The Impact of Global Warming on the Pacific Decadal Oscillation and the Possible Mechanism简

The response of the Pacific Decadal Oscillation （PDO） to global warming according to the Fast Ocean Atmosphere Model （FOAM） and global warming comparison experiments of 11 IPCC AR4 models is investigated. The results show that North Pacific ocean decadal variability, its dominant mode （i.e., PDO）, and atmospheric decadal variability, have become weaker under global warming, but with PDO shifting to a higher frequency. The SST decadal variability reduction maximum is shown to be in the subpolar North Pacific Ocean and western North Pacific （PDO center）. The atmospheric decadal variability reduction maximum is over the PDO center. It was also found that oceanic baroclinic Rossby waves play a key role in PDO dynamics, especially those in the subpolar ocean. As the frequency of ocean buoyancy increases under a warmer climate, oceanic baroclinic Rossby waves become faster, and the increase in their speed ratio in the high latitudes is much larger than in the low latitudes. The faster baroclinic Rossby waves can cause the PDO to shift to a higher frequency, and North Pacific decadal variability and PDO to become weaker.     

The relative importance of tropical variability forced from the North Pacific through ocean pathways

To what extent is tropical variability forced from the North Pacific through ocean pathways relative to locally generated variability and variability forced through the atmosphere? To address this question, in this study we use an anomaly-coupled model, consisting of a global, atmospheric general circulation model and a 4½-layer, reduced-gravity, Pacific-Ocean model. Three solutions are obtained; with coupling over the entire basin (CNT), with coupling confined to the tropics and wind stress and heat fluxes in the North and South Pacific specified by climatology (TP), and with coupling confined to the Tropics and wind stress and heat fluxes in the North Pacific specified by output from CNT (NPF). It is found that there are two distinct signals forced in the North Pacific that can impact the tropics through ocean pathways. These two signals are forced by wind stress and surface heat flux anomalies in the subtropical North Pacific. The first signal is relatively fast, impacts tropical variability less than a year after forcing, is triggered from November to March, and propagates as a first-mode baroclinic Rossby wave. The second signal is only triggered during springtime when buoyancy forcing can effectively generate higher-order baroclinic modes through subduction anomalies into the permanent thermocline, and it reaches the equator 4–5 years after forcing. The slow signal is found to initiate tropical variability more efficiently than the fast signal with one standard deviation in subtropical zonal wind stress forcing tropical SST anomalies centered on the equator at 135°W of approximately 0.5°C. Allowing extratropically forced tropical variability is found to shift primarily 2-year ENSO variability in a tropics-alone simulation to a more realistic range of 2–6 years.     

大气季节内振荡对印度洋—西太平洋地区热带低压/气旋生成的影响

文中利用EOF分析大气季节内振荡 (MJO)的时空变化的方法 ,研究了 1996年 9月～ 1997年 6月间的MJO活动对生成在印度洋—西太平洋海域的热带低压 /气旋的影响。结果发现 ,除西北太平洋之外 ,发生在其他区域的热带低压 /气旋有半数以上生成在向东移动的MJO的湿位相中。伴随MJO的向东传播 ,热带低压 /气旋平均生成位置也随之向东移动 ,而生成在西北太平洋的热带低压 /气旋分别受到向东和向西传播的MJO影响     

1月份两类ENSO的海气环流模态分析

本文对1950～2001年1月份的大气风场和大洋流场做了联合复EOF（Empirical Orthogonal Function）分解,用以探讨1月份两类ENSO（El Ni?o-Southern Oscillation）的海气环流及耦合情况,所得结果主要有:该分解第1、2模态空间场分别相应于东部型、中部型ENSO,前者在赤道太平洋东部和中部都有海温动力异常,并以东部异常最强,后者仅在中部存在此异常,两模态的时间系数都与ENSO有很好相关,为此第1、2模态可分别称为东部型、中部型ENSO的风场流场（异常）模态。东部型ENSO模态具有3～6年的年际变化和13～14年的年代际变化,中部型则有明显的7年年际变化和12、17年的年代际变化,两者中约13年的周期与冬季北太平洋NPGO（North Pacific Gyre Oscillation）的周期相同。东、中部型El Ni?o期间,沃克环流上升支分别从印尼东移至赤道西、中太平洋,并有所减弱;南、北支哈得莱环流则分别位于日界线以东及该线附近,且均有所加强,从而使南、北太平洋副热带高压偏强;而在5°S的南美沿岸则分别有垂直运动上升和下沉异常。在海气耦合上,两类ENSO模态在赤道中太平洋均存在西风异常与海洋赤道Kelvin波和Rossby波的波包解耦合,而海温动力异常对大气的影响则都起到负反馈作用,从而有利于ENSO的维持和稳定。     

冬季北太平洋海气环流年代际异常的统计动力诊断

本文对冬季北太平洋大气和大洋环流做了联合复经验正交函数(CEOF)分解和小波分析, 并分别讨论了第一、二模态的年代际变化及其与海表温度异常(SSTA)年代际变化(PDO、NPGO模态)的关系, 得到以下主要结论:第一、二模态对时间系数的分析显示, 其与PDO、NPGO指数的相关性较高, 且小波分析表明其分别具有明显的准22、12年的年代际变化周期, 这与PDO、NPGO模态的周期相同;第一、二模态时间系数对北太平洋SSTA的回归分析表明, 其回归系数场的空间分布分别与PDO、NPGO的十分接近。第一、二模态空间场中大气环流异常分别类似于海平面气压异常(SLPA)的AL、NPO模态, 可分称其为AL、NPO的风场模;而大洋环流异常则分别相应于SSTA的PDO、NPGO模态, 可称其为PDO、NPGO的流场模。由第一、二模态近表层流场异常得到的垂直运动空间分布分别与PDO、NPGO的空间结构相似, 说明海洋上层海盆尺度大洋环流引起的垂直运动所导致的海温动力变化是形成PDO、NPGO的重要原因, 而大洋环流异常扮演着中介角色。     

Impact of the winter North-Atlantic weather regimes on subtropical sea-surface height variability

Interannual variability of subtropical sea-surface-height (SSH) anomalies, estimated by satellite and tide-gauge data, is investigated in relation to wintertime daily North-Atlantic weather regimes. Sea-level anomalies can be viewed as proxies for the subtropical gyre intensity because of the intrinsic baroclinic structure of the circulation. Our results show that the strongest correlation between SSH and weather regimes is found with the so-called Atlantic-Ridge (AR) while no significant values are obtained for the other regimes, including those related to the North Atlantic Oscillation (NAO), known as the primary actor of the Atlantic dynamics. Wintertime AR events are characterized by anticyclonic wind anomalies off Europe leading to a northward shift of the climatological wind-stress curl. The latter affects subtropical SSH annual variability by altered Sverdrup balance and ocean Rossby wave dynamics propagating westward from the African coast towards the Caribbean. The use of a simple linear planetary geostrophic model allows to quantify those effects and confirms the primary importance of the winter season to explain the largest part of SSH interannual variability in the Atlantic subtropical gyre. Our results open new perspectives in the comprehension of North-Atlantic Ocean variability emphasizing the role of AR as a driver of interannual variability at least of comparable importance to NAO.     

Interdecadal Modulation of AMO on the Winter North Pacific Oscillation-Following Winter ENSO Relationship

It is known that the wintertime North Pacific Oscillation (NPO) is an important extratropical forcing for the occurrence of an El Ni?o?Southern Oscillation (ENSO) event in the subsequent winter via the “seasonal footprinting mechanism” (SFM). This study reveals that the Atlantic Multidecadal Oscillation (AMO) can notably modulate the relationship between the winter NPO and the following winter ENSO. During the negative AMO phase, the winter NPO has significant impacts on the following winter ENSO via the SFM. In contrast, the influence of the winter NPO on ENSO is not robust at all during the positive AMO phase. Winter NPO-generated westerly wind anomalies over the equatorial western Pacific during the following spring are much stronger during negative than positive AMO phases. It is suggested that the AMO impacts the winter NPO-induced equatorial westerly winds over the western Pacific via modulating the precipitation climatology over the tropical central Pacific and via modulating the connection of the winter NPO with spring sea surface temperature in the tropical North Atlantic.     

冬季北太平洋海温主模态在1990年前后调整及其成因初探?

太平洋年代际振荡(PDO)和北太平洋涡旋振荡(NPGO)是北太平洋(20°~60°N,120°E~120°W)海温(SST)的EOF前两个模态,本文通过比较1990年前后北太平洋冬季SST EOF前两个模态,揭示了PDO和NPGO在1990年前后特征,并从关键区海温变化、北太平洋涛动(NPO)、赤道太平洋中部变暖(CPW)和北极涛动(AO)的影响,揭示了北太平洋主模态在1990年之后调整的成因。我们发现,1990年之前,北太平洋SST场的EOF前两个模态与PDO和NPGO的空间结构类似,但是在1990年之后,SST的EOF第一模态的最大荷载中心向日界线移动,40°N以北的太平洋被正的SST异常控制,表现出与NPGO模态的负位相相似的空间分布特征,而EOF第二模态由偶极子演变成了三极子结构。北太平洋中部(28°~36°N,152°~178°W)和北太平洋北部(44°~49°N,151°~177°W)海温距平在1990年之后呈显著的负相关变化,是导致在1990年之后冬季NPGO成为主模态的内部原因,而NPO在1990年之后的显著增强则是重要的外部原因。分析显示,NPO在1980年开始表现出增强趋势,通过风生流机制,NPO可以增强北太平洋45°N附近的气压梯度和西风异常幅度,从而导致了1990年之后NPGO海温模态的加强。虽然CPW和AO对NPO的南支(夏威夷)和北支(阿拉斯加)的海平面气压异常中心加强有贡献,但是上述两个因子与NPGO之间的关系在1990年之前并不明显。因此,CPW和AO与NPGO之间并不存在稳定的物理联系。     

Impact of the Winter North Pacific Oscillation on the Surface Air Temperature over Eurasia and North America: Sensitivity to the Index Definition

This study analyzes the impact of the winter North Pacific Oscillation(NPO) on the surface air temperature(SAT)variations over Eurasia and North America based on six different NPO indices. Results show that the influences of the winter NPO on the SAT over Eurasia and North America are sensitive to the definition of the NPO index. The impact of the winter NPO on the SAT variations over Eurasia(North America) is significant(insignificant) when the anticyclonic anomaly associated with the NPO index over the North Pacific midlatitudes shifts westward and pronounced northerly wind anomalies appear around Lake Baikal. By contrast, the impact of the winter NPO on the SAT variations over Eurasia(North America)is insignificant(significant) when the anticyclonic anomaly over the North Pacific related to the NPO index shifts eastward and the associated northerly wind anomalies to its eastern flank extend to North America. The present study suggests that the NPO definition should be taken into account when analyzing the impact of the winter NPO on Eurasian and North American SAT variations.     

东亚-太平洋型季节内演变和维持机理研究

利用850hPa的纬向风异常建立一个逐候东亚-太平洋(East Asian Pacific,EAP)型指数,研究其季节内演变特征,发现东亚-太平洋型经向波列是东亚夏季风季节内变化的主要模态.其演变过程为:扰动首先出现在北太平洋中部,并通过正压不稳定过程从基本气流中获得能量而发展,在高层罗斯贝波能量向南频散,激发热带对流异常和赤道罗斯贝波,并相互锁相,因赤道罗斯贝波受β效应影响而共同向西移动.热带对流和环流异常在菲律宾附近达到最强,此时在东亚沿岸出现经向三极型波列,此后中低纬度异常继续向西北方向移动,使降水异常在长江流域能维持较长时间.东亚-太平洋型在东亚发展和维持有以下原因:首先,菲律宾暖水上空的对流和低层环流之间存在正反馈;其次,由于海陆热力差异导致暖大陆和冷海洋之间存在特殊的纬向温度梯度和北风垂直切变,东亚-太平洋型在经向上有向北倾斜的斜压结构,能通过斜压能量转换从平均有效位能中获得能量,同时,也能从经向温度梯度的平均有效位能中获得能量.     

北太平洋冬季上层海温异常的NPGO模态

利用较高分辨率的全球海洋同化分析系统(SODA)资料, 对冬季北太平洋上层的海温异常做了整层经验正交函数(EOF)分析, 并主要讨论了第二模态的结果。该模态空间结构与经典的北太平洋涡旋振荡(NPGO)一致, 且时间系数与NPGO指数也吻合, 故北太平洋上层海温异常EOF第二模态可称为NPGO模态。这说明NPGO现象不单纯反映在海表面温度异常上, 在上层海温中该现象也存在。在该模态空间场上, 水深100 m以上25°N～30°N的副热带处, 沿纬圈从120°E向东延伸至中东太平洋均为海温正异常带, 其北面则为负异常带, 两者构成双带系统;其中在170°W附近分别有正、负异常大值区, 中心构成南北偶极子;在本州岛以东海域, 从海表直到海洋上层底则有小范围的海温强异常。该模态空间结构的形成与大气NPO模态关系密切, 并与中纬度西风大值带上的风应力异常有关, 是造成NPGO的直接原因。该模态表现出明显的准13年年代际变化, 且对其进行5年滑动平均后发现, 从20世纪70年代中期以来, 该序列的振幅越来越大, 1976/1977年和1988/1989年的两次气候年代际突变均处该序列峰值处。引入了冬季北太平洋上层海温异常的NPGO指数, 其能更好反映海洋上层的NPGO现象及其年代际变化。     

Stable and unstable Rossby waves in the North Pacific current as inferred from the mean stratification

We have determined free Rossby waves in the North Pacific Current by numerical methods. We have found only two stable solutions — the barotropic and first-order baroclinic Rossby shear modes. The influence of the current on the dispersion features of these waves is small for the barotropic shear mode, but is significant for the baroclinic shear mode. An explicit comparison of the dispersion relations for the baroclinic wave in case of vanishing and non-vanishing current is given. We have found at most one unstable solution per wave number. The unstable wave with largest growth rate has an e-folding time of 1.1 year. We have calculated vertical profiles of the stream function and the temperature for the various shear modes at various wave numbers. The temperature shear modes have been calculated for later usage in a Rossby wave model to be fitted to observed temperature data from the North Pacific Current area.     

On the predictability of decadal changes in the North Pacific

 The predictability of decadal changes in the North Pacific is investigated with an ocean general circulation model forced by simplified and realistic atmospheric conditions. First, the model is forced by a spatially fixed wind stress anomaly pattern characteristic for decadal North Pacific climate variations. The time evolution of the wind stress anomaly is chosen to be sinusoidal, with a period of 20 years. In this experiment different physical processes are found to be important for the decadal variations: baroclinic Rossby waves dominate the response. They move westward and lead to an adjustment of the subtropical and subpolar gyre circulations in such a way that anomalous temperatures in the central North Pacific develop as a delayed response to the preceding wind stress anomalies. This delayed response provides not only a negative feedback but also bears the potential for long-term predictions of upper ocean temperature changes in the central North Pacific. It is shown by additional experiments that once these Rossby waves have been excited, decadal changes of the upper ocean temperatures in the central North Pacific evolve without any further anomalous atmospheric forcing. In the second part, the model is forced by surface heat flux and wind stress observations for the period 1949–1993. It is shown that the same physical processes which were found to be important in the simplified experiments also govern the evolution of the upper ocean in this more realistic simulation. The 1976/77 cooling can be mainly attributed to anomalously strong horizontal advection due to the delayed response to persistent wind stress curl anomalies in the early 1970s rather than local anomalous atmospheric forcing. This decadal change could have been predicted some years in advance. The subsequent warming in the late 1980s, however, cannot be mainly explained by advection. In this case, local anomalous atmospheric forcing needs to be considered. Received: 6 July 1998 / Accepted: 16 October 1999     

On the contribution of Rossby waves driven by surface buoyancy fluxes to low-frequency North Atlantic steric sea surface height variations

Previous studies have shown that wind-forced baroclinic Rossby waves can capture a large portion of low-frequency steric sea surface height (SSH) variations in the North Atlantic. In this paper, the classical wind-driven Rossby wave model derived in a 1.5-layer ocean is extended to include surface buoyancy forcing, and the new model is then used to assess the contribution from buoyancy-forced Rossby waves to low-frequency North Atlantic steric SSH variations. Buoyancy forcing is determined from surface heating as freshwater fluxes are negligible. It is found that buoyancy-forced Rossby waves are important in only a few regions belonging to the subtropical-to-midlatitude and eastern subpolar North Atlantic. In these regions, the new Rossby wave model accounts for 25%–70% of low-frequency steric SSH variations. Furthermore, as part of the analysis it is also shown that a simple static model driven by local surface heat fluxes captures 60%–75% of low-frequency steric SSH variations in the Labrador Sea, which is a region where Rossby waves are found to have no influence on the steric SSH.     

Source of low frequency modulation of ENSO amplitude in a CGCM

We study the relationship between changes in equatorial stratification and low frequency El Niño/Southern Oscillation (ENSO) amplitude modulation in a coupled general circulation model (CGCM) that uses an anomaly coupling strategy to prevent climate drifts in the mean state. The stratification is intensified at upper levels in the western and central equatorial Pacific during periods of high ENSO amplitude. Furthermore, changes in equatorial stratification are connected with subsurface temperature anomalies originating from the central south tropical Pacific. The correlation analysis of ocean temperature anomalies against an index for the ENSO modulation supports the hypothesis of the existence of an oceanic “tunnel” that connects the south tropical Pacific to the equatorial wave guide. Further analysis of the wind stress projection coefficient onto the oceanic baroclinic modes suggests that the low frequency modulation of ENSO amplitude is associated with a significant contribution of higher-order modes in the western and central equatorial Pacific. In the light of these results, we suggest that, in the CGCM, change in the baroclinic mode energy distribution associated with low frequency ENSO amplitude modulation have its source in the central south tropical Pacific.     

北太平洋海平面气压场变化与海温的关系

利用SVD（singular value decomposition）方法分析了1948年1月—2002年12月北太平洋海平面气压场与海温的关系。结果表明,SVD第1对异类相关分布型反映出,当东北太平洋副热带高压加强（减弱）时,Namias海区海温升高（降低）,而加利福尼亚海流区海温降低（升高）。SVD第2对异类相关分布型表明,当阿留申低压加深、北太平洋副热带地区气压升高时,黑潮暖流区海温升高,而北太平洋高、低纬海温降低;反之亦然。时滞相关表明,北太平洋大气环流异常超前海温1个月的相关最好,海温变化对大气环流异常分布型具有维持作用。NCAR CCSM3模拟结果很好地验证了上述结论,即在海气相互作用过程中,东北太平洋副热带高压和NPO（North Pacific Oscillation）与北太平洋海温存在密切联系。     

High-resolution modeling study of the Kuroshio path variations south of Japan

A high-resolution ocean general circulation model （OGCM） is used to investigate the Kuroshio path variations south of Japan. The model reproduces many important features of the Kuroshio system including its interannual bimodal variability south of Japan. A decreasing trend of the spatial averaged relative vorticity is detected when the Kuroshio takes the non-large meander （NLM） path, and during the transition period from the NLM to the large meander （LM）, a sudden release of velocity shear corresponds well to the weakening of the Shikoku recirculation gyre （SRG）, which plays a key role in modulating the Kuroshio path variations. Analysis of eddy energetics indicates that baroclinic instability is mainly responsible for the formation of the LM. In addition, further analysis shows that the strength of the SRG could be largely influenced by the baroclinic Rossby wave adjustment process, forced by the wind stress curl anomalies in the North Pacific basin, based on the model investigation. It is suggested that the cyclonic disturbances might account for the weakening of the SRG, and act as a remote trigger for the baroclinic instability of the Kuroshio south of Japan.     

北方涛动同北半球温带大气环流的遥相关(一)——基本结构

本文利用1951—1980年逐季的平均值资料(共120个季)讨论了北方涛动和与其相联系的北太平洋海温与北半球海平面气压场、500hPa位势高度场遥相关的基本结构,并与南方涛动和赤道东太平洋海温的结果进行了对比分析.发现北太平洋Namias海区和加利福尼亚海流区海温的变化与北方涛动具有很密切的联系;北方涛动和这两个海区的海温同北半球中高纬度大气环流特别是PNA型和NAO型环流异常存在明显的遥相关关系;南方涛动和赤道太平洋海温同WP型或NPO型环流异常关系比较密切,而与PNA型和NAO型的关系不如北方涛动和Namias海区及加利福尼亚海流区海温的显著.     

An Observed Connection Between Wintertime Temperature Anomalies over Northwest China and Weather Regime Transitions in North Atlantic

In this study, the association between wintertime temperature anomalies over Northwest China and the weather regime transitions in North Atlantic on synoptic scale is analyzed by using observational surface air temperature(SAT) data and atmospheric reanalysis data. Daily SAT anomaly and duration time are used in order to define SAT anomaly cases. Differences with regard to the circulation anomalies over the Ural Mountains and the upstream North Atlantic area are evident. It is found that the colder than normal SAT is caused by the enhanced Ural high and associated southward flow over Northwest China. Time-lagged composites reveal possible connections between the SAT anomalies and the different development phases of the North Atlantic Oscillation(NAO). The Ural highs tend to be strengthened during the negative phase of NAO(NAO–) to Atlantic ridge transition, which are closely related to the downstream-propagating Rossby wave activity. The opposite circulation patterns are observed in the warm SAT cases. A cyclonic circulation anomaly is distinctly enhanced over the Urals during the positive phase of NAO(NAO+) to Scandinavian blocking transition, which would cause warmer SAT over Northwest China. Further analyses suggest that the intensified zonal wind over North Atlantic would favor the NAO– to Atlantic ridge transition, while the weakened zonal wind may be responsible for the transition between NAO+ and Scandinavian blocking.