太平洋年代际振荡对冬季北半球两大洋风暴轴协同变化的可能影响

利用NCEP/NCAR提供的再分析资料和NOAA提供的海温资料分析太平洋年代际振荡(Pacific Decadal Oscillation,PDO)不同位相的年代际背景下北半球海气耦合关系的异常与风暴轴协同变化的联系,主要结果如下:1)冬季太平洋年代际振荡与北半球两大洋风暴轴协同变化之间存在显著的相关关系,当PDO暖位相时,对应两大洋风暴轴南北位置反向的异常变化,其中北太平洋风暴轴偏南且中东部减弱,北大西洋风暴轴偏北且中东部增强,PDO冷位相时相反。2)PDO为暖位相时,对应El Niňo型海温异常,北大西洋海温呈三极型,平均槽脊加强,经向环流增强,极涡收缩,北太平洋风暴轴南压,大西洋风暴轴则北抬,此时欧亚大陆北部和北美大陆大部分地区温度异常升高,亚洲南部、非洲北部及巴伦支海以北的高纬温度异常降低,北美西南部和格陵兰岛附近温度也为异常降低,PDO冷位相时相反。     

中国南方多雪年环流特征及对关键区海温的响应

根据NECP/NCAR月平均再分析资料,采用回归分析、合成分析等方法,探讨了La Nia（El Nio）气候背景下,大气外强迫因子北大西洋中纬度（45°N,30°W）、赤道印度洋、我国南海和东海海域海温异常对亚洲环流及我国南方降雪多寡的影响。分析发现,La Nia气候背景下,北大西洋中纬（45°N,30°W）海温为正异常,有利于500 hPa高度场亚洲中高纬叶尼塞地区（50°N, 90°E）位势高度偏高并出现阻塞型,也有利于东亚冬季风环流加强;赤道印度洋、我国南海和东海附近海温出现负异常,850 hPa高度东亚大陆沿海低纬地区风距平场出现偏北风距平,我国南方多雪,温度相对偏低。El Nio背景下,北大西洋中纬度（45°N,30°W）海温为负异常,亚洲中高纬叶尼塞地区位势高度偏低,东亚冬季风环流偏弱;赤道印度洋、我国南海和东海附近海温出现正异常,850 hPa高度东亚大陆沿海低纬风距平场出现偏南风距平,我国南方多雪,温度相对偏高。回归分析指出,冬季500 hPa亚洲中高纬叶尼塞地区（90°E附近）的位势高度正（负）异常与北大西洋中纬度（45°N,30°W）附近海温正（负）异常有关;冬季850 hPa东亚大陆沿海低纬地区偏南（北）风距平与赤道印度洋、我国南海和东海地区的海温正（负）异常有关。La Nia气候背景下的2008年1月我国南方低温、雨雪、冰冻极端灾害事件的发生可能与北大西洋中纬度（45°N,30°W）附近以及赤道印度洋、我国南海和东海海温都出现正异常有关。     

El Ni?o衰退年夏季西北太平洋异常反气旋季节内演变特征及其机制

基于多种再分析资料和观测资料,分析El Ni?o衰退年北半球夏季西北太平洋异常反气旋(NWPAC)季节内演变特征及其机制。结果表明,较之El Ni?o衰退年6月,NWPAC在7月与8月北移,且其强度在7月与8月显著增强。NWPAC通过影响对流层低层水汽通量散度对东亚夏季季节内降水产生影响,使东亚夏季异常雨带随NWPAC北移而逐渐北移;与NWPAC相伴随的降水异常减弱与入射太阳短波辐射增强,可引起夏季中南半岛、菲律宾及我国南部地区地表气温正异常,且随着NWPAC北移,东亚地表气温异常也随之北移。局地海气相互作用过程可能是NWPAC夏季季节内北移的成因之一。NWPAC北侧短波辐射的减弱和海表潜热与感热释放的增强会造成其下海温负异常,而海温负异常有利于NWPAC的维持。与之相反,NWPAC南侧短波辐射的增强与海表潜热与感热释放的减弱会造成其下海温正异常,而海温正异常可能会激发对流不利于NWPAC的维持。     

Sea surface temperature anomaly in the tropical North Atlantic during El Nino decaying years

基于1979年到2016年多种再分析资料,本文分析了El Ni?o衰减年热带北大西洋的海温异常.结果表明,热带北大西洋海温在此期间呈显著变暖趋势.10次El Ni?o事件的合成结果表明热带北大西洋海温异常在El Ni?o事件峰值之后的春季达到最大值,并持续到夏季.一般而言,这种异常与三个因子有关,即El Nino,北大西洋涛动和长期趋势,能分别导致局地海温上升0.4℃,0.3℃和0.35℃.1983年和2005年的对比分析表明,尽管El Ni?o强度对春季北大西洋海温起到决定性作用,与长期趋势密切相关的前冬海温也很重要.此外,超前-滞后相关结果表明北大西洋涛动超前海温约2-3个月.比较两个冬季相反位相北大西洋涛动的年份(即1992年和2010年),表明北大西洋涛动也能调制北大西洋海温异常.冬季负位相北大西洋涛动能显著增强El Ni?o的强迫影响,反之亦然.换言之,如果北大西洋涛动与El Ni?o位相相合,衰减年北大西洋海温异常才更为显著.因此,为全面理解热带北大西洋海温变化,除长期趋势外,还必须考虑El Ni?o和北大西洋涛动的综合影响.     

多因子协同作用对1992年和1998年黄淮地区夏季降水异常的影响

1991年5月和1997年4月赤道中东太平洋均发生了El Ni?o事件,但是1992年夏季黄淮地区降水异常偏少,而1998年夏季却异常偏多。分析结果显示,1992年夏季西北太平洋副热带高压（以下简称副高）偏东,中高纬阻塞高压偏弱,黄淮地区降水异常偏少;而1998年夏季,副高偏西,中高纬阻塞高压活动频繁,黄淮地区降水异常偏多。对海温外强迫信号的诊断和数值模式试验显示:当西太平洋对流活动偏弱时,有利于副高西伸;鄂霍茨克海及以东海温偏高时,其上空的阻塞高压增强;北大西洋中纬度地区海温偏高时,有利于后期乌拉尔山高压脊明显增强。即在赤道中东太平洋发生El Ni?o事件的背景下,西太平洋对流、鄂霍茨克海附近亲潮区域和北大西洋中纬度区域海温异常可能是导致黄淮区域1992年夏季和1998年夏季降水差异大的主要原因。该工作显示仅根据El Ni?o事件的发生时间和强度无法完全预测黄淮地区夏季降水变化,需要综合考虑西太平洋对流、鄂霍茨克海附近海域和北大西洋中纬度区域海温异常对季风环流的影响,从多因子协同作用的角度诊断和预测黄淮地区夏季降水异常趋势,提高预测能力。     

不同PDO位相下El Niño发展年和La Niña年东亚夏季风的季节内变化

基于1957~2017年观测和再分析资料,合成分析了北太平洋年代际振荡(Pacific decadal oscillation,PDO)不同位相下El Ni?o发展年和La Nina年东亚夏季风的环流、降水特征及季节内变化。结果表明,PDO正、负位相作为背景场,分别对El Ni?o发展年、La Nina年东亚夏季风及夏季降水具有加强作用。PDO正位相一方面可增强El Ni?o发展年夏季热带中东太平洋暖海温异常信号,另一方面通过冷海温状态加强中高纬东亚大陆与西北太平洋的环流异常,从而在一定程度上增强了东亚夏季风环流的异常程度;反之,PDO负位相则增强了La Nina年热带海气相互作用以及中高纬环流(如东北亚反气旋)的异常。在季节内变化方面,El Ni?o发展年6月贝湖以东反气旋性环流为东亚地区带来稳定的北风异常,东北亚位势高度减弱;7月开始,环流形势发生调整,日本以东洋面出现气旋性异常,东亚大陆偏北风及位势高度负异常均得到加强;8月,随着东亚夏季风季节进程和El Ni?o发展,西太平洋出现气旋性环流异常,东亚副热带位势高度进一步降低,西北太平洋副热带高压(简称副高)明显东退。La Nina年6月异常较弱,主要环流差异自7月西北太平洋为大范围气旋性异常控制开始,东亚-太平洋遥相关型显著,副高于季节内始终偏弱偏东。上述两种情况下,均造成东亚地区夏季降水总体上偏少,尤其是中国北方降水显著偏少。     

冬季海温异常影响北太平洋东部型风暴轴的数值试验

最强中心出现在160°W以东地区的北太平洋风暴轴定义为东部型风暴轴,针对这种短期气候异常现象,利用大气环流模式CAM 3.0对其可能的外部强迫机制进行探究,主要关注赤道中东部和黑潮海区正、负海温异常的影响。结果表明:赤道海区负海温异常对风暴轴东部型的出现有重要意义,当该海区海温为负(正)异常且黑潮海区海温正(负)异常时,风暴轴表现为东(西)部型。风暴轴在中、东太平洋地区低层斜压性的增强,是太平洋风暴轴中、东端天气尺度涡动活动增强的主要原因。当出现东部型时,北太平洋东部区域急流强度增强,涡动斜压增长偏强,涡动的热量和动量输送加强,风暴轴和急流的反馈也加强;反之亦然。冬季赤道海区引起的大气响应范围较广,而黑潮海区的影响较为局地,尤其是黑潮海区的负异常主要影响风暴轴入口区域,表现为关于海温异常强迫的符号非对称性。     

不同太平洋年代际振荡和ENSO位相下大气水分收支变化对北半球冬季太平洋蒸发量的影响

本文利用OAFlux资料研究了1958~2015年北半球冬季太平洋蒸发量在不同厄尔尼诺—南方涛动（ENSO）和太平洋年代际振荡（PDO）位相下的分布特征,并从水汽收支的角度分析了蒸发量异常的成因,结果表明:ENSO主要影响热带东太平洋、副热带西北太平洋和中纬度北太平洋中部的蒸发量。El Ni?o（La Ni?a）时水汽在北太平洋中部异常辐散（辐合）,有利于当地大气水汽含量减小（增大）,造成蒸发量增大（减小）;副热带西北太平洋异常的水汽辐合（辐散）有利于蒸发量减小（增大）;除此以外,蒸发量在热带东太平洋蒸发量增大（减小）则主要是降水量增大（减小）导致。与此同时,ENSO对上述海区蒸发量的影响还受到PDO的调控,当PDO处于暖（冷）位相时,El Ni?o（La Ni?a）造成蒸发量异常程度在中纬度北太平洋中部显著增大,这主要是由降水量增大（减小）引起的大气水汽含量减小（增大）所致,此时对应着风暴轴异常增大（减小）;当PDO处于冷（暖）位相时,El Ni?o（La Ni?a）造成的蒸发量异常程度在副热带西北太平洋和热带东太平洋显著增大,而这与湿度变化引起的水汽平流异常程度增大紧密相关。     

北半球冬季风暴轴与ENSO循环的关系及其能量平衡特征

利用欧洲中期天气预报中心的再分析资料,分析了北半球冬季对流层上层300 hPa面上风暴轴与ENSO循环的关系以及风暴轴的能量平衡特征.研究表明,由于El Nino年北太平洋的哈德莱环流增强,导致北太平洋西风急流向赤道和向东伸展,进而引起了北太平洋风暴轴的增强并向赤道和向东伸展;而La Nina年事件期间情形正好相反.在北大西洋,El Nino年其西风急流中心最大值有所减小,但整个西风急流区域有所扩大并向西和向赤道伸展,相应北大西洋风暴轴强度在El Nino年也有所减弱并向西和向赤道方向伸展;La Nina事件的情形正好相反.能量分析表明,斜压转换的正值中心位于风暴轴及其上游区域.在El Nino年,北太平洋斜压转换的正值中心向赤道和向下游伸展,而北大西洋斜压转换的正值中心向赤道和向西伸展,这与北太平洋风暴轴和北大西洋风暴的变化趋势是一致的.行星尺度扰动对斜压转换的贡献比天气尺度扰动的贡献要小,而行星尺度-天气尺度扰动的相互作用项对斜压转换的贡献更小.研究还表明,能量正压转换的正中心化于风暴轴的上游,负中心位于风暴轴区域.相对于La Nina年,El Nino年北太平洋东部的正压转换负值中心偏北,北美的正值中心位置偏南,而北大西洋的负值中心强度减弱范围变小.这些变化与从La Nina年剑El Nino年北太平洋和北大西洋风暴轴的变化是一致的.在能量的正压转换过程中,天气尺度扰动的贡献要大于行星尺度扰动,而行星尺度-天气尺度扰动的相互作用项的贡献更小.     

中国东部夏季分区降水对海温异常响应特征的研究

用刘征宇最新发展的广义平衡反馈方法结合经验正交分析和旋转经验正交分析法,研究中国东部6个分区夏季降水异常对各海盆海温异常(SSTA)模态的响应特征,探讨了东北区降水异常对SSTA的响应机制。结果表明:各分区降水异常对SSTA的响应特征有明显差异。东北区降水异常对热带太平洋SSTA的回应显著,当SSTA表现为类似El Niño模时,该区夏季降水增加;江淮区降水异常同时受到中纬度以及热带SSTA的强迫,对热带太平洋SSTA的类似La Niña Modoki模、北大西洋SSTA三极型模等回应显著;西南地区的降水异常主要与中纬度SSTA有密切关联。对流层高度场对热带太平洋SSTA类似El Niño模的直接回应在热带太平洋上空为显著正异常。通过东亚—太平洋型遥相关波列使东北地区上空高度场出现负异常,并在其西侧形成负异常中心,对流层低层为气旋性环流异常。这一环流回应与东北地区夏季降水偏多时的环流特征相近。     

Dynamic Linkage between the North Pacific and the Tropical Pacific: Atmosphere-Ocean Coupling

In this study, dynamic linkage of atmosphere-ocean coupling between the North Pacific and the tropical Pacific was demonstrated using a large number of ensemble perturbed initial condition experiments in a fully coupled fast ocean-atmosphere model (FOAM). In the FOAM model, an idealized mixed layer warming was initiated in the Kuroshio-Oyashio extension region, while the ocean and atmosphere remained fully coupled both locally and elsewhere. The modeling results show that the warm anomalies are associated with anomalous cyclonic winds, which induce initial warming anomalies extending downstream in the following winter. Then, the downstream warming spreads southwestward and induces SST warming in the equatorial Pacific via surface wind-evaporation-SST feedback. Warming in the tropical Pacific is further reinforced by Bjerknes’ feedback.     

Influence of the Eastern Pacific and Central Pacific Types of ENSO on the South Asian Summer Monsoon

Based on observational and reanalysis data,the relationships between the eastern Pacific(EP)and central Pacific(CP)types of El Ni?o?Southern Oscillation(ENSO)during the developing summer and the South Asian summer monsoon(SASM)are examined.The roles of these two types of ENSO on the SASM experienced notable multidecadal modulation in the late 1970s.While the inverse relationship between the EP type of ENSO and the SASM has weakened dramatically,the CP type of ENSO plays a far more prominent role in producing anomalous Indian monsoon rainfall after the late 1970s.The drought-producing El Ni?o warming of both the EP and CP types can excite anomalous rising motion of the Walker circulation concentrated in the equatorial central Pacific around 160°W to the date line.Accordingly,compensatory subsidence anomalies are evident from the Maritime Continent to the Indian subcontinent,leading to suppressed convection and decreased precipitation over these regions.Moreover,anomalously less moisture flux into South Asia associated with developing EP El Ni?o and significant northwesterly anomalies dominating over southern India accompanied by developing CP El Ni?o,may also have been responsible for the Indian monsoon droughts during the pre-1979 and post-1979 sub-periods,respectively.El Ni?o events with the same“flavor”may not necessarily produce consistent Indian monsoon rainfall anomalies,while similar Indian monsoon droughts may be induced by different types of El Ni?o,implying high sensitivity of monsoonal precipitation to the detailed configuration of ENSO forcing imposed on the tropical Pacific.     

Impacts of the central and eastern Pacific types of ENSOon sea surface temperature in the South Pacific

The present study examines the relationship between two types of El Niño–Southern Oscillation (ENSO), the central Pacific (CP) ENSO and the eastern Pacific (EP) ENSO, and the sea surface temperature (SST) variability over the South Pacific (SP) (20° S–60° S, 145° E–70° W) using NOAA OI SST for the period 1982–2006. The SP SST variability associated with the two types of ENSO varies with season. These two types of ENSO can excite different atmospheric patterns associated with the Pacific–South American mode, through which they influence the SP SST variability. Both the surface turbulent air–sea heat fluxes and the heat advection by Ekman currents (i.e., Ekman heat fluxes) have an important impact on the SST variability. An analysis of the surface mixed layer heat budget indicates that the heat fluxes (the sum of turbulent heat fluxes and Ekman heat fluxes) can effectively explain much of the SST variability related to the two types of ENSO.     

北太平洋副热带高压与赤道东部海温的相互作用

本文根据1957—1976年太平洋的月平均海平面气压和海温资料,分析了赤道东部海温与北太平洋海平面气压场的时滞相关。发现赤道海温的变化,就北太平洋而言,主要是受该洋东南部低层副热带及气旋的控制,而赤道海温对副热带高压的反馈,主要是发生在太平洋中部对流层副热带高压中心附近地区。赤道海温向副热带反气旋调整的时间平均为二个月左右,而副热带高压向赤道海温调整的时间平均为四个月左右。前者比后者更快。它们之间相互影响、相互调整形成了一种闭合的负反馈过程。整个过程约历时22个月左右,大致相当于北太平洋副热带高压和赤道海     

Linkages between the North Pacific Oscillation and central tropical Pacific SSTs at low frequencies

The North Pacific Oscillation (NPO) recently (re-)emerged in the literature as a key atmospheric mode in Northern Hemisphere climate variability, especially in the Pacific sector. Defined as a dipole of sea level pressure (SLP) between, roughly, Alaska and Hawaii, the NPO is connected with downstream weather conditions over North America, serves as the atmospheric forcing pattern of the North Pacific Gyre Oscillation (NPGO), and is a potential mechanism linking extratropical atmospheric variability to El Ni?o events in the tropical Pacific. This paper explores further the forcing dynamics of the NPO and, in particular, that of its individual poles. Using observational data and experiments with a simple atmospheric general circulation model (AGCM), we illustrate that the southern pole of the NPO (i.e., the one near Hawaii) contains significant power at low frequencies (7–10?years), while the northern pole (i.e., the one near Alaska) has no dominant frequencies. When examining the low-frequency content of the NPO and its poles separately, we discover that low-frequency variations (periods >7?years) of the NPO (particularly its subtropical node) are intimately tied to variability in central equatorial Pacific sea surface temperatures (SSTs) associated with the El Ni?o-Modoki/Central Pacific Warming (CPW) phenomenon. This result suggests that fluctuations in subtropical North Pacific SLP are important to monitor for Pacific low-frequency climate change. Using the simple AGCM, we also illustrate that variability in central tropical Pacific SSTs drives a significant fraction of variability of the southern node of the NPO. Taken together, the results highlight important links between secondary modes (i.e., CPW-NPO-NPGO) in Pacific decadal variability, akin to already established relationships between the primary modes of Pacific climate variability (i.e., canonical El Ni?o, the Aleutian Low, and the Pacific Decadal Oscillation).     

Joint impact of North Pacific Victoria mode and South Pacific Quadrapole mode on Pacific ITCZ summer precipitation

Climate Dynamics - This study investigates the joint impact of the boreal late winter–early spring North Pacific Victoria mode (VM) and South Pacific Quadrapole (SPQ) mode on the following...     

Interdecadal Change in the Relationship Between the North Pacific Oscillation and the Pacific Meridional Mode and Its Impact on ENSO

Two leading but independent modes of Northern Pacific atmospheric circulation: the North Pacific Oscillation (NPO) and the Pacific Meridional Mode (PMM), are known external triggers of the El Niño-Southern Oscillation (ENSO) by the sequential migration of sea surface temperature (SST) anomalies into the tropics possibly by means of wind-evaporation-SST (WES) feedbacks. Because of the similar roles of NPO and PMM, most previous studies have explored them with no separation. Here, we investigate their independent and combined effects in triggering ENSO, and find that when the NPO and PMM occur simultaneously during spring, ENSO or ENSO-like SST anomalies are generated during the following winter; whereas when either the NPO or PMM occur alone, ENSO events rarely occur. Furthermore, the relationship between NPO and PMM shows noticeable interdecadal variability, which is related to decadal changes in the mean upper-level jet stream over the North Pacific. Changes in the upper-level jet stream modify the location of the center of the Aleutian Low, which plays a role in bridging the NPO and PMM processes, especially when it migrates to the southwest. The period when NPO and PMM are well correlated coincides somewhat with the active ENSO period, and vice versa, indicating that a more efficient trigger due to combined NPO-PMM processes results in a higher variation of ENSO. Finally, analysis of the coupled model control simulations strongly supports our observational analysis results.     

Secular variation of the Pacific Decadal Oscillation,the North Pacific Oscillation and climatic jumps in a multi-millennial simulation

Outputs from a 10,000-year simulation with a coupled global climatic model for present climatic conditions have been used to investigate the behaviour of the Pacific Decadal Oscillation (PDO), the North Pacific Oscillation (NPO) and related phenomena. The analysis reveals a wide range of temporal variability for these Oscillations, suggesting that observations to date provide only a limited sample of possible outcomes. In addition, the simulation suggests that the current observed phase relation between the PDO and NPO may not be typical of longer-term variability. Climatic jumps appear to be a ubiquitous feature of climatic variability, and while, as observed, the most common interval between such jumps is about 20 years, intervals of up to 100 years occur in the simulation. The probability density functions of the PDO and NPO are very close to Gaussian, with the PDO being represented by an auto-regressive function of order one, while the NPO consisted of white noise. An FFT analysis of PC1 of the PDO revealed periodicities concentrated near 10 years, while for the NPO the principal periodicities were decadal to bidecadal. Global distributions of the distributions of the correlations between PC1 or the NPO and selected climatic variables were similar, and in agreement with observations. These correlations highlight the inter-relationships between these two Oscillations. The above correlations were not necessarily stable in time for a given geographical point, with transitions occurring between positive and negative extremes. Climatic jumps were identified with transitions of both the PDO and NPO, with magnitudes of importance as regards climatic perturbations. Spatial patterns of the changes associated with such jumps have global scales, and the need to consider the implications of these jumps in regard to greenhouse induced climatic change is noted.     

Operational hindcasting of the tropical Pacific

Procedures have been implemented at the Climate Analysis Center of the National Meteorological Center (CAC/NMC) to provide montly hindcasts of oceanographic conditions in the tropical Pacific. A central component of this system is a primitive equation ocean general circulation model that was developed at the Geophysical Fluid Dynamics Laboratory (GFDL). This is forced with monthly mean fields for wind stress and net heat flux. Until recently the former were derived from ship reports available on the Global Telecommunication System (GTS). The heat fluxes are slightly modified climatological fluxes from Esbensen and Kushnir. To correct for errors in the simulations, thermal data in the upper 450 and surface-temperature data are assimilated montly.Numerical experiments were run to examine the sensitivity of the simulations to small changes in the stress fields. Variations of the drag coefficient by 15% result in differences in sea-surface temperature (SST) and subsurface thermal structure in the eastern Pacific that are comparable with the observed annual and interannual variability. Comparisons with simulations in which the wind stresses were derived from operational atmospheric analyses show sensitivities of the same magnitude. Comparisons of simulations forced either with these of ship-recorded winds to a run with data assimilation show that significant errors are found in both, especially in the off-equatorial regions. Consequently, until forcing fields are improved, accurate simulations will require the use of data assimilation.