Winter AO/NAO modifies summer ocean heat content and monsoonal circulation over the western Indian Ocean

This paper analyzes the possible influence of boreal winter Arctic Oscillation/North Atlantic Oscillation (AO/ NAO) on the Indian Ocean upper ocean heat content in summer as well as the summer monsoonal circulation. The strong interannual co-variation between winter 1000-hPa geopotential height in the Northern Hemisphere and summer ocean heat content in the uppermost 120 m over the tropical Indian Ocean was investigated by a singular decomposition analysis for the period 1979–2014. The second paired-modes explain 23.8% of the squared covariance, and reveal an AO/NAO pattern over the North Atlantic and a warming upper ocean in the western tropical Indian Ocean. The positive upper ocean heat content enhances evaporation and convection, and results in an anomalous meridional circulation with ascending motion over 5°S–5°N and descending over 15°–25°N. Correspondingly, in the lower troposphere, significantly anomalous northerly winds appear over the western Indian Ocean north of the equator, implying a weaker summer monsoon circulation. The off-equator oceanic Rossby wave plays a key role in linking the AO/NAO and the summer heat content anomalies. In boreal winter, a positive AO/NAO triggers a down-welling Rossby wave in the central tropical Indian Ocean through the atmospheric teleconnection. As the Rossby wave arrives in the western Indian Ocean in summer, it results in anomalous upper ocean heating near the equator mainly through the meridional advection. The AO/NAO-forced Rossby wave and the resultant upper ocean warming are well reproduced by an ocean circulation model. The winter AO/NAO could be a potential season-lead driver of the summer atmospheric circulation over the northwestern Indian Ocean.     

Interannual linkage between Arctic/North Atlantic Oscillation and tropical Indian Ocean precipitation during boreal winter

In the study authors analyzed the interannual relationship between the Arctic Oscillation (AO)/North Atlantic Oscillation (NAO) and the tropical Indian Ocean (TIO) precipitation in boreal winter for the period 1979–2009. A significant simultaneous teleconnection between them is found. After removing the El Niño/Southern Oscillation and Indian Ocean dipole signals, the AO/NAO and the TIO precipitation (0°–10°S, 60°–80°E) yield a correlation of +0.56, which is also consistent with the AO/NAO-outgoing longwave radiation correlation of ?0.61. The atmospheric and oceanic features in association with the AO/NAO-precipitation links are investigated. During positive AO/NAO winter, the Rossby wave guided by westerlies tends to trigger persistent positive geopotential heights in upper troposphere over about 20°–30°N and 55°–70°E, which is accompanied by a stronger Middle East jet stream. Meanwhile, there are anomalous downward air motions, strengthening the air pressure in mid-lower troposphere. The enhanced Arabian High brings anomalous northern winds over the northern Indian Ocean. As a result the anomalous crossing-equator air-flow enhances the intertropical convergence zone (ITCZ). On the other hand, the anomalous Ekman transport convergence by the wind stress curl over the central TIO deepens the thermocline. Both the enhanced ITCZ and the anomalous upper ocean heat content favor in situ precipitation in the central TIO. The AO/NAO-TIO precipitation co-variations in the IPCC AR4 historical climate simulation (1850–1999) of Bergen Climate Model version 2 were investigated. The Indian Ocean precipitation anomalies (particularly the convective precipitation along the ITCZ), in conjunction with the corresponding surface winds and 200 hPa anticyclonic atmospheric circulation and upper ocean heat contents were well reproduced in simulation. The similarity between the observation and simulation support the physical robustness of the AO/NAO-TIO precipitation links.     

我国短期气候预测的物理基础及其预测思路

短期气候预测依据大气科学原理,运用气候动力学、统计学等手段,在研究气候异常成因的基础上对未来气候趋势进行预测。虽然目前我国短期气候预测的水平还不高,但短期气候预测是国家经济发展和防灾减灾的迫切需求,提高预测准确率是气象科研和业务人员的重要任务。该文从海洋、积雪等外强迫信号及大气环流大尺度变动等大气内部特性等角度概述了短期气候预测的物理基础,简要回顾了近60年来我国短期气候预测的发展历程,并介绍了作者近十几年来研制短期气候预测客观统计学及统计与动力学相结合预测模型的主要思路。     

Arctic Oscillation and Antarctic Oscillation in Internal Atmospheric Variability with an Ensemble AGCM Simulation

In this study, we investigated the features of Arctic Oscillation （AO） and Antarctic Oscillation （AAO）, that is, the annular modes in the extratropics, in the internal atmospheric variability attained through an ensemble of integrations by an atmospheric general circulation model （AGCM） forced with the global observed SSTs. We focused on the interannual variability of AO/AAO, which is dominated by internal atmospheric variability. In comparison with previous observed results, the AO/AAO in internal atmospheric variability bear some similar characteristics, but exhibit a much clearer spatial structure： significant correlation between the North Pacific and North Atlantic centers of action, much stronger and more significant associated precipitation anomalies, and the meridional displacement of upper-tropospheric westerly jet streams in the Northern/Southern Hemisphere. In addition, we examined the relationship between the North Atlantic Oscillation （NAO）/AO and East Asian winter monsoon （EAWM）. It has been shown that in the internal atmospheric variability, the EAWM variation is significantly related to the NAO through upper-tropospheric atmospheric teleconnection patterns.     

Interannual Variations of the Blocking High over the Ural Mountains and Its Association with the AO/NAO in Boreal Winter

This paper analyzes interannual variations of the blocking high over the Ural Mountains in the boreal winter and their association with the Arctic Oscillation/North Atlantic Oscillation (AO/NAO).In Jan...     

Natural forcing of climate during the last millennium: fingerprint of solar variability

The variability of the climate during the last millennium is partly forced by changes in total solar irradiance (TSI). Nevertheless, the amplitude of these TSI changes is very small so that recent reconstruction data suggest that low frequency variations in the North Atlantic Oscillation (NAO) and in the thermohaline circulation may have amplified, in the North Atlantic sector and mostly in winter, the radiative changes due to TSI variations. In this study we use a state-of-the-art climate model to simulate the last millennium. We find that modelled variations of surface temperature in the Northern Hemisphere are coherent with existing reconstructions. Moreover, in the model, the low frequency variability of this mean hemispheric temperature is found to be correlated at 0.74 with the solar forcing for the period 1001?C1860. Then, we focus on the regional climatic fingerprint of solar forcing in winter and find a significant relationship between the low frequency TSI forcing and the NAO with a time lag of more than 40?years for the response of the NAO. Such a lag is larger than the around 20-year lag suggested in other studies. We argue that this lag is due, in the model, to a northward shift of the tropical atmospheric convection in the Pacific Ocean, which is maximum more than four decades after the solar forcing increase. This shift then forces a positive NAO through an atmospheric wave connection related to the jet-stream wave guide. The shift of the tropical convection is due to the persistence of anomalous warm SST forcing the anomalous precipitation, associated with the advection of warm SST by the North Pacific subtropical gyre in a few decades. Finally, we analyse the response of the Atlantic meridional overturning circulation to solar forcing and find that the former is weakened when the latter increases. Changes in wind stress, notably due to the NAO, modify the barotropic streamfunction in the Atlantic 50?years after solar variations. This implies a wind-driven modification of the oceanic circulation in the Atlantic sector in response to changes in solar forcing, in addition to the variations of the thermohaline circulation.     

Northern poleward edge of regional Hadley cell over western Pacific during boreal winter: year-to-year variability,influence factors and associated winter climate anomalies

Compared to the zonal-mean Hadley cell (HC), our knowledge of the characteristics, influence factors and associated climate anomalies of the regional HC remains quite limited. Here, we examine interannual variability of the northern poleward HC edge over western Pacific (WPHCE) during boreal winter. Results suggest that interannual variability of the WPHCE is impacted by the El Niño-Southern Oscillation (ENSO) Modoki, North Pacific Oscillation (NPO) and North Atlantic Oscillation (NAO). The WPHCE tends to shift poleward during negative phase of the ENSO Modoki, and positive phases of the NPO and NAO, which highlights not merely the tropical forcing but also the extratropical signals that modulate the WPHCE. ENSO modoki, NPO and NAO modulate the WPHCE via inducing atmospheric anomalies over the western North Pacific. We further investigate the climatic impacts of the WPHCE on East Asia. The poleward shift of the northern descending branch of the WPHC results in anomalous upward (downward) motions and upper-level divergence (convergence) anomalies over south-central China (northern East-Asia), leading to increased (decreased) rainfall there. Moreover, pronounced cold surface air temperature anomalies appear over south-central China when the sinking branch of the WPHC moves poleward. Based on the temperature diagnostic analysis, negative surface temperature tendency anomalies over central China are mostly attributable to the cold zonal temperature advection and ascent-induced adiabatic cooling, while the negative anomalies over South China are largely due to the cold meridional temperature advection. These findings could improve our knowledge of the WPHCE variability and enrich the knowledge of forcing factors for East Asian winter climate.

     

The Role of the Solar Cycle in the Relationship Between the North Atlantic Oscillation and Northern Hemisphere Surface Temperatures

The North Atlantic Oscillation （NAO） is one of the leading modes of climate variability in the Northern Hemisphere. It has been shown that it clearly relates to changes in meteorological variables, such as surface temperature, at hemispherical scales. However, recent studies have revealed that the NAO spatial pattern also depends upon solar forcing. Therefore, its effects on meteorological variables must vary depending upon this factor. Moreover, it could be that the Sun affects climate through variability patterns, a hypothesis that is the focus of this study. We find that the relationship between the NAO/AO and hemispheric temperature varies depending upon solar activity. The results show a positive significant correlation only when solar activity is high. Also, the results support the idea that solar activity influences tropospheric climate fluctuations in the Northern Hemisphere via the fluctuations of the stratospheric polar vortex .     

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.     

Transition of Zonal Asymmetry of the Arctic Oscillation and the Antarctic Oscillation at the End of 1970s

In this study,the interdecadal changes in the zonal symmetry of both Arctic Oscillation(AO) and Antarctic Oscillation(AAO) were analyzed.To describe the zonal asymmetry,a local index of AO and AAO was defined using the normalized sea level pressure(SLP) differences between 40° and 65°(latitudes) in both hemispheres.The zonal covariability of local AO and AAO can well represent the zonal symmetry of AO and AAO.Results show that the zonal asymmetry of both AO and AAO significantly changed in the late 1970s.AO was less asymmetric in the zonal direction in the boreal winter season during the latter period,while in the boreal summer it became more asymmetric after 1979.The zonal symmetry of AAO in both austral summer and winter has also significantly decreased since the late 1970s.These changes may imply interdecadal transition in the atmospheric circulation at middle and high latitudes,which is of vital importance to understanding climate variability and predictability across the globe,including the African-Asian-Australian monsoon regions.     

On non-linearities in a forced North Atlantic Oscillation

Large ensembles of simulations (ensemble size of 500 members) are performed using a simplified atmospheric general circulation model (AGCM) in order to investigate the non-linearities in the response to composite sea surface temperature (SST) anomaly forcings that are constant in time. The SST composite corresponds to the observed anomaly associated with the atmospheric North Atlantic Oscillation (NAO). The integration length is 90 days for each ensemble (covering January, February and March). A non-linearity is found in the mean response to the SST-forcing, with the negative SST-NAO forcing leading to a stronger and more clear atmospheric NAO response. These non-linearities appear to be due to asymmetries in the heating anomalies induced by the SST-forcing and asymmetries in the transient eddy vorticity forcing. Further non-linearities are due to initial period dependences of the response to the same SST-forcing. As a consequence, a pre-existing negative atmospheric NAO is much more persistent due to SST-feedback than a positive NAO.     

Tropical/extratropical forcing on wintertime variability of the extratropical temperature and circulation

The secular trends and interannual variability of wintertime temperatures over northern extratropical lands and circulations over the northern hemisphere are examined using the NCEP/NCAR reanalysis from 1961 to 2010. A primitive equation dry atmospheric model, driven by time-averaged forcing in each winter diagnosed from the NCEP reanalysis, is then employed to investigate the influences of tropical and extratropical forcing on the temperature and circulation variability. The model has no topography and the forcing is thus model specific. The dynamic and thermodynamic maintenances of the circulation and temperature anomalies are also diagnosed. Distinct surface temperature trends over 1961–1990 and 1991–2010 are found over most of the extratropical lands. The trend is stronger in the last two decades than that before 1990, particularly over eastern Canadian Arctic, Greenland, and Asia. The exchange of midlatitude and polar air supports the temperature trends. Both the diagnosed extratropical and tropical forcings contribute to the temperature and circulation trends over 1961–1990, while the extratropical forcing dominates tropical forcing for the trends over 1991–2010. The contribution of the tropical forcing to the trends is sensitive to the period considered. The temperature and circulation responses to the diagnosed tropical and extratropical forcings are approximately additive and partially offsetting. Covariances between the interannual surface temperature and 500-hPa geopotential anomalies for the NCEP reanalysis from 1961 to 2010 are dominated by two leading modes associated with the North Atlantic Oscillation (NAO) and Pacific-North American (PNA) teleconnection patterns. The diagnosed extratropical forcing accounts for a significant part of the NAO and PNA associated variability, including the interannual variability of stationary wave anomalies, as well as dynamically and thermodynamically synoptic eddy feedbacks over the North Atlantic and North Pacific. The tropical forcing contributes to the PNA related temperature and circulation variability, but has a small contribution to the NAO associated variability. Additionally, relative contributions of tropical Indian and Pacific forcings are also assessed.     

北极涛动的年代际变化及其气候影响

北极涛动(Arctic Oscillation,AO)是北半球热带外地区大气环流变率的主导模态,对北半球以及区域尺度气温变化具有重要影响.AO可在没有外强迫条件下通过d波流相互作用形成,因此它被认为是全球气候系统内部变率的重要组成部分.研究年代际尺度上AO的变化及其气候影响,可加深对当前北半球气候变化规律的物理理解,也...     

On the separability of the North Atlantic oscillation and Arctic oscillation

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.     

The North Atlantic Oscillation as a source of stochastic forcing of the wind-driven ocean circulation

Observations show that at middle and high latitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that of the seasonal cycle and will likely exert a significant influence on the ocean circulation. The focus of this work will be the contribution of the North Atlantic Oscillation (NAO) to the stochastic forcing in the North Atlantic and its influence on the large-scale, wind-driven ocean circulation. To this end, a QG model of the North Atlantic Ocean was forced with the stochastic component of wind stress curl associated with the NAO signal. The ocean response is localized primarily in the western boundary region and can be conveniently understood using generalized stability analysis. Much of the variability is associated with the nonnormal influence of the bathymetry and inhomogeneities in the western boundary flow on the large-scale circulation. A more traditional statistical analysis of the circulation, however, reveals that there are very small and insignificant correlations between the NAO forcing and the ocean response within the western boundary region. This suggests that the dynamics of the ocean response to stochastic forcing may obscure any obvious coherence between the forcing and the response which is equally difficult to identify from observations.     

The North Atlantic Oscillation as a source of stochastic forcing of the wind-driven ocean circulation

Observations show that at middle and high latitudes, the magnitude of stochastic wind stress forcing due to atmospheric weather is comparable to that of the seasonal cycle and will likely exert a significant influence on the ocean circulation. The focus of this work will be the contribution of the North Atlantic Oscillation (NAO) to the stochastic forcing in the North Atlantic and its influence on the large-scale, wind-driven ocean circulation. To this end, a QG model of the North Atlantic Ocean was forced with the stochastic component of wind stress curl associated with the NAO signal. The ocean response is localized primarily in the western boundary region and can be conveniently understood using generalized stability analysis. Much of the variability is associated with the nonnormal influence of the bathymetry and inhomogeneities in the western boundary flow on the large-scale circulation. A more traditional statistical analysis of the circulation, however, reveals that there are very small and insignificant correlations between the NAO forcing and the ocean response within the western boundary region. This suggests that the dynamics of the ocean response to stochastic forcing may obscure any obvious coherence between the forcing and the response which is equally difficult to identify from observations.     

Arctic or North Atlantic Oscillation? Arguments based on the principal component analysis methodology

Summary The Arctic Oscillation (AO) appears as the leading unrotated mode of principal component analysis (PCA) of monthly mean sea level pressure anomalies, whereas the North Atlantic Oscillation (NAO) results from rotated PCA, regardless of the number of PCs rotated. Three criteria are employed to decide whether the interpretation in terms of the NAO or AO should be preferred: the degree of simple structure, the similarity between the PC loadings and correlation/covariance maps, and the sensitivity to spatial subsampling. All these criteria favour, to a different extent, the interpretation in terms of the NAO. This is further supported by more general arguments. Therefore, the statistical arguments suggest that in interpreting the Northern Hemisphere circulation variability, the sectorial view, i.e. the NAO, should be preferred to the hemispheric view, i.e. the AO. Our analysis supports the idea expressed in other studies that the AO is rather a statistical artifact.     

冬季北半球大气对秋冬季巴伦支海海冰异常的敏感性研究

基于ERA-Interim再分析资料,借助大气模式CAM4,分析了北半球冬季不同月份的平均大气对巴伦支海不同振幅及不同季节海冰扰动的敏感性,并考察了中高纬度典型大气模态的分布变化情况.结果表明,冬季巴伦支海海冰的减少,会导致湍流热通量异常向上、局地异常变暖及水汽含量的异常升高,且相关异常的强度和范围随着海冰减少幅度的减...     

Changes of interannual NAO variability in response to greenhouse gases forcing

Observations show that there was change in interannual North Atlantic Oscillation (NAO) variability in the mid-1970s. This change was characterized by an eastward shift of the NAO action centres, a poleward shift of zonal wind anomalies and a downstream extension of climate anomalies associated with the NAO. The NAO interannual variability for the period after the mid-1970s has an annular mode structure that penetrates deeply into the stratosphere, indicating a strengthened relationship between the NAO and the Arctic Oscillation (AO) and strengthened stratosphere-troposphere coupling. In this study we have investigated possible causes of these changes in the NAO by carrying out experiments with an atmospheric GCM. The model is forced either by doubling CO₂, or increasing sea surface temperatures (SST), or both. In the case of SST forcing the SST anomaly is derived from a coupled model simulation forced by increasing CO₂. Results indicate that SST and CO₂ change both force a poleward and eastward shift in the pattern of interannual NAO variability and the associated poleward shift of zonal wind anomalies, similar to the observations. The effect of SST change can be understood in terms of mean changes in the troposphere. The direct effect of CO₂ change, in contrast, can not be understood in terms of mean changes in the troposphere. However, there is a significant response in the stratosphere, characterized by a strengthened climatological polar vortex with strongly enhanced interannual variability. In this case, the NAO interannual variability has a strong link with the variability over the North Pacific, as in the annular AO pattern, and is also strongly related to the stratospheric vortex, indicating strengthened stratosphere-troposphere coupling. The similarity of changes in many characteristics of NAO interannual variability between the model response to doubling CO₂ and those in observations in the mid-1970s implies that the increase of greenhouse gas concentration in the atmosphere, and the resulting changes in the stratosphere, might have played an important role in the multidecadal change of interannual NAO variability and its associated climate anomalies during the late twentieth century. The weak change in mean westerlies in the troposphere in response to CO₂ change implies that enhanced and eastward extended mid-latitude westerlies in the troposphere might not be a necessary condition for the poleward and eastward shift of the NAO action centres in the mid-1970s.     

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.