEM）。在此基础上,利用英国哈得来中心逐月海表温度资料、美国NCEP/NCAR月平均再分析数据集、美国国家海洋和大气管理局（NOAA）逐月降水资料（CMAP）,通过在太平洋海表温度异常中扣除厄尔尼诺-Modoki信号后,在Nino1+2区域上定义了东太平洋型海表温度异常指数EPNI（IEPN）。据此,由IEPN和IEM可构成描述热带太平洋海表温度异常变化的一对指数。分析了两个指数相应的海气状态及对海洋性大陆区域气候异常的影响。结果表明,厄尔尼诺-Modoki和东太平洋型海表温度异常及其影响存在显著差异。在北半球夏季,当IEM处于正位相时,热带太平洋海表温度异常呈现“负-正-负”的结构,海洋性大陆大部分区域海表温度异常为负,此时对流层低层太平洋地区辐合,海洋性大陆地区辐散,对流层高层太平洋地区辐散,海洋性大陆地区辐合。对应于辐合辐散中心,存在着自赤道中太平洋分别向赤道东太平洋和海洋性大陆中东部地区的异常垂直环流圈,同时也存在自海洋性大陆西部向印度洋西部的垂直环流。大气在海洋性大陆区域北部加热,南部冷却;在太平洋地区西部加热而东部冷却;在海洋性大陆区域10°N以南降水偏少,而10°N以北降水偏多。当IEPN处于正位相时,热带太平洋海表温度异常呈现“西负东正”分布型,海洋性大陆区域海表温度异常呈现“西正东负”分布,对流层低层海洋性大陆地区辐散中心范围偏大、位置偏东、强度偏强,太平洋地区辐合中心范围偏小、位置偏东,热带环流异常在垂直方向上呈斜压结构,海洋性大陆区域北部大气加热而南部冷却,太平洋地区大气均呈加热正异常,海洋性大陆大部分区域降水均偏少,赤道太平洋降水偏多。以上这些结果有利于深刻理解热带太平洋海表温度异常的特征及其对海洋性大陆区域气候的影响。 相似文献
15.
夏季印度洋海盆模与MC区域降水异常联系的进一步分析 总被引：1，自引：0，他引：1
利用英国哈德莱中心的逐月海表温度资料及NCEP/NCAR月平均再分析资料等,通过在印度洋海盆模IOBM指数(IIOB)中扣除长期趋势和两类ENSO的同期信号后,得到了修正的IOBM指数(Im IOB),并由此分析了IOBM的变化及与海洋性大陆区域降水异常的联系。结果表明:印度洋IOBM为暖位相时,不同季节的印度洋地区均呈现异常偏暖,但大气是上升还是下沉运动则在印度洋不同季节和不同区域存在很大变化。就夏季而言,印度洋大部分地区存在上升运动,这与海温异常偏暖有关。在北半球夏季,指数Im IOB存在3~5 a的周期变化。当IOBM处于正位相时,印度洋至我国东海地区大范围海温偏暖。MC(Maritime Continent,海洋性大陆)区域西部降水正异常,而MC区域东北部降水为负异常。造成这种降水分布的原因是:当指数为正时,在MC区域的西部对流层低层辐合、高层辐散,上升运动增强,且水汽辐合,而MC区域的东北部对流层低层辐散、高层辐合,上升运动不明显,水汽辐散,不易形成降水。而在对流层低层与西太平洋辐散中心对应,南北半球出现关于赤道对称的反气旋对,赤道印度洋上的异常加热激发东传的Kelvin波,加强东风异常,同时加强了KMC(海洋性大陆的核心区域)之外南北半球热带地区的这对Rossby波型。以上这些结果有利于深刻理解MC降水异常成因及热带海陆气相互作用过程。 相似文献
16.
Hengyi Weng Karumuri Ashok Swadhin K. Behera Suryachandra A. Rao Toshio Yamagata 《Climate Dynamics》2007,29(2-3):113-129
Present work uses 1979–2005 monthly observational data to study the impacts of El Niño Modoki on dry/wet conditions in the Pacific rim during boreal summer. The El Niño Modoki phenomenon is characterized by the anomalously warm central equatorial Pacific flanked by anomalously cool regions in both west and east. Such zonal SST gradients result in anomalous two-cell Walker Circulation over the tropical Pacific, with a wet region in the central Pacific. There are two mid-tropospheric wave trains passing over the extratropical and subtropical North Pacific. They contain a positive phase of a Pacific-Japan pattern in the northwestern Pacific, and a positive phase of a summertime Pacific-North American pattern in the northeastern Pacific/North America region. The western North Pacific summer monsoon is enhanced, while the East Asian summer monsoon is weakened. In the South Pacific, there is a basin-wide low in the mid-latitude with enhanced Australian high and the eastern South Pacific subtropical high. Such an atmospheric circulation pattern favors a dry rim surrounding the wet central tropical Pacific. The El Niño Modoki and its climate impacts are very different from those of El Niño. Possible geographical regions for dry/wet conditions influenced by El Niño Modoki and El Niño are compared. The two phenomena also have very different temporal features. El Niño Modoki has a large decadal background while El Niño is predominated by interannual variability. Mixing-up the two different phenomena may increase the difficulty in understanding their mechanisms, climate impacts, and uncertainty in their predictions. 相似文献
17.
The present paper selects the northern winter of December 1995–February 1996 for a case study on the impact of sea surface temperature (SST) anomalies on the atmospheric circulation over the North Atlantic and Western Europe. In the Atlantic, the selected winter was characterized by positive SST anomalies over the northern subtropics and east of Newfoundland, and negative anomalies along the US coast. A weak La Niña event developed in the Pacific. The North Atlantic Oscillation (NAO) index was low, precipitation over the Iberian Peninsula and northern Africa was anomalously high, and precipitation over northern Europe was anomalously low. The method of study consists of assessing the sensitivity of ensemble simulations by the UCLA atmospheric general circulation model (UCLA AGCM) to SST anomalies from the observation, which are prescribed either in the World Oceans, the Atlantic Ocean only, or the subtropical North Atlantic only. The results obtained are compared with a control run that uses global, time-varying climatological SST. The ensemble simulations with global and Atlantic-only SST anomalies both produce results that resemble the observations over the North Atlantic and Western Europe. It is suggested that the anomalous behavior of the atmosphere in the selected winter over those regions, therefore, was primarily determined by conditions within the Atlantic basin. The simulated fields in the tropical North Atlantic show anomalous upward motion and lower (upper) level convergence (divergence) in the atmosphere overlying the positive SST anomalies. Consistently, the subtropical jet intensifies and its core moves equatorward, and precipitation increases over northern Africa and southern Europe. The results also suggest that the SST anomalies in the tropical North Atlantic only do not suffice to produce the atmospheric anomalies observed in the basin during the selected winter. The extratropical SST anomalies would provide a key contribution through increased transient eddy activity, which causes an extension of the subtropical jet eastward from the coast of North America. 相似文献
18.
Coupled ocean-atmosphere surface variability and its climate impacts in the tropical Atlantic region
This study examines time evolution and statistical relationships involving the two leading ocean-atmosphere coupled modes
of variability in the tropical Atlantic and some climate anomalies over the tropical 120 °W–60 °W region using selected historical
files (75-y near global SSTs and precipitation over land), more recent observed data (30-y SST and pseudo wind stress in the
tropical Atlantic) and reanalyses from the US National Centers for Environmental Prediction (NCEP/NCAR) reanalysis System
on the period 1968–1997: surface air temperature, sea level pressure, moist static energy content at 850 hPa, precipitable
water and precipitation. The first coupled mode detected through singular value decomposition of the SST and pseudo wind-stress
data over the tropical Atlantic (30 °N–20 °S) expresses a modulation in the thermal transequatorial gradient of SST anomalies
conducted by one month leading wind-stress anomalies mainly in the tropical north Atlantic during northern winter and fall.
It features a slight dipole structure in the meridional plane. Its time variability is dominated by a quasi-decadal signal
well observed in the last 20–30 ys and, when projected over longer-term SST data, in the 1920s and 1930s but with shorter
periods. The second coupled mode is more confined to the south-equatorial tropical Atlantic in the northern summer and explains
considerably less wind-stress/SST cross-covariance. Its time series features an interannual variability dominated by shorter
frequencies with increased variance in the 1960s and 1970s before 1977. Correlations between these modes and the ENSO-like
Nino3 index lead to decreasing amplitude of thermal anomalies in the tropical Atlantic during warm episodes in the Pacific.
This could explain the nonstationarity of meridional anomaly gradients on seasonal and interannual time scales. Overall the
relationships between the oceanic component of the coupled modes and the climate anomaly patterns denote thermodynamical processes
at the ocean/atmosphere interface that create anomaly gradients in the meridional plane in a way which tends to alter the
north–south movement of the seasonal cycle. This appears to be consistent with the intrinsic non-dipole character of the tropical
Atlantic surface variability at the interannual time step and over the recent period, but produces abnormal amplitude and/or
delayed excursions of the intertropical convergence zone (ITCZ). Connections with continental rainfall are approached through
three (NCEP/NCAR and observed) rainfall indexes over the Nordeste region in Brazil, and the Guinea and Sahel zones in West
Africa. These indices appear to be significantly linked to the SST component of the coupled modes only when the two Atlantic
modes+the ENSO-like Nino3 index are taken into account in the regressions. This suggests that thermal forcing of continental
rainfall is particularly sensitive to the linear combinations of some basic SST patterns, in particular to those that create
meridional thermal gradients. The first mode in the Atlantic is associated with transequatorial pressure, moist static energy
and precipitable water anomaly patterns which can explain abnormal location of the ITCZ particularly in northern winter, and
hence rainfall variations in Nordeste. The second mode is more associated with in-phase variations of the same variables near
the southern edge of the ITCZ, particularly in the Gulf of Guinea during the northern spring and winter. It is primarily linked
to the amplitude and annual phase of the ITCZ excursions and thus to rainfall variations in Guinea. Connections with Sahel
rainfall are less clear due to the difficulty for the model to correctly capture interannual variability over that region
but the second Atlantic mode and the ENSO-like Pacific variability are clearly involved in the Sahel climate interannual fluctuations:
anomalous dry (wet) situations tend to occur when warmer (cooler) waters are present in the eastern Pacific and the gulf of
Guinea in northern summer which contribute to create a northward (southward) transequatorial anomaly gradient in sea level
pressure over West Africa.
Received: 14 April 1998 / Accepted: 24 December 1998 相似文献
19.
J. Otterman R. Atlas J. Ardizzone D. Starr J. C. Jusem J. Terry 《Theoretical and Applied Climatology》1999,64(3-4):201-211
Summary The relationship between European surface temperature and winds over the eastern North Atlantic are investigated for the
years 1988 to 1997. Daily Special Sensor Microwave Imager SSM/I observations are used to evaluate a monthly surface wind index
that quantifies the influence of southwesterly flow. Our wind index and the monthly-mean surface-air temperatures in late
winter and early spring over France and northern-latitude Europe are highly correlated. In February, the year-to-year increases/decreases
match every year for France (correlation of 0.82 with the Index); and every year with just one exception for Europe (correlation
with the Index of 0.76 for a longitudinal strip through Europe 45–50° N, and 0.73 for the 50–60° N strip). In March, the increases/decreases
of the wind Index and of the temperatures for France also match, but the correlation with the Index is lower, 0.65.
The high correlation between our Index and the large interannual fluctuations in the monthly temperature in late winter and
early spring indicate that the onset of the spring conditions in Europe is significantly influenced by the wind patterns over
the eastern North Atlantic.
Coinciding with the fluctuations from warm-Europe/high-Index winter to the opposite conditions, we observe “seesaw” effects,
fluctuations over the North Atlantic, in opposite directions in the east (25–5° W), and the west (65–45° W). In the low-Index
years we find that: (a) the surface-air temperatures in the west are appreciably higher than in the east (but slightly lower
in the high-Index year), and (b) the difference between the 500 mb meridional wind in the west and that in the east is positive
and large, exceeding 10 m s−1 (but it becomes negative and small in the high-Index years). The “seesaw” effects suggest that a positive feedback exits
between these cross-Atlantic temperature differences and the surface winds.
Received August 7, 1998 Revised April 23, 1999 相似文献
20.
Extratropical cyclone variability in the Northern Hemisphere winter from the NCEP/NCAR reanalysis data 总被引：9，自引：0，他引：9
The winter climatology of Northern Hemisphere cyclone activity was derived from 6-hourly NCEP/NCAR reanalysis data for the
period from 1958 to 1999, using software which provides improved accuracy in cyclone identification in comparison to numerical
tracking schemes. Cyclone characteristics over the Kuroshio and Gulfstream are very different to those over continental North
America and the Arctic. Analysis of Northern Hemisphere cyclones shows secular and decadal-scale changes in cyclone frequency,
intensity, lifetime and deepening rates. The western Pacific and Atlantic are characterized by an increase in cyclone intensity
and deepening during the 42-year period, although the eastern Pacific and continental North America demonstrate opposite tendencies
in most cyclone characteristics. There is an increase of the number of cyclones in the Arctic and in the western Pacific and
a downward tendency over the Gulf Stream and subpolar Pacific. Decadal scale variability in cyclone activity over the Atlantic
and Pacific exhibits south-north dipole-like patterns. Atlantic and Pacific cyclone activity associated with the NAO and PNA
is analyzed. Atlantic cyclone frequency demonstrates a high correlation with NAO and reflects the NAO shift in the mid 1970s,
associated with considerable changes in European storm tracks. The PNA is largely linked to the eastern Pacific cyclone frequencies,
and controls cyclone activity over the Gulf region and the North American coast during the last two decades. Assessment of
the accuracy of the results and comparison with those derived using numerical algorithms, shows that biases inherent in numerical
procedures are not negligible.
Received: 7 July 2000 / Accepted: 30 November 2000 相似文献