The North Atlantic Oscillation and oceanic precipitation variability

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.     

The influence of sea surface temperature anomalies on low-frequency variability of the North Atlantic Oscillation

The influence of sea surface temperature anomalies (SSTA) on multi-year persistence of the North Atlantic Oscillation (NAO) during the second half of the twentieth century is investigated using the Center for Ocean-Land-Atmosphere Studies (COLA) Atmospheric GCM (AGCM) with an emphasis on isolating the geographic location of the SSTA that produce this influence. The present study focuses on calculating the atmospheric response to the SSTA averaged over 1988–1995 (1961–1968) corresponding to the observed period of strong persistence of the positive (negative) phase of the decadal NAO. The model response to the global 1988–1995 average SSTA shows a statistically significant large-scale pattern characteristic of the positive phase of the NAO. Forcing with the global 1961–1968 average SSTA generates a NAO of the opposite polarity compared to observations. However, all large-scale features both in the model and observations during this period are weaker in magnitude and less significant compared to 1988–1995. Additional idealized experiments show that over the northern center of the NAO the non-linear component of the forced response appears to be quite important and acts to enhance the positive NAO signal. On the other hand, over the southern center where the model response is the strongest, it is also essentially linear. The 1988–1995 average SSTA restricted to the western tropical Pacific region produce a positive NAO remarkably similar in structure but stronger in magnitude than the model response to the global and tropical Indo-Pacific 1988–1995 forcing. A 200-hPa geopotential height response in these experiments shows a positive anomaly over the southern center of the NAO embedded in the Rossby wave trains propagating from the western tropical Pacific. Indian Ocean SSTA lead to much weaker positive NAO primarily through the effect on its northern center. SST forcing confined to the North Atlantic north of equator does not produce a response statistically different from the control simulation, suggesting that it is not strong enough to significantly affect the phase of the decadal NAO. Inclusion of the South Atlantic north of 45° south does not change this result.

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A new North Atlantic Oscillation index and its variability

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     

Subseasonal variability and predictability of the Arctic Oscillation/North Atlantic Oscillation in BCC_AGCM2.2

The subseasonal variability and predictability of the Arctic Oscillation/North Atlantic Oscillation (AO/NAO) is evaluated using a full set of hindcasts generated from the Beijing Climate Center Atmospheric General Circulation Model version 2.2 (BCC_AGCM2.2). It is shown that the predictability of the monthly mean AO/NAO index varies seasonally, with the highest predictability during winter (December–March) and the lowest during autumn (August–November), with respect to both observations and BCC_AGCM2.2 results. As compared with the persistence prediction skill of observations, the model skillfully predicts the monthly mean AO/NAO index with a one-pentad lead time during all winter months, and with a lead time of up to two pentads in December and January. During winter, BCC_AGCM2.2 exhibits an acceptable skill in predicting the daily AO/NAO index of ∼9 days, which is higher than the persistence prediction skill of observations of ∼4 days. Further analysis suggests that improvements in the simulation of storm track activity, synoptic eddy feedback, and troposphere–stratosphere coupling in the Northern Hemisphere could help to improve the prediction skill of subseasonal AO/NAO variability by BCC_AGCM2.2 during winter. In particular, BCC_AGCM2.2 underestimates storm track activity intensity but overestimates troposphere–stratosphere coupling, as compared with observations, thus providing a clue to further improvements in model performance.     

冬季北大西洋涛动对中国春季降水异常的影响

利用中国397个测站降水资料和NCEP/NCAR再分析资料,采用相关分析、合成分析等方法,研究了冬季北大西洋涛动(NAO)对我国春季降水的影响。结果表明,我国春季降水与前期冬季NAO关系密切,冬季NAO偏强(弱)时,我国东部南方地区春季降水偏多(少),北方地区春季降水偏少(多)。冬季NAO信号通过波列形式传播至东亚地区,使得春季东亚副热带急流和温带急流发生变化,冬季NAO偏强(弱)时,春季东亚副热带西风急流增强(减弱),温带急流减弱(增强)。进一步分析表明,冬季NAO异常会引起春季乌拉尔山高压脊和东亚大槽的变化,导致东亚对流层上层的温度发生变化,并由此产生经向温度梯度异常,这可能是NAO影响东亚高空急流的原因之一。春季东亚对流层上层的气温变冷(暖),使得东亚地区30°-40°N区域产生下沉(上升)运动,20°-30°N区域产生上升(下沉)运动,最终导致我国东部南方地区春季降水偏多(少)、北方地区春季降水偏少(多)。     

Stochastic models and spectra of interannual variability of mean annual sea surface temperature in the North Atlantic

Estimates of one- and two-variate autoregressive models of mean annual sea surface temperature (SST) in five Smed squares in the North Atlantic are obtained by analysing time series of SST, 1881–1970. Year-to-year variations of SST are shown to follow the AR model of order one with a regression parameter of 0.5 so that their generalized spectrum decreases monotonically and relatively fast with frequency while the limits of statistical predictability amount up to two years. Two-variate models of SST reveal frequency-dependent time lags up to three years and possess slightly better statistical predictability. A feedback in the system of warm and cold currents is found with a characteristic time scale of about six years, which plays an important role in the system's energy budget.     

冬季北大西洋涛动极端异常变化与东亚冬季风

依据资料分析发现,冬季北大西洋涛动指数与冬季西伯利亚高压范围呈反向变化关系,冬季北大西洋涛动指数异常偏高（低）时期,30～50oN的亚洲大陆中部气压显著偏低（高）,致使冬季西伯利亚高压和东亚冬季风减弱（增强）以及亚洲大陆北部气温显著偏高（低）。冬季西伯利亚高压范围异常变化对北大西洋涛动没有显著的影响,其对北半球海平面气压、850 hPa温度的影响也明显要弱于北大西洋涛动的影响。     

Weakening of Winter North Atlantic Oscillation Signal in Spring Precipitation over Southern China

In this study,the relationship between the North Atlantic Oscillation(NAO)in winter(December–February)and the precipitation over southern China(SCP)in the following spring(March–May)was investigated.Results showed an interdecadal change,from strong to weak connection,in their connection.Before the early1980s,they were highly correlated,with a strong(weak)winter NAO followed by an increased(decreased)spring SCP.However,after the early 1980s,their relationship was weakened significantly.This unstable relationship may be linked to the climatological change of East Asian jet.Before the early 1980s,the wave train along the Asian jet propagated the NAO signal eastward to East Asia and affected local upper-tropospheric atmospheric circulation.A strong NAO in winter led to an anomalous anticyclonic circulation at the south side of 30 N in East Asia in spring,resulting in an increase of SCP.In contrast,after the early1980s,the wave train pattern along the Asian jet extended eastward due to strengthening of the climatological East Asian jet.Correspondingly,the NAO-related East Asian atmospheric circulations in the upper troposphere shifted eastward,thereby weakening the linkage between the spring SCP and the winter NAO.     

Interannual and long-term variability in the North Atlantic Oscillation and Indian Summer monsoon rainfall

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     

Multidecadal variability of summer temperature over Romania and its relation with Atlantic Multidecadal Oscillation

We investigate the multidecadal variability of summer temperature over Romania as measured at 14 meteorological stations with long-term observational records. The dominant pattern of summer temperature variability has a monopolar structure and shows pronounced multidecadal variations. A correlation analysis reveals that these multidecadal variations are related with multidecadal variations in the frequency of four daily atmospheric circulation patterns from the North Atlantic region. It is found that on multidecadal time scales, negative summer mean temperature (TT) anomalies are associated with positive sea level pressure (SLP) anomalies centered over the northern part of the Atlantic Ocean and Scandinavia and negative SLP anomalies centered over the northern part of Africa. It is speculated that a possible cause of multidecadal fluctuations in the frequency of these four patterns are the sea surface temperature (SST) anomalies associated to the Atlantic Multidecadal Oscillation (AMO). These results have implications for predicting the evolution of summer temperature over Romania on multidecadal time scales.     

Modeled winter sea ice variability and the North Atlantic Oscillation: a multi-century perspective

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.     

Reclassification of rainfall regions of Turkey by K-means methodology and their temporal variability in relation to North Atlantic Oscillation (NAO)

Rainfall over Turkey portrays highly variable character both spatially and temporally. The aim of this study is to redefine main rainfall clusters of Turkey by using k-means methodology and investigate spatial shifts in the redefined rainfall clusters in subsequent periods with respect to North Atlantic Oscillation (NAO) patterns. Initially, monthly rainfall totals are subjected to k-means clustering by taking into consideration 148 stations covering the 1977?C2006 period. Considering the maximum silhouette value and lowest negative silhouette number, six rainfall clusters are determined as optimum classifications for this climate period. The results indicate that Aegean?CMarmara and Eastern Anatolia?CCentral Anatolia geographic regions are characterized as single rainfall cluster contrary to the conventional geographical regions. The Mediterranean region is characterized with two separate sub-regions indicating highly variable rainfall distribution characters over the region. The study further adapts a similar classification for 10-year sub-periods to determine spatial shifts of the redefined rainfall clusters for the last 30?years. From one decade to another, temporally drier and wetter clusters are observed with underlying shifting causes in relation to NAO patterns. Parallel to other studies in the literature, NAO is found to be partially useful in explaining the temporally dry trends while less useful in justifying wet periods. On the other hand, coefficient of variation (COV) is introduced in order to explain the temporal shifts in the clusters. Strong relations are obtained between the regions with the higher COV numbers and highest cluster shifts, while smaller COV numbers are associated with the most stable clusters.     

Enhancement of the summer North Atlantic Oscillation influence on Northern Hemisphere air temperature

This study investigates the relationship between the summer North Atlantic Oscillation (SNAO) and the simultaneous Northern Hemisphere (NH) land surface air temperature (SAT) by using the Climate Research Unit (CRU) data. The results show that the SNAO is related to NH land SAT, but this linkage has varied on decadal timescales over the last 52 years, with a strong connection appearing after the late 1970s, but a weak connection before. The mechanism governing the relationship between the SNAO and NH land SAT is discussed based on the NCEP/NCAR reanalysis data. The results indicate that such a variable relationship may result from changes of the SNAO mode around the late 1970s. The SNAO pattern was centered mainly over the North Atlantic before the late 1970s, and thus had a weak influence on the NH land SAT. But after the late 1970s, the SNAO pattern shifted eastward and its southern center was enhanced in magnitude and extent, which transported the SNAO signal to the North Atlantic surrounding continents and even to central East Asia via an upper level wave train along the Asian jet.     

Temporal structures of the North Atlantic Oscillation and its impact on the regional climate variability

In this study, the temporal structure of the variation of North Atlantic Oscillation （NAO） and its impact on regional climate variability are analyzed using various datasets. The results show that blocking formations in the Atlantic region are sensitive to the phase of the NAO. Sixty-seven percent more winter blocking days are observed during the negative phase compared to the positive phase of the NAO. The average length of blocking during the negative phase is about 11 days, which is nearly twice as long as the 6-day length observed during the positive phase of the NAO. The NAO-related differences in blocking frequency and persistence are associated with changes in the distribution of the surface air temperature anomaly, which, to a large extent, is determined by the phase of the NAO. The distribution of regional cloud amount is also sensitive to the phase of the NAO. For the negative phase, the cloud amounts are significant, positive anomalies in the convective zone in the Tropics and much less cloudiness in the mid latitudes. But for the positive phase of the NAO, the cloud amount is much higher in the mid-latitude storm track region. In the whole Atlantic region, the cloud amount shows a decrease with the increase of surface air temperature. These results suggest that there may be a negative feedback between the cloud amount and the surface air t.emperature in the Atlantic region.     

北大西洋涛动指数变化与北半球冬季阻塞活动

线性回归分析表明北大西洋涛动(NAO)主要与大西洋、欧洲及乌拉尔山地区阻塞的频率和强度的变化存在显著相关关系.在NAO负位相时期阻塞活动在大西洋地区较为频繁且强度较强,正位相时期大西洋地区阻塞活动减少,强度减弱,而欧洲阻塞加强,频率增加,同时乌拉尔山地区的阻塞活动也显著减少.NAO正指数的增强和减弱对应于大西洋和欧洲阻...     

北极涛动与北大西洋涛动的差异

根据北极涛动和北大西洋涛动指数的时间序列,选取两者差异较大的13个年份进行合成分析。结果表明：除北太平洋地区外,北极涛动与北大西洋涛动差异最显著的区域是西欧-地中海区域和亚洲东北部地区。北极涛动高指数阶段,对流层中层为纬向二波的驻波型,分别对应于极地-欧亚遥相关型和太平洋-北美遥相关型。同时,纬向平均纬向风偶极型使西风急流向极地偏移,与增强的中纬度经圈环流相互作用,引导对流层上层异常信号向下传播,形成高低空耦合机制。进一步分析发现,这种中纬度经圈环流异常和高低空耦合形势的差异主要表现在欧亚大陆地区;在北大西洋区域差异并不显著。     

Simulating the winter North Atlantic Oscillation: the roles of internal variability and greenhouse gas forcing

Analysis of simulations with seven coupled climate models demonstrates that the observed variations in the winter North Atlantic Oscillation (NAO), particularly the increase from the 1960s to the 1990s, are not compatible with either the internally generated variability nor the response to increasing greenhouse gas forcing simulated by these models. The observed NAO record can be explained by a combination of internal variability and greenhouse gas forcing, though only by the models that simulate the strongest variability and the strongest response. These models simulate inter-annual variability of the NAO index that is significantly greater than that observed, and can no longer explain the observed record if the simulated NAO indices are scaled so that they have the same high-frequency variance as that observed. It is likely, therefore, that other external forcings also contributed to the observed NAO index increase, unless the climate models are deficient in their simulation of inter-decadal NAO variability or their simulation of the response to greenhouse gas forcing. These conclusions are based on a comprehensive analysis of the control runs and transient greenhouse-gas-forced simulations of the seven climate models. The simulations of mean winter circulation and its pattern of inter-annual variability are very similar to the observations in the Atlantic half of the Northern Hemisphere. The winter atmospheric circulation response to increasing greenhouse gas forcing shows little inter-model similarity at the regional scale, and the NAO response is model-dependent and sensitive to the index used to measure it. At the largest scales, however, sea level pressure decreases over the Arctic Ocean in all models and increases over the Mediterranean Sea in six of the seven models, so that there is an increase of the NAO in all models when measured using a pattern-based index.