Interdecadal variability of the ENSO teleconnection to the wintertime North Pacific

The El Niño/Southern Oscillation (ENSO) strongly influences the large-scale atmospheric circulation over the extratropical North Pacific during boreal winter, which has an important impact on North American winter climate. This study analyses the interdecadal variability of the ENSO teleconnection to the wintertime extratropical North Pacific, over the period 1900–2010, using a range of observationally derived datasets and an ensemble of atmospheric model simulations. The observed teleconnection strength is found to vary substantially over the 20th century. Specifically, 31-year periods in the early-century (1912–1942), mid-century (1946–1976) and the late-century (1980–2010) are identified in the observations when the ENSO teleconnection to the North Pacific circulation are found to be particularly strong, weak and strong respectively. The ENSO teleconnection to the North Pacific in the atmospheric model ensemble is weak in the mid-century period and substantially stronger in the late-century, closely following the variability in the observed ENSO-North Pacific teleconnection. In the early-century, however, the atmospheric model also exhibits a weak teleconnection to the North Pacific, unlike in observations. In a subset of the model realisations that exhibit similar ENSO-North Pacific teleconnection as in observations during the early-century period there are large differences in extratropical circulation but not in equatorial Pacific precipitation anomalies, in contrast to the late-century period. This suggests that the high correlation in the early century period is largely due to internal extratropical variability. The important implications of these results for seasonal predictability and the assessment of seasonal forecasting systems are discussed.

     

Seasonal prediction skill of ECMWF System 4 and NCEP CFSv2 retrospective forecast for the Northern Hemisphere Winter

The seasonal prediction skill for the Northern Hemisphere winter is assessed using retrospective predictions (1982–2010) from the ECMWF System 4 (Sys4) and National Center for Environmental Prediction (NCEP) CFS version 2 (CFSv2) coupled atmosphere–ocean seasonal climate prediction systems. Sys4 shows a cold bias in the equatorial Pacific but a warm bias is found in the North Pacific and part of the North Atlantic. The CFSv2 has strong warm bias from the cold tongue region of the eastern Pacific to the equatorial central Pacific and cold bias in broad areas over the North Pacific and the North Atlantic. A cold bias in the Southern Hemisphere is common in both reforecasts. In addition, excessive precipitation is found in the equatorial Pacific, the equatorial Indian Ocean and the western Pacific in Sys4, and in the South Pacific, the southern Indian Ocean and the western Pacific in CFSv2. A dry bias is found for both modeling systems over South America and northern Australia. The mean prediction skill of 2 meter temperature (2mT) and precipitation anomalies are greater over the tropics than the extra-tropics and also greater over ocean than land. The prediction skill of tropical 2mT and precipitation is greater in strong El Nino Southern Oscillation (ENSO) winters than in weak ENSO winters. Both models predict the year-to-year ENSO variation quite accurately, although sea surface temperature trend bias in CFSv2 over the tropical Pacific results in lower prediction skill for the CFSv2 relative to the Sys4. Both models capture the main ENSO teleconnection pattern of strong anomalies over the tropics, the North Pacific and the North America. However, both models have difficulty in forecasting the year-to-year winter temperature variability over the US and northern Europe.     

Comparison of Wintertime North American Climate Impacts Associated with Multiple ENSO Indices

This analysis compares the climate impacts over North America during winter associated with various El Niño–Southern Oscillation (ENSO) indices, including the Niño 3.4 index, the leading tropical Pacific outgoing longwave radiation and sea surface temperature (OLR-SST) covariability, and the eastern Pacific (EP) and central Pacific (CP) types of ENSO identified from both partial-regression–empirical orthogonal function (EOF) and regression–EOF approaches. The traditional Niño 3.4 SST index is found to be optimal for monitoring the tropical Pacific OLR-SST covariability and for the tropical SST impact on North America. The circulation anomalies associated with the Niño 3.4 index project on both the Pacific/North American (PNA) and Tropical/Northern Hemisphere (TNH) patterns. The ENSO associated with the PNA tends to come from both the EP and CP ENSOs, whereas that associated with the TNH comes more from the EP ENSO. The variability of ENSO significantly affects North American temperature and precipitation, as well as temperature and precipitation extremes. For either the EP or CP types of ENSO, qualitatively similar patterns of climate and climate extreme anomalies are apparent associated with the indices identified by the two EOF approaches, with differences mainly in the anomalous amplitude. The anomalous patterns are generally field significant over North America for the EP ENSO but not field significant for the CP ENSO.

The circulation anomalies associated with ENSO are reinforced and maintained by synoptic vorticity fluxes in the upper troposphere. The anomalous surface temperature is mainly determined by the anomalies in surface radiative heating in the face of upward surface longwave radiative damping. The precipitation anomalies are supported by the vertically integrated moisture transport. The differences in atmospheric circulation, surface temperature, and precipitation among the various ENSO indices, including the intensity and spatial structure of the fields, can be attributed to the corresponding differences in synoptic eddy vorticity forcing, surface radiative heating, and vertically integrated moisture transport.     

Response of the Asian summer monsoon to weakening of Atlantic thermohaline circulation

Various paleoclimate records have shown that the Asian monsoon was punctuated by numerous suborbital time-scale events, and these events were coeval with those that happened in the North Atlantic. This study investigates the Asian summer monsoon responses to the Atlantic Ocean forcing by applying an additional freshwater flux into the North Atlantic. The simulated results indicate that the cold North Atlantic and warm South Atlantic induced by the weakened Atlantic thermohaline circulation （THC） due to the freshwater flux lead to significantly suppressed Asian summer monsoon. The authors analyzed the detailed processes of the Atlantic Ocean forcing on the Asian summer monsoon, and found that the atmospheric teleconnection in the eastern and central North Pacific and the atmosphere-ocean interaction in the tropical North Pacific play the most crucial role. Enhanced precipitation in the subtropical North Pacific extends the effects of Atlantic Ocean forcing from the eastern Pacific into the western Pacific, and the atmosphere-ocean interaction in the tropical Pacific and Indian Ocean intensifies the circulation and precipitation anomalies in the Pacific and East Asia.     

A transition-phase teleconnection of the Pacific quasi-decadal oscillation

The atmospheric circulation patterns associated with the Pacific quasi-decadal oscillation (QDO) are investigated using available observational data from 1948 to 2007. Previous studies indicate that the Pacific QDO is characterized by a distinct lifecycle in the form of sea surface temperature (SST) patterns. In the warm and cool phases of the Pacific QDO, the SST patterns resemble those associated with the El Ni?o-Southern Oscillation (ENSO). During the warm?Ccool and cool?Cwarm transitions of the Pacific QDO, recurrent SST patterns are also clearly visible. The rotated empirical orthogonal function analysis on the 10?C15?year filtered data shows that the evolutions of SST and atmospheric circulation are well coupled. While the warm-/cool-phase Pacific QDO generates an ENSO-like circulation pattern, the transition phases form a distinct short-wave train emanating from Southeast Asia towards North America. This short-wave train is particularly robust in the streamfunction of water vapor flux. Diagnostic analyses of the heat budget, the stationary wave flux, and a barotropic model indicate that this short-wave train is thermodynamically maintained and is likely forced by diabatic heating near Southeast Asia. Additional modulations of forcing sources in the western and eastern tropical Pacific on this short-wave teleconnection are also discussed.     

ENSO teleconnections in projections of future climate in ECHAM5/MPI-OM

The teleconnections of the El Niño/Southern Oscillation (ENSO) in future climate projections are investigated using results of the coupled climate model ECHAM5/MPI-OM. For this, the IPCC SRES scenario A1B and a quadrupled CO₂ simulation are considered. It is found that changes of the mean state in the tropical Pacific are likely to condition ENSO teleconnections in the Pacific North America (PNA) region and in the North Atlantic European (NAE) region. With increasing greenhouse gas emissions the changes of the mean states in the tropical and sub-tropical Pacific are El Niño-like in this particular model. Sea surface temperatures in the tropical Pacific are increased predominantly in its eastern part and redistribute the precipitation further eastward. The dynamical response of the atmosphere is such that the equatorial east–west (Walker) circulation and the eastern Pacific inverse Hadley circulation are decreased. Over the subtropical East Pacific and North Atlantic the 200 hPa westerly wind is substantially increased. Composite maps of different climate parameters for positive and negative ENSO events are used to reveal changes of the ENSO teleconnections. Mean sea level pressure and upper tropospheric zonal winds indicate an eastward shift of the well-known teleconnection patterns in the PNA region and an increasing North Atlantic oscillation (NAO) like response over the NAE region. Surface temperature and precipitation underline this effect, particularly over the North Pacific and the central North Atlantic. Moreover, in the NAE region the 200 hPa westerly wind is increasingly related to the stationary wave activity. Here the stationary waves appear NAO-like.     

Interdecadal Variability of the ENSO–North Pacific Atmospheric Circulation in Winter

The interdecadal change in the relationship between the El Niño–Southern Oscillation (ENSO) and atmospheric circulation over the North Pacific is investigated using both observational data and an atmospheric general circulation model. There are two prominent modes of winter mid-latitude atmospheric variability in the North Pacific: the West Pacific (WP) teleconnection and the Aleutian Low (AL). The relationship between ENSO and the WP-AL patterns changed notably around the late 1970s. From 1957 to 1975, during the mature phase of ENSO, significant sea surface temperature anomalies (SSTAs) occurred, mainly in the equatorial eastern Pacific Ocean; the associated atmospheric circulation anomaly pattern resembles the negative phase of a WP teleconnection pattern. In contrast, for the 1978–2011 period, significant negative SSTAs were observed in the western and extratropical Pacific in both hemispheres, with some significant positive SSTAs appearing over the eastern Pacific. This is in agreement with the defined regions of a mega-ENSO, the associated atmospheric circulation anomaly pattern resembles the AL mode. Further analysis suggests that a negative–positive anomaly pattern in the 500?hPa geopotential height throughout the entire North Pacific, possibly enhanced by the SSTAs in the extratropical North Pacific associated with the mature phase of ENSO, is responsible for modulating the relationship between ENSO and the North Pacific atmospheric circulation.     

Impacts of Sea Surface Temperature on the Interannual Variability of Winter Haze Days in Guangdong Province

The impact of sea surface temperature (SST) on winter haze in Guangdong province (WHDGD) was analyzed on the interannual scale. It was pointed out that the northern Indian Ocean and the northwest Pacific SST play a leading role in the variation of WHDGD. Cold (warm) SST anomalies over the northern Indian Ocean and the Northwest Pacific stimulate the eastward propagation of cold (warm) Kelvin waves through the Gill forced response, causing Ekman convergence (divergence) in the western Pacific, inducing abnormal cyclonic (anticyclonic) circulation. It excites the positive (negative) Western Pacific teleconnection pattern (WP), which results in the temperature and the precipitation decrease (increase) in Guangdong and forms the meteorological variables conditions that are conducive (not conducive) to the formation of haze. ENSO has an asymmetric influence on WHDGD. In El Ni?o (La Ni?a) winters, there are strong (weak) coordinated variations between the northern Indian Ocean, the northwest Pacific, and the eastern Pacific, which stimulate the negative (positive) phase of WP teleconnection. In El Ni?o winters, the enhanced moisture is attributed to the joint effects of the horizontal advection from the surrounding ocean, vertical advection from the moisture convergence, and the increased atmospheric apparent moisture sink (Q2) from soil evaporation. The weakening of the atmospheric apparent heat source (Q1) in the upper layer is not conducive to the formation of inversion stratification. In contrast, in La Ni?a winters, the reduced moisture is attributed to the reduced upward water vapor transport and Q2 loss. Due to the Q1 increase in the upper layer, the temperature inversion forms and suppresses the diffusion of haze.     

Iran's precipitation analysis using synoptic modeling of major teleconnection forces (MTF)

In this relatively unprecedented study, the effects of thirty-four leading teleconnection Patterns (indices) of atmospheric circulation- on regional-scale for the Middle East- along with precipitation over Iran have been investigated. Different types of data including teleconnection Indices from NOAA (NCEP/NCAR, BOM) and monthly precipitation data from thirty-six synoptic stations of Iran were applied. The data have been investigated with various types of statistical and synoptical methods. The results indicate that El Niño–Southern Oscillation (ENSO) is the most effective factor and it could possibly influence the spatiotemporal variation of precipitation on all types of climate regimes in Iran. ENSO (nino3.4), The Atlantic Multidecadal Oscillation (AMO) and The Arctic oscillation (AO) are known as the first three important indices, determined by Principal component analysis (PCA) method. The research has clarified that a combination of warm phase of ENSO and a predominant Southeastern (SE) wind over the Indian Ocean can result in a significant moisture transport from the Indian Ocean to the Middle East and to Iran; a combination of cold phase of ENSO and a predominant Northwestern (NW) wind over the Indian Ocean can be followed by a widespread drought over the Middle East and Iran. The results also indicate that a combination of the first three important above-mentioned indices and the Indian Ocean Dipole (IOD) can provide a much better explanation for spatial and temporal variation of precipitation of Iran. Finally, the results of this study will enable us to present a new approach and new graphical-conceptual modeling, called "Teleconnection-Synoptic Method (TSM)"to clarify the underlying mechanism that can explain the spatial and temporal variations of global atmospheric circulation and precipitation of Iran. According to the correlation of different patterns with precipitation, the strongest relationships are related to the Scandinavia Index (SCN), Pressure Change in East Pacific (dPEPac) and Trade Wind Index at the 850 hPa (TrdWnd850), respectively.     

Spring-summer temperatures since AD 1780 reconstructed from stable oxygen isotope ratios in white spruce tree-rings from the Mackenzie Delta,northwestern Canada

High-latitude δ¹⁸O archives deriving from meteoric water (e.g., tree-rings and ice-cores) can provide valuable information on past temperature variability, but stationarity of temperature signals in these archives depends on the stability of moisture source/trajectory and precipitation seasonality, both of which can be affected by atmospheric circulation changes. A tree-ring δ¹⁸O record (AD 1780–2003) from the Mackenzie Delta is evaluated as a temperature proxy based on linear regression diagnostics. The primary source of moisture for this region is the North Pacific and, thus, North Pacific atmospheric circulation variability could potentially affect the tree-ring δ¹⁸O-temperature signal. Over the instrumental period (AD 1892–2003), tree-ring δ¹⁸O explained 29 % of interannual variability in April–July minimum temperatures, and the explained variability increases substantially at lower-frequencies. A split-period calibration/verification analysis found the δ¹⁸O-temperature relation was time-stable, which supported a temperature reconstruction back to AD 1780. The stability of the δ¹⁸O-temperature signal indirectly implies the study region is insensitive to North Pacific circulation effects, since North Pacific circulation was not constant over the calibration period. Simulations from the NASA-GISS ModelE isotope-enabled general circulation model confirm that meteoric δ¹⁸O and precipitation seasonality in the study region are likely insensitive to North Pacific circulation effects, highlighting the paleoclimatic value of tree-ring and possibly other δ¹⁸O records from this region. Our δ¹⁸O-based temperature reconstruction is the first of its kind in northwestern North America, and one of few worldwide, and provides a long-term context for evaluating recent climate warming in the Mackenzie Delta region.     

The variation of warm pool in the equatorial western pacific and its impacts on climate

1.IntroductionSincetheEINinoeventwasregardedasaresultoftheair--seainteraction(Bjerknes,1969;RasmussonandWallace,1983;Philander,1990),thetropicalPacifichasbeenPaidmuchattentionbymeteorologistsintheclimaticstudies.Particularly,thereisthehighestoceantemperatureintheequatorialwesternPacific,theuwarmpool",andthestrongestconvectionandatmosphericheatingareovertheequatorialwesternPacific,sothattheequatorialwesternPacificisveryimportanttotheclimaticvariationintheglobe.Thenumericalsimulationwithasi…     

ENSO forcing of the Northern Hemisphere climate in a large ensemble of model simulations based on a very long SST record

The January–March (JFM) climate response of the Northern Hemisphere atmosphere to observed sea surface temperature (SST) anomalies for the period 1855–2002 is analysed from a 35-member ensemble made with SPEEDY, an atmospheric general circulation model (AGCM) of intermediate complexity. The model was run at the T30-L8 resolution, and initial conditions and the early stage of model runs differ among ensemble members in the definition of tropical diabatic heating. SST anomalies in the Niño3.4 region were categorised into five classes extending from strong cold to strong warm. Composites based on such a categorisation enabled an analysis of the influence of the tropical Pacific SST on the Northern Hemisphere atmospheric circulation with an emphasis on the Pacific-North America (PNA) and the North Atlantic-Europe (NAE) regions. As expected, the strongest signal was detected over the PNA region. An “asymmetry” in the model response was found for the opposite polarity of the Niño3.4 index; however, this asymmetry stems mainly from the difference in the amplitude of model response rather than from the phase shift between responses to warm and cold El Niño-Southern Oscillation (ENSO) events. The extratropical signal associated with warm ENSO events was found to be stronger than that related to cold events. The results also reveal that, for the PNA region, the amplitude of the response is positively correlated with the strength of ENSO, irrespective of the sign of ENSO. With almost no phase shift between model responses to El Niño and La Niña, the linear component of the response is much stronger than the non-linear component. Although the model climate response over the NAE region is much weaker than that over the PNA region, some striking similarities with the PNA are found. Both sea level pressure and precipitation responses are positively correlated with the strength of ENSO. This is not true for the 200-hPa geopotential heights, and no plausible explanation for such a result could be offered. An appreciable linear component in model response over the NAE was also found. The model results over the NAE region agree reasonably well with observational studies. An additional analysis of the remote atmospheric response to very weak ENSO forcing (defined from the interval between 0.5σ and 1.0σ of the interannual variance) was also carried out. A discernible model response in the Northern Hemisphere to such a weak SST forcing was found.     

ENSO influence on Europe during the last centuries

El Niño/Southern Oscillation (ENSO) affects climate not only in the Pacific region and the tropics, but also in the North Atlantic-European area. Studies based on twentieth-century data have found that El Niño events tend to be accompanied in late winter by a negative North Atlantic Oscillation index, low temperatures in northeastern Europe and a change in precipitation patterns. However, many questions are open, for example, concerning the stationarity of this relation. Here we study the relation between ENSO and European climate during the past 500 years based on statistically reconstructed ENSO indices, early instrumental station series, and reconstructed fields of surface air temperature, sea-level pressure, precipitation, and 500 hPa geopotential height. After removing years following tropical volcanic eruptions (which systematically mask the ENSO signal), we find a consistent and statistically significant ENSO signal in late winter and spring. The responses to El Niño and La Niña are close to symmetric. In agreement with studies using twentieth-century data only, the ENSO signal in precipitation is different in fall than in late winter. Moving correlation analyses confirm a stationary relationship between ENSO and late winter climate in Europe during the past 300 years. However, the ENSO signal is modulated significantly by the North Pacific climate. A multi-field cluster analysis for strong ENSO events during the past 300 years yields a dominant pair of clusters that is symmetric and represents the ‘classical’ ENSO effects on Europe.     

中纬度北太平洋依赖于ENSO事件及独立于ENSO事件的变率特征:观测事实与海气耦合模式模拟

观测事实显示,在E1 Ni(?)o发生期间,伴随着赤道中东太平洋的增暖,中纬度北大平洋中部表层海温（SST）常出现冷距平,而北美大陆西海岸SST则出现暖距平。借助观测资料分析和海气耦合模式模拟两种手段,检验了北太平洋对ENSO事件的上述响应。观测证据和数值模拟都支持有关学者提出的“大气桥”概念,即大气对赤道中东太平洋SST异常增暖的响应,随后强迫中纬度北太平洋,并导致那里SST的变冷,从而起到了连接热带和热带外特别是中纬度北太平洋的“桥梁”的作用。关于其机制,本文认为主要是海洋对大气强迫的动力响应导致那里的SST变冷,尽管潜热通量的贡献也很显著。至少模式结果证明短波辐射、长波辐射和感热通量的贡献都是次要的。进一步的分析揭示,北太平洋存在着线性独立于ENSO事件的所谓“北太平洋模态”,在空间型上,它和线性地依赖于ENSO事件的模态非常相近,即它们的纬向结构都呈现出扁平的“双极”型,只是彼此间SST距平极大值的中心位置不同。模拟结果表明,北太平洋模态与大气的耦合作用,主要是通过海气热通量交换实现的,其中短波辐射和长波辐射的作用居主导地位,而潜热通量的贡献则基本可以忽略。     

冬季北太平洋多尺度水汽输送和大气河的变化特征及其与PDO和ENSO的联系

利用1950-2015年NCEP/NCAR大气再分析资料和NOAA海表面温度资料,通过回归分析和合成分析的方法研究了冬季北太平洋不同尺度水汽输送和大气河(Atmospheric River,AR)的变化特征及其与PDO、ENSO的联系.结果表明,在PDO正位相时期,时间平均的水汽输送将大量水汽从太平洋北部和中部地区输送...     

冬季亚洲—太平洋涛动年际变率与东亚气候异常

利用1948—2011年NCEP/NCAR月平均再分析资料和1951—2010年我国160站降水量资料,研究了冬季亚洲—太平洋区域的大气遥相关及其与东亚冬季风和降水的关系。结果表明:冬季在亚洲—西太平洋与中、东太平洋中低纬度对流层上层扰动温度之间存在类似于夏季的亚洲—太平洋涛动 (APO) 现象,即当东亚中低纬度对流层中、上层偏暖时,中东太平洋中低纬度对流层中上层温度偏冷,反之亦然。冬季APO可以反映冬季亚洲—太平洋东西向热力差异强度变化,与夏季相比,冬季APO遥相关在亚洲的中心位置略偏南、偏东,且冬季APO与大气环流关系与夏季也有所不同;当冬季APO指数偏高时,对流层上层东亚大槽位置偏西,而东亚热带地区的高压向北伸展,导致我国南方对流层为深厚的异常反气旋系统所控制,此时南方地区对流层低层盛行异常的偏东北气流,并伴随水汽辐散和异常下沉运动,南方降水偏少;冬季APO指数与ENSO有紧密联系。     

夏季长江中下游冷热、旱涝和东部近海台风活动及其与ENSO关系的研究

利用1951~1995年气温、降水和台风年鉴等资料, 在划分长江中下游冷、热夏年标准的基础上, 分别统计分析了我国近海台风活动异常和季风雨型特征及其与ENSO的关系.指出, 冷夏年和热夏年我国东部近海台风活动差异显著; 与之相对应的季风雨型同样十分悬殊; 同时赤道太平洋海温SSTA场的配置亦迥然不同.最后给出了遥响应过程图像的物理概念模型.     

Teleconnection–extreme precipitation relationships over the Mediterranean region

The relationship between five teleconnection patterns (North Atlantic Oscillation (NAO), Arctic Oscillation (AO), East Atlantic/Western Russian (EAWR) pattern, Scandinavian (SCAND) pattern, and El Niño Southern Oscillation (ENSO)) and the frequency of occurrence of days (per month) with extreme precipitation in the Euro-Mediterranean region is investigated with National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis data. To quantify the teleconnection–precipitation relationships over the Euro-Mediterranean region, linear correlations are calculated between the monthly teleconnection indices for the five patterns and time series at each grid point of the monthly frequency of days with extreme precipitation, focusing on daily precipitation amounts that exceed a particular threshold value (a 90 % threshold is used). To evaluate dynamical processes, the teleconnection indices are also correlated with the frequencies of days with extreme values of dynamic tropopause pressure and precipitable water. The former quantity is used as a proxy for potential vorticity intrusions and the latter to identify regions of enhanced moisture. The results of this analysis indicates positive, statistically significant correlations between the NAO, AO, and SCAND indices and the frequency of extreme precipitation in the western Mediterranean; positive (negative) correlations between the EAWR index and the extreme precipitation frequency in the eastern (western) Mediterranean; and a positive correlation between the Niño3.4 index and the extreme precipitation frequency over the Iberian Peninsula and the Middle East. For all of the teleconnection patterns other than ENSO, the dynamic tropopause pressure correlation patterns resemble those for the precipitation. In contrast, similar precipitation and precipitable water correlation patterns are observed only for ENSO. These findings suggest that the teleconnections affect the interannual variation of the frequency of days with extreme precipitation over a large part of the Euro-Mediterranean region through their impact on the spatial distribution of regions with enhanced potential vorticity and air moisture.     

1990年代末东亚北部地区夏季水汽输送年代际变化特征及其影响机制

利用1983～2011年降水量、环流和海温的再分析资料,探讨了东亚北部地区夏季水汽输送的年代际变化特征,并分析了前冬北大西洋海温对东亚北部地区夏季水汽输送与大气环流的可能影响。研究结果表明,20世纪90年代末期东亚北部地区夏季整层水汽与降水年代际的变化特征相一致,整层水汽通量的年代际变化主要是由于纬向水汽输送异常作用的结果。东亚北部地区（35°～55°N,90°～145°E）西边界的水汽输送通量由多变少,东边界的水汽输送通量由少变多特征则直接导致了该地区降水由偏多转为偏少的年代际变化。就外强迫海温角度来说,前冬北大西洋海温跟东亚北部地区夏季500 hPa高度场、850 hPa风场和850 hPa比湿均显著相关。同时,在20世纪90年代中后期前冬北大西洋海温也表现出由偏低向偏高转变的年代际变化特征,且由于海温自身的记忆性前冬的海温异常一直延续到夏季。并在夏季激发出横跨北大西洋和欧亚大陆中高纬度地区的大西洋-欧亚（AEA）遥相关结构,并进一步影响东亚北部地区夏季水汽输送。     

Atmospheric teleconnection patterns and severity of winters in the Laurentian Great Lakes basin

Abstract

We analyzed the relationship between an index of Great Lakes winter severity (winters 1950–1998) and atmospheric circulation characteristics. Classification and Regression Tree analysis methods allowed us to develop a simple characterization of warm, normal and cold winters in terms of teleconnection indices and their combinations. Results are presented in the form of decision trees. The single most important classifier for warm winters was the Polar/Eurasian index (POL). A majority of warm winters (12 out of 15) occurred when this index was substantially positive (POL > 0.23). There were no cold winters when this condition was in place. Warm winters are associated with a positive phase of the Western Pacific pattern and El Niño events in the equatorial Pacific. The association between cold winters and La Niña events was much weaker. Thus, the effect of the El Niño/Southern Oscillation (ENSO) on severity of winters in the Great Lakes basin is not symmetric. The structure of the relationship between the index of winter severity and teleconnection indices is more complex for cold winters than for warm winters. It takes two or more indices to successfully classify cold winters. In general, warm winters are characterized by a predominantly zonal type of atmospheric circulation over the Northern Hemisphere (type W1). Within this type of circulation it is possible to distinguish two sub‐types, W2 and W3. Sub‐type W2 is characterized by a high‐pressure cell over North America, which is accompanied by enhanced cyclonic activity over the eastern North Pacific. Due to a broad southerly “anomalous” flow, surface air temperatures (SATs) are above normal almost everywhere over the continent. During the W3 sub‐type, the polar jet stream over North America, instead of forming a typical ridge‐trough pattern, is almost entirely zonal, thus effectively blocking an advection of cold Arctic air to the south. Cold winters tend to occur when the atmospheric circulation is more meridional (type C1). As with warm winters, there are two sub‐types of circulation, C2 and C3. In the case of C2, the jet stream loops southward over the western part of North America, but its northern excursion over the eastern part is suppressed. In this situation, the probability of a cold winter is higher for Lake Superior than for the lower Great Lakes. Sub‐type C3 is characterized by an amplification of the climatological ridge over the Rockies and the trough over the East Coast. The strongest negative SAT anomalies are located south of the Great Lakes basin, so that the probability of a cold winter is higher for the lower Great Lakes than for Lake Superior.