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1.
The role of El Niño/Southern Oscillation (ENSO) and the mechanism through which ENSO influences the precipitation variability over northwest India and the adjoining (NWIA) region is well documented. In this study, the relative role of North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) and ENSO in modulating the Asian jet stream in the Northern Hemisphere winter and their relative impact on the precipitation variability over the region have been estimated through analysis of observed data. It is seen that interannual variations of NWIA precipitation are largely influenced by ENSO. An empirical orthogonal function (EOF) analysis has been carried out to understand dominant modes of interannual variability of zonal wind at 200 hPa of the Northern Hemisphere. The EOF-1 pattern in the tropical region is similar to that of an ENSO pattern, and the principal component (PC) time series corresponds to the ENSO time series. The EOF-2 spatial pattern resembles that of NAO/AO with correlation of PC time series with AO and NAO being 0.74 and 0.62, respectively. The precipitation anomaly time series over the region of interest has marginally higher correlation with the PC-2 time series as compared to that of PC-1. Regression analysis of precipitation and circulation parameters indicates a larger contribution of the second mode to variability of winds and precipitation over the NWIA. Moisture transport from the Arabian Sea during the active phase of NAO/AO and the presence of a cyclonic anomaly lead to higher precipitation over the NWIA region.  相似文献   

2.
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   

3.
 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   

4.
A new North Atlantic Oscillation index and its variability   总被引：31，自引：4，他引：27  
A new North Atlantic Oscillation (NAO) index, the NAOI, is defined as the differences of normalized sea level pressures regionally zonal-averaged over a broad range of longitudes 80°W-30°E. A comprehensive comparison of six NAO indices indicates that the new NAOI provides a more faithful representation of the spatial-temporal variability associated with the NAO on all timescales. A very high signal-to-noise ratio for the NAOI exists for all seasons, and the life cycle represented by the NAOI describes well the seasonal migration for action centers of the NAO. The NAOI captures a larger fraction of the variance of sea level pressure over the North Atlantic sector (20°-90°N, 80°W-30°E), on average 10% more than any other NAO index. There are quite different relationships between the NAOI and surface air temperature during winter and summer. A novel feature, however, is that the NAOI is significantly negative correlated with surface air temperature over the North Atlantic Ocean between 10°-25°N and  相似文献   

5.
Based on daily precipitation data from 524 meteorological stations in China during the period 1960–2009, the climatology and the temporal changes (trends, interannual, and decadal variations) in the proportion of seasonal precipitation to the total annual precipitation were analyzed on both national and regional scales. Results indicated that (1) for the whole country, the climatology in the seasonal distribution of precipitation showed that the proportion accounted for 55 % in summer (June–August), for around 20 % in both spring (March–May) and autumn (September–November), and around 5 % in winter (December–February). But the spatial features were region-dependent. The primary precipitation regime, “summer–autumn–spring–winter”, was located in central and eastern regions which were north of the Huaihe River, in eastern Tibet, and in western Southwest China. The secondary regime, “summer–spring–autumn–winter”, appeared in the regions south of the Huaihe River, except Jiangnan where spring precipitation dominated, and the southeastern Hainan Island where autumn precipitation prevailed. (2) For the temporal changes on the national scale, first, where the trends were concerned, the proportion of winter precipitation showed a significantly increasing trend, while that of the other three seasons did not show any significant trends. Second, for the interannual variation, the variability in summer was the largest among the four seasons and that in winter was the smallest. Then, on the decadal scale, China experienced a sharp decrease only in the proportion of summer precipitation in 2000. (3) For the temporal changes on the regional scale, all the concerned 11 geographic regions of China underwent increasing trends in the proportion of winter precipitation. For spring, it decreased over the regions south of the Yellow River but increased elsewhere. The trend in the proportion of summer precipitation was generally opposite to that of spring. For autumn, it decreased over the other ten regions except Inner Mongolia with no trend. It is noted that the interannual variability of precipitation seasonality is large over North China, Huanghuai, and Jianghuai; its decadal variability is large over the other regions, especially over those regions south of the Yangtze River.  相似文献   

6.
In this paper,the data of summer precipitation in Northwest China were expanded by means of EOF.According tomajor eigenvectors in expansion the area of Northwest China was divided into four natural rainfall regions.Amongthem the region of greatest precipitation variability is found over the East Qinghai-North Shaanxi region,includingEast Qinghai,Central and East Gansu,Ningxia and North Shaanxi.There is apparent teleconnection between the firstand second time-dependent coefficients in EOF expansion and the tropical Pacific SST in the corresponding period andearlier months.The variation of the east tropical Pacific SST in winter and spring is able to predict precipitation trend ofNorthwest China next summer.Moreover,in the El Nino years precipitation trend is opposite to the following year,andthe region from East Qinghai to North Shaanxi is most sensitive.  相似文献   

7.
春季北大西洋三极型海温异常变化及其与NAO和ENSO的联系   总被引：1，自引：0，他引：1  
利用1951—2016年HadISST逐月海表温度(Sea Surface Temperature,SST)资料,NCEP/NCAR再分析资料以及1958—2016年美国伍兹霍尔海洋研究所(Woods Hole Oceanographic Institution,WHOI)提供的OAFlux数据集,运用经验正交函数分解(Empirical Orthogonal Function,EOF)和偏相关分析等统计方法,研究了春季北大西洋海温异常的主要特征及其与春季NAO和前期冬季ENSO联系。结果表明:春季北大西洋海温异常EOF的第一模态是自北而南出现的三极结构的海温距平型,其方差贡献率为35.7%。春季北大西洋三极型海温异常的形成主要受到春季NAO主导作用,还受到前期冬季热带中东太平洋海温异常的影响。消除前期冬季Niňo3.4的影响后,春季北大西洋三极型海温异常指数与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数的偏相关系数分别为0.50,通过了99%置信度水平的显著性检验。消除春季NAO的影响后,春季北大西洋三极型海温异常指数与前期冬季Niňo3.4指数的偏相关系数为-0.26,通过了95%信度水平的显著性检验。春季NAO正(负)位相引起的海表风场和海表湍流热通量的异常,进而激发出正(负)位相的北大西洋三极型海温异常。前期冬季ENSO事件可以引起春季大气环流异常和热带外海温异常,进而调制春季NAO对北大西洋三极型海温异常的影响。  相似文献   

8.
Summary The main characteristics of the spatial and temporal variability of winter and summer precipitation observed at 30 stations in Serbia and Montenegro were analysed for the period 1951–2000. The rainfall series were examined spatially by means of Empirical Orthogonal Functions (EOF) and temporally by means of the Mann-Kendall test and spectral analysis. The Alexandersson test was used to detect the inhomogeneity of the data set.The EOF analysis gave three winter and summer dominant modes of variations, which explained 89.7% and 70.4% of the variance, respectively. The time series associated with the first pattern showed a decreasing trend in winter precipitation. The spectral analysis showed a 16-year oscillation for the dominant winter pattern, around a 3-year oscillation for the dominant summer pattern, and a quasi-cycle of 2.5 years for the winter third pattern.  相似文献   

9.
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   

10.
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   

11.
热带太平洋海温与中国西北夏季降水的关系   总被引：18，自引：1，他引：18  
本文将中国西北(陕、甘、宁、青四省区)夏季降水资料用经验正交函数(EOF)方法展开,根据展开后的主要特征向量把西北划分为四个自然降水区,其中降水变率最大的区域是包括青海省东部、甘肃省中东部、宁夏全区和陕西省北部在内的海东—陕北区。西北夏季降水经EOF方法展开后的第一、二时间系数与同期和前期热带太平洋海温有着明显的遥相关。热带东太平洋海温冬、春季的冷暖变化程度,可以预示后期西北夏季降水的趋势变化。另外,在厄尼诺现象发生的当年和次年,西北夏季降水具有明显的相反变化,其中海东—陕北区最敏感。  相似文献   

12.
北极海冰变化的时间和空间型   总被引：14，自引：0，他引：14  
汪代维  杨修群 《气象学报》2002,60(2):129-138
利用 4 4a(195 1～ 1994年 )北极海冰密度逐月资料 ,分析提出了一种与北极冰自然季节变化相吻合的分季法 ,并根据这种分季法 ,使用EOF分解 ,揭示了北极各季海冰面积异常的特征空间型及其对应的时间变化尺度。结果表明 :(1)北极冰面积异常变化的关键区 ,冬季 (2～ 4月 )主要位于北大西洋一侧的格陵兰海、巴伦支海和戴维斯海峡以及北太平洋一侧的鄂霍次克海和白令海 ,夏季 (8～ 10月 )则主要限于从喀拉海、东西伯利亚海、楚科奇海到波佛特海的纬向带状区域内 ,格陵兰海和巴伦支海是北极海冰面积异常变化的最重要区域 ;(2 )春 (5～ 7月 )、秋 (11月～次年 1月 )季各主要海区海冰面积异常基本呈同相变化 ,夏季东西伯利亚海、楚科奇海、波佛特海一带海冰面积异常和喀拉海呈反相变化 ,而冬季巴伦支海、格陵兰海海冰面积异常和戴维斯海峡、拉布拉多海、白令海、鄂霍次克海的海冰变化呈反相变化 ;(3)北极冰总面积过去 4 4a来确实经历了一种趋势性的减少 ,并且叠加在这种趋势变化之上的是年代尺度变化 ,其中春季 (5～ 7月 )海冰面积异常变化对年平均北极冰总面积异常变化作出了主要贡献 ;(4)位于北太平洋一侧极冰面积异常型基本具有半年的持续性 ,而位于北大西洋一侧极冰面积异常型具有半年至一年的持续性  相似文献   

13.
The aim of the present paper is to detect recurring fluctuations in the course of annual and seasonal total precipitation in Pomerania. The basic material consisted of monthly sums of atmospheric precipitation obtained from 11 IMGW weather stations from 1951-2010. The analysis comprised both annual and seasonal precipitation recorded in spring (March-May), summer (June-August), autumn (September-November), and winter (December-February). The results of spectral analysis obtained using module functional series revealed a significant cyclical nature of rainfall occurrence. Annual and seasonal total precipitation recorded at 11 stations representative for Pomerania that occurred in the period 1951-2010, have cycles of variable length, i.e., 10-year period for annual sums, 30-year period for autumn, and quasi 7-year period for winter sums. However, the detected cyclical elements differed depending on the station, and their lengths varied greatly depending on the season: approximately from 6 to 30 years. The greatest variability in cyclicality was recorded in summer precipitation, whereas during the calendar spring the changes in total precipitation recorded at most of the hstations in periods of the same length.  相似文献   

14.
In this study the behaviour of the North Atlantic Oscillation (NAO) and its impact on the surface air temperature in Europe 1891-1990 is analysed using statistical time series analysis techniques. For this purpose, both the NAO index (NAOI) and the surface air temperature time series from 41 European stations are split up into typical variation components. Various measures of correlation indicate that the NAOI-temperature relationships are approximately linear and most pronounced in winter. The spatial correlation patterns show a correlation decrease from North West to South East (winter) exceeding correlation coefficients of 0.6 in the Scotland-South Norwegian area. In summer, these correlations are very weak, in spring and autumn stronger but smaller than in winter. These correlations change significantly in time indicating increasing correlations in Central and North Europe and decreasing correlations in the North West. Low-frequent episodic components represented by related polynomials of different order are very outstanding in both NAO and temperatures showing up in all seasons, except summer, relative maxima roughly 1900 and in recent times, relative minima in the beginning ( ca . 1870) and roughly 1960-1970. Periodogramm analysis reveals a dominant cycle of 7.5 years (NAOI and a majority of temperature time series) whereas in case of the polynomial component one may speculate about a 80-90 year cycle.  相似文献   

15.
塞罕坝地区是我国生态文明建设的典范区,本文对塞罕坝地区多年气温和降水的大尺度环流异常进行合成分析,研究其环流特征及关键环流因子。结果表明：1962-2020年塞罕坝地区气温总体呈偏高趋势,春、秋、冬季降水呈偏多趋势,夏季降水呈偏少趋势。大尺度环流背景下,各季节影响塞罕坝地区气温和降水的主要因子不同,通过挑选最优因子分析得出：塞罕坝地区受大气环流指数影响明显,春季太平洋-北美遥相关型指数（PNA）、夏季北大西洋涛动指数（NAO）、秋季西太平洋遥相关型指数（WP）、冬季高原季风指数（PMI）与该地气温和降水有较好的关系。  相似文献   

16.
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   

17.
The NCEP twentieth century reanalyis and a 500-year control simulation with the IPSL-CM5 climate model are used to assess the influence of ocean-atmosphere coupling in the North Atlantic region at seasonal to decadal time scales. At the seasonal scale, the air-sea interaction patterns are similar in the model and observations. In both, a statistically significant summer sea surface temperature (SST) anomaly with a horseshoe shape leads an atmospheric signal that resembles the North Atlantic Oscillation (NAO) during the winter. The air-sea interactions in the model thus seem realistic, although the amplitude of the atmospheric signal is half that observed, and it is detected throughout the cold season, while it is significant only in late fall and early winter in the observations. In both model and observations, the North Atlantic horseshoe SST anomaly pattern is in part generated by the spring and summer internal atmospheric variability. In the model, the influence of the ocean dynamics can be assessed and is found to contribute to the SST anomaly, in particular at the decadal scale. Indeed, the North Atlantic SST anomalies that follow an intensification of the Atlantic meridional overturning circulation (AMOC) by about 9 years, or an intensification of a clockwise intergyre gyre in the Atlantic Ocean by 6 years, resemble the horseshoe pattern, and are also similar to the model Atlantic Multidecadal Oscillation (AMO). As the AMOC is shown to have a significant impact on the winter NAO, most strongly when it leads by 9 years, the decadal interactions in the model are consistent with the seasonal analysis. In the observations, there is also a strong correlation between the AMO and the SST horseshoe pattern that influences the NAO. The analogy with the coupled model suggests that the natural variability of the AMOC and the gyre circulation might influence the climate of the North Atlantic region at the decadal scale.  相似文献   

18.
Using monthly independently reconstructed gridded European fields for the 500 hPa geopotential height, temperature, and precipitation covering the last 235 years we investigate the temporal and spatial evolution of these key climate variables and assess the leading combined patterns of climate variability. Seasonal European temperatures show a positive trend mainly over the last 40 years with absolute highest values since 1766. Precipitation indicates no clear trend. Spatial correlation technique reveals that winter, spring, and autumn covariability between European temperature and precipitation is mainly influenced by advective processes, whereas during summer convection plays the dominant role. Empirical Orthogonal Function analysis is applied to the combined fields of pressure, temperature, and precipitation. The dominant patterns of climate variability for winter, spring, and autumn resemble the North Atlantic Oscillation and show a distinct positive trend during the past 40 years for winter and spring. A positive trend is also detected for summer pattern 2, which reflects an increased influence of the Azores High towards central Europe and the Mediterranean coinciding with warm and dry conditions. The question to which extent these recent trends in European climate patterns can be explained by internal variability or are a result of radiative forcing is answered using cross wavelets on an annual basis. Natural radiative forcing (solar and volcanic) has no imprint on annual European climate patterns. Connections to CO2 forcing are only detected at the margins of the wavelets where edge effects are apparent and hence one has to be cautious in a further interpretation. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

19.
A gridded monthly precipitable water (PW) data for 1979?C2007 from the NCEP/NCAR reanalysis are used to investigate summertime interannual PW variability over Europe and its relation to the key climate parameters in the region. During summer season the first EOF mode of PW, explaining 27?C41% of its total variance, demonstrates significant month-to-month changes in its structure, thus, implying its essential non-stationarity. The second EOF mode of PW is also non-stationary during the summer season. In contrast to precipitation, both leading modes of PW are not associated with the North Atlantic Oscillation (NAO), as well as with other regional teleconnections, suggesting relatively minor role of the atmospheric dynamics in atmospheric moisture variability over Europe during summer season. Analysis of links between leading EOF modes of regional PW and air temperature (AT) has revealed a strong link between PW and AT over Europe, persisting during entire summer season. Locally, these links imply that positive (negative) AT anomalies result in enhanced (decreased) PW over particular region. Revealed links between leading modes of PW and AT highlight important role of thermodynamics in summertime PW variability over Europe. Detected relatively weak and unstable links between leading modes of PW and precipitation over Europe were somewhat expected since in contrast to atmospheric moisture, regional precipitation variability is largely driven by the atmospheric dynamics (particularly, the NAO).  相似文献   

20.
Abstract

Past research has unveiled important variations in total precipitation, often related to large‐scale shifts in atmospheric circulation, and consistent with projected responses to enhanced greenhouse warming. More recently, however, it has been realized that important and influential changes in the variability of daily precipitation events have also occurred in the past, often unrelated to changes in total accumulation.

This study aims to uncover variations in daily precipitation intensity over Canada and to compare the observed variations with those in total accumulation and two dominant modes of atmospheric variability, namely the North Atlantic Oscillation (NAO) and the Pacific/North America teleconnection pattern (PNA). Results are examined on both annual and seasonal bases, and with regions defined by similarities in monthly variability.

Seasonally increasing trends in total precipitation that result from increases in all levels of event intensity during the 20th century are found in southern areas of Canada. During the latter half of the century increases are concentrated in heavy and intermediate events, with the largest changes occurring in Arctic areas. Variations in precipitation intensity can, however, be unrelated to variations in the total accumulation. Consistent with these differences, the precipitation responses to the NAO and PNA are often found to occur only at specific levels of event intensity. Precipitation responses to the NAO occur in northeastern regions in summer and winter with the intensity affected in both seasons. The PNA strongly influences precipitation in many regions of the country during autumn and winter. In particular, it strongly influences variations in southern British Columbia and the Prairies, affecting the intensity in only some areas. However, it only influences the frequency of heavier events in autumn and winter in Ontario and southern Quebec, where this response is actually more robust than the response in total accumulation. During these seasons a negative PNA generally leads to more extreme precipitation events.  相似文献   

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