Storm track sensitivity to sea surface temperature resolution in a regional atmosphere model

A high resolution regional atmosphere model is used to investigate the sensitivity of the North Atlantic storm track to the spatial and temporal resolution of the sea surface temperature (SST) data used as a lower boundary condition. The model is run over an unusually large domain covering all of the North Atlantic and Europe, and is shown to produce a very good simulation of the observed storm track structure. The model is forced at the lateral boundaries with 15–20 years of data from the ERA-40 reanalysis, and at the lower boundary by SST data of differing resolution. The impacts of increasing spatial and temporal resolution are assessed separately, and in both cases increasing the resolution leads to subtle, but significant changes in the storm track. In some, but not all cases these changes act to reduce the small storm track biases seen in the model when it is forced with low-resolution SSTs. In addition there are several clear mesoscale responses to increased spatial SST resolution, with surface heat fluxes and convective precipitation increasing by 10–20% along the Gulf Stream SST gradient.     

北大西洋海表面温度锋与大西洋风暴路径及大气大尺度异常的关系研究

运用NCEP、Had ISST再分析资料,北大西洋涛动(NAO)月指数序列,探讨了海表面温度(SST)锋的时空变化特征,揭示了北大西洋SST锋的主要气候变率及其与北大西洋风暴轴和大气大尺度环流异常的关系。研究表明,剔除季节循环后的SST锋显示其最主要变率为锋区的向南/北摆动,其对应的风暴轴发生相应的西南/东北移动,并同时在北大西洋上空对应一个跨海盆的位势高度负/正异常。这种环流异常可引起高纬度海平面气压(SLP)的反气旋/气旋式环流,这有利于增强海表面风对大洋副极地环流的负/正涡度异常输入,进一步减弱/加强了高纬度上层冷水向SST锋区的输送。北大西洋SST锋的另一主要模态为锋区在南北方向的分支和合并。当SST锋异常在40°N~45°N以单支形式加强时,对流层位势高度场和SLP南北梯度增大,对应NAO正位相,此时风暴轴也为单支型;同时SLP异常场促使冰岛附近具有气旋式风应力异常,亚速尔地区具有反气旋式风应力异常,导致副极地环流和副热带环流均加强,增加高纬度冷水和低纬度暖水在锋区的输入,从而进一步增强40°N~45°N附近的SST锋区。当SST锋异常在40°N~45°N纬带南北发生分支时,风暴轴也同时出现北强南弱的南北分支,此时对应了负位相NAO,来自北南的冷暖水输送减弱,SST锋也发生减弱分支。此外,位于大洋内区的SST锋东端也存在一个偶极子型的模态,尽管其解释方差相对较小,但仍与偏东北的NAO型具有显著相关。谱分析表明,北大西洋SST锋与风暴轴具有1~3年和年代际共振,与中高纬大尺度环流也存在周期1~3年的共变信号,其中准一年共变信号体现了SST锋和NAO之间的对应关系。进一步诊断分析表明,SST锋上空的近表层大气斜压性和经向温度梯度随着SST锋的增强而增强,经向热通量的向北输送导致涡动有效位能的增加;海洋的非绝热加热产生更强的垂直热量通量,这有利于涡动有效位能释放成为涡动动能,从而表现为该区域的风暴轴加强,并进一步影响风暴轴中的天气尺度扰动与下游大尺度环流异常的相互作用过程。     

A NUMERICAL STUDY OF THE INFLUENCE OF SEA SURFACE TEMPERATURES WITH DIFFERENT TEMPORAL RESOLUTIONS ON TYPHOON DUJUAN OVER THE SOUTH CHINA SEA

Daily and weekly sea surface temperature data of Tropical Rainfall Measuring Mission (TRMM) Microwave Imager and Advanced Microwave Scanning Radiometer-Earth Observing System sensors are used as forcing of the underlying sea surface in the mesoscale numerical model to simulate Typhoon Dujuan that moved across the South China Sea in 2003. The numerical results show that different SSTs near the typhoon center result in differences in the atmospheric wind field, indicating that the model has a fast and obvious response to SSTs. Different SST influences the intensity and track of Dujuan to some degree and has significant impacts on its precipitation and latent heat flux near the eye. The SST influence on Dujuan is mainly fulfilled by changing the latent heat flux between the ocean surface and the atmosphere above.     

Storm track response to ocean fronts in a global high-resolution climate model

Synoptic atmospheric eddies are affected by lower tropospheric air-temperature gradients and by turbulent heat fluxes from the surface. In this study we examine how ocean fronts affect these quantities and hence the storm tracks. We focus on two midlatitude regions where ocean fronts lie close to the storm tracks: the north-west Atlantic and the Southern Ocean. An atmospheric climate model of reasonably high resolution (~50 km) is applied in a climate-length (60 year) simulation in order to obtain stable statistics. Simulations with frontal structure in the sea surface temperature (SST) in one of the regions are compared against simulations with globally smoothed SST. We show that in both regions the ocean fronts have a strong influence on the transient eddy heat and moisture fluxes, not just in the boundary layer, but also in the free troposphere. Local differences in these quantities between the simulations reach 20–40 % of the maximum values in the simulation with smoothed SST. Averaged over the entire region of the storm track over the ocean the corresponding differences are 10–20 %. The effect on the transient eddy meridional wind variance is strong in the boundary layer but relatively weak above that. The potential mechanisms by which the ocean fronts influence the storm tracks are discussed, and our results are compared against previous studies with regional models, Aquaplanet models, and coarse resolution coupled models.     

冬季北太平洋风暴轴的年代际变化特征及其可能影响机制

利用1958-2002年的ERA-40再分析资料,用谐波变换和EOF方法分析了冬季北太平洋风暴轴在年代际时间尺度上的变化特征,并通过回归分析的方法初步探讨了风暴轴年代际变化的可能影响机制.结果表明,在年代际时间尺度上,北太平洋风暴轴有两种主要模态,第1模态是风暴轴在其气候平均位置增强或减弱的主体一致变化型,其年代际变化受到上游涡旋强迫的影响,北大西洋强(弱)的涡旋活动,使得冬季北太平洋西风急流减弱(增强)、变宽(窄)、北抬(南压),同期北太平洋风暴轴活动偏强(弱),黑潮延续体区海表温度有偏暖(冷)的响应;第2模态是风暴轴中东部在气候平均位置南北两侧振荡的经向异常型,与太平洋年代际振荡(PDO)循环的暖(冷)位相相联系,下垫面海温非绝热加热的作用,激发加强(减弱)大气中类太平洋/北美遥相关型(PNA)的响应,引起大气斜压性异常偏南(北),使得风暴轴整体南压(北抬),且中东部向东南(北)方向移动.因此,冬季北太平洋风暴轴的年代际变化不仅是局地波-流相互作用的结果,还应考虑上游涡旋活动和海温热力强迫的作用.     

Mesoscale structures of two types of cold-air outbreaks over the East China Sea and the effect of coastal sea surface temperature

Two types of cold-air outbreaks over the Yellow and East China Seas are investigated using a regional mesoscale model. Distinct patterns of surface turbulent heat fluxes and precipitation are identified between the two cases. The sea surface heat flux and frictional velocity are strongly influenced by mesoscale differences between high- and low-resolution datasets of sea surface temperature (SST). The influence of the SST difference on atmospheric water is spread to the downstream area of the outbreak with the phase transition of water. The air mass transformation around 800?hPa over the Kuroshio is partly influenced by the upstream SST difference. In particular, the SST difference near the mouth of the Yanzi River strongly modifies the air mass around Taiwan. Thus, in addition to the Kuroshio front, the mesoscale Chinese coastal SST structure is also important in the air mass transformation over the East China Sea.     

Impacts of Oceanic Fronts and Eddies in the Kuroshio-Oyashio Extension Region on the Atmospheric General Circulation and Storm Track

This paper reviews the progress in our understanding of the atmospheric response to midlatitude oceanic fronts and eddies,emphasizing the Kuroshio-Oyashio Extension(KOE)region.Oceanic perturbations of interest consist of sharp oceanic fronts,temperature anomalies associated with mesoscale eddies,and to some extent even higher-frequency submesoscale variability.The focus is on the free atmosphere above the boundary layer.As the midlatitude atmosphere is dominated by vigorous transient eddy activity in the storm track,the response of both the time-mean flow and the storm track is assessed.The storm track response arguably overwhelms the mean-flow response and makes the latter hard to detect from observations.Oceanic frontal impacts on the mesoscale structures of individual synoptic storms are discussed,followed by the role of oceanic fronts in maintaining the storm track as a whole.KOE fronts exhibit significant decadal variability and can therefore presumably modulate the storm track.Relevant studies are summarized and intercompared.Current understanding has advanced greatly but is still subject to large uncertainties arising from inadequate data resolution and other factors.Recent modeling studies highlighted the importance of mesoscale eddies and probably even submesoscale processes in maintaining the storm track but confirmation and validation are still needed.Moreover,the atmospheric response can potentially provide a feedback mechanism for the North Pacific climate.By reviewing the above aspects,we envision that future research shall focus more upon the interaction between smaller-scale oceanic processes(fronts,eddies,submesoscale features)and atmospheric processes(fronts,extratropical cyclones etc.),in an integrated way,within the context of different climate background states.     

冬季黑潮延伸体海区海洋锋强度变化及其与北太平洋风暴轴的关系

利用英国哈德莱中心（Hadley Center）1949~2014年表层海温资料,将黑潮延伸体海区海温经向梯度的大值区域的平均值定义为黑潮延伸体及其北部海洋锋强度指数（KEFI）,利用数据分析方法研究了各季节KEFI的变化特征及其与北太平洋风暴轴的关系。分析表明,各季节的KEFI存在明显的年际和年代际变化特征,同时在冬季与北太平洋瞬时方差有显著正相关,两者在风暴轴主体位置的相关性最为显著,并且这种相关性在KEFI超前一个月时就所显现,同时对后期风暴轴也有一定影响,即冬季黑潮延伸体海区海洋锋的强度会影响风暴轴区域瞬时方差的变化。之后主要分析了这种影响的可能机制,发现在冬季KEFI高值年,由于海洋锋两侧的热量输送差异更加明显,导致海洋锋附近的近表面气温经向梯度增强,维持了近表面的斜压性,促进涡动热量的向极输送和海洋锋南侧的向上输送,有利于瞬时涡旋的发展。另外大尺度环流场与冬季黑潮延伸体海洋锋也有关系,具体表现为,在海洋锋强年,阿留申低压加深,副热带高压略有加强,对应的对流层低层位势高度场在40°N以北有负变高,以南有正变高,同时高空极锋急流加强,副热带西风急流减弱加宽北抬,海洋锋偏弱年的变化则相反。因此,冬季黑潮延伸体及其北部的海洋锋主要通过两侧海表热量输送差异不断产生气温梯度,进而维持斜压性以促进上层风暴轴的发展。     

春季北太平洋风暴轴的年际与年代际变化特征及其与太平洋海温异常的关系

利用20世纪大气再分析资料和欧洲中心海温资料研究了春季西北太平洋风暴轴的年（代）际变化特征以及在不同年代际背景下风暴轴与太平洋海温关系的转变。结果表明,春季西北太平洋风暴轴主要存在两种空间变化模态,即反映其强度变化的第1模态和反映其南北位置变化的第2模态。年代际及以上时间尺度上,风暴轴强度、位置与太平洋海温的关系主要表现为大气对海洋的强迫作用。在不同年代际背景下,风暴轴与太平洋海温的关系则存在明显的年代际转变：1977年以后,风暴轴强度与太平洋海温的关系主要表现为大气对海洋的强迫作用,而在1977年之前则主要表现为海洋对大气的强迫作用,特别是同期冬季日本以东黑潮和黑潮延伸区海温异常的强迫作用;风暴轴南北位置与太平洋海温异常的关系,在1977年以后表现为大气对海洋的强迫作用,主要表现为对北太平洋中部海温的影响,但在1977年以前表现为海洋和大气的共同作用,风暴轴南北位置的变化还与同期的赤道中东太平洋海温异常有关,表明ENSO可能对风暴轴的位置变化存在影响。     

Isolating mesoscale coupled ocean–atmosphere interactions in the Kuroshio Extension region

The Kuroshio Extension region is characterized by energetic oceanic mesoscale and frontal variability that alters the air–sea fluxes that can influence large-scale climate variability in the North Pacific. We investigate this mesoscale air-sea coupling using a regional eddy-resolving coupled ocean–atmosphere (OA) model that downscales the observed large-scale climate variability from 2001 to 2007. The model simulates many aspects of the observed seasonal cycle of OA coupling strength for both momentum and turbulent heat fluxes. We introduce a new modeling approach to study the scale-dependence of two well-known mechanisms for the surface wind response to mesoscale sea surface temperatures (SSTs), namely, the ‘vertical mixing mechanism’ (VMM) and the ‘pressure adjustment mechanism’ (PAM). We compare the fully coupled model to the same model with an online, 2-D spatial smoother applied to remove the mesoscale SST field felt by the atmosphere. Both VMM and PAM are found to be active during the strong wintertime peak seen in the coupling strength in both the model and observations. For VMM, large-scale SST gradients surprisingly generate coupling between downwind SST gradient and wind stress divergence that is often stronger than the coupling on the mesoscale, indicating their joint importance in OA interaction in this region. In contrast, VMM coupling between crosswind SST gradient and wind stress curl occurs only on the mesoscale, and not over large-scale SST gradients, indicating the essential role of the ocean mesocale. For PAM, the model results indicate that coupling between the Laplacian of sea level pressure and surface wind convergence occurs for both mesoscale and large-scale processes, but inclusion of the mesoscale roughly doubles the coupling strength. Coupling between latent heat flux and SST is found to be significant throughout the entire seasonal cycle in both fully coupled mode and large-scale coupled mode, with peak coupling during winter months. The atmospheric response to the oceanic mesoscale SST is also studied by comparing the fully coupled run to an uncoupled atmospheric model forced with smoothed SST prescribed from the coupled run. Precipitation anomalies are found to be forced by surface wind convergence patterns that are driven by mesoscale SST gradients, indicating the importance of the ocean forcing the atmosphere at this scale.     

北太平洋风暴轴对黑潮延伸体系统变异的响应及其能量转换机制

西北太平洋纬向扰动海温经验正交函数（EOF）分解第一和第三模态、第二和第四模态分别代表同期黑潮延伸体和亲潮强弱的配置关系,将两者的典型位相合成,可以分别得到延伸体收缩和扩张状态时的典型模态海温,本文以此及气候态海温作为初始海温强迫场,利用CESM1.2.0模式,讨论了延伸体的系统变异对北太平洋风暴轴的影响及其在不同能量转换过程的主要影响机制,结果表明,延伸体收缩状态下,北太平洋风暴轴强度整体加强,而扩张模态下强度减弱。空间分布上,收缩模态下,风暴轴主要体现为经向方向的变化,中心及其以北强度加强,中心以南减弱;扩张状态下,则主要表现为纬向方向的差异,中心及以西强度减弱明显,中心以东有所增强。对能量转换的诊断分析表明,正压能量转换过程对涡动动能的变化贡献很小,且在风暴轴中心附近,其作用主要为消耗涡动动能,延伸体收缩状态下其消耗作用增强,而扩张状态下消耗作用减弱,这一差异主要是由于不同海温异常强迫下瞬变涡旋的形变不同造成;斜压有效位能释放比正压能量转换大一个量级以上,该过程几乎全部通过基流的经向温度梯度和经向涡动热量输送的相互作用完成,在这一过程中大气斜压性（经向温度梯度）起了关键性作用,大气斜压性异常、基流经向温度梯度异常、斜压有效位能释放异常与风暴轴异常的空间分布均具有较好的对应关系,该过程可能也是延伸体海温异常影响北太平洋风暴轴的主要物理过程;涡动有效位能需要进一步转换为涡动动能才能产生瞬变涡旋运动,涡动有效位能释放的量级与斜压有效位能的释放相当,但数值要小,这一过程通过冷暖空气的上升下沉运动完成,延伸体异常模态下,扰动垂直速度和扰动温度的负相关性的变化与涡动有效位能向涡动动能转换的变化也有较好的对应关系。     

黑潮延伸体中尺度涡年代际变化与北太平洋风暴轴变化之间的关系

利用18年带通滤波的卫星高度计资料,通过引入黑潮延伸体中尺度涡能量(EKE)的面积指数,分析了黑潮延伸体中尺度涡EKE的强度和位置的年代际变化特征,并使用回归分析等方法分析了它们与北太平洋风暴轴之间的关系。结果表明,黑潮延伸体中尺度涡增强与北太平洋风暴轴的增强相对应,而EKE位置偏北(南)时对应的北太平洋风暴轴也偏北(南),同时当EKE的位置偏东(偏西)时北太平洋风暴轴则西退(东移)。此外,北太平洋风暴轴的变化对黑潮延伸体也可能有一定的反馈作用,黑潮延伸体中尺度涡EKE强度的变化与北太平洋风暴轴EOF第一和第三个模态(第二个模态)回归的海表面高度距平模态有明显的3~4年滞后的正(负)相关,而黑潮延伸体中尺度涡EKE位置的变化则相反。这种滞后相关可能是通过北太平洋风暴轴驱动的遥相关型环流改变海表面风应力旋度并强迫出的海表面高度距平的西传导致的。     

Mechanisms of the atmospheric response to North Atlantic multidecadal variability: a model study

The atmospheric circulation response to decadal fluctuations of the Atlantic meridional overturning circulation (MOC) in the IPSL climate model is investigated using the associated sea surface temperature signature. A SST anomaly is prescribed in sensitivity experiments with the atmospheric component of the IPSL model coupled to a slab ocean. The prescribed SST anomaly in the North Atlantic is the surface signature of the MOC influence on the atmosphere detected in the coupled simulation. It follows a maximum of the MOC by a few years and resembles the model Atlantic multidecadal oscillation. It is mainly characterized by a warming of the North Atlantic south of Iceland, and a cooling of the Nordic Seas. There are substantial seasonal variations in the geopotential height response to the prescribed SST anomaly, with an East Atlantic Pattern-like response in summer and a North Atlantic oscillation-like signal in winter. In summer, the response of the atmosphere is global in scale, resembling the climatic impact detected in the coupled simulation, albeit with a weaker amplitude. The zonally asymmetric or eddy part of the response is characterized by a trough over warm SST associated with changes in the stationary waves. A diagnostic analysis with daily data emphasizes the role of transient-eddy forcing in shaping and maintaining the equilibrium response. We show that in response to an intensified MOC, the North Atlantic storm tracks are enhanced and shifted northward during summer, consistent with a strengthening of the westerlies. However the anomalous response is weak, which suggests a statistically significant but rather modest influence of the extratropical SST on the atmosphere. The winter response to the MOC-induced North Atlantic warming is an intensification of the subtropical jet and a southward shift of the Atlantic storm track activity, resulting in an equatorward shift of the polar jet. Although the SST anomaly is only prescribed in the Atlantic ocean, significant impacts are found globally, indicating that the Atlantic ocean can drive a large scale atmospheric variability at decadal timescales. The atmospheric response is highly non-linear in both seasons and is consistent with the strong interaction between transient eddies and the mean flow. This study emphasizes that decadal fluctuations of the MOC can affect the storm tracks in both seasons and lead to weak but significant dynamical changes in the atmosphere.     

冬季黑潮延伸体区域海表温度锋对北太平洋风暴轴的影响

利用NOAA最优插值逐日海表温度资料和NCEP/NCAR的逐日大气再分析资料,分析了冬季黑潮延伸体区域海表温度锋的变化及其对北太平洋风暴轴的影响。结果表明,冬季黑潮延伸体区域海表温度锋强度和纬度位置既存在年际变化,也存在年代际变化,且强度和位置的变化是相互独立的。冬季黑潮延伸体区域海表温度锋强度的年际变化对北太平洋风暴轴没有显著的影响,而其年代际变化则对北太平洋风暴轴具有非常显著的影响,当冬季海表温度锋偏强时,大气斜压性在鄂霍次克海及阿拉斯加附近区域上空增强,而在海表温度锋下游至东太平洋区域上空显著减弱,平均有效位能向涡动有效位能的斜压能量转换在45°N以北的太平洋区域上空有所增多,而在30°-45°N的太平洋区域上空有所减少,涡动有效位能向涡动动能的斜压能量转换在35°N以北的西太平洋区域以及45°N以北的东太平洋区域都显著增加,而仅在其南部边缘存在东西带状的减弱区域,导致40°N以北海区北太平洋风暴轴增强,40°N以南海区北太平洋风暴轴减弱,冬季海表温度锋偏弱时则有与之相反的结果。冬季黑潮延伸体区域海表温度锋纬度位置的变化对北太平洋风暴轴也存在较显著的影响,当海表温度锋位置偏北时,在其下游45°N以南的太平洋区域上空大气斜压性减弱,45°N以南的中东太平洋区域上空区域平均有效位能向涡动有效位能、以及涡动有效位能向涡动动能的斜压能量转换都减少;而在45°N以北的太平洋区域上空大气斜压性增强,在阿拉斯加湾附近上空尤其显著,在黑潮延伸体区域附近以及45°N以北的中东太平洋上空平均有效位能向涡动有效位能、以及涡动有效位能向涡动动能的斜压能量转换都显著增加,导致北太平洋风暴轴在其气候平均态轴线两侧呈现北正南负的偶极子形态;海表温度锋位置偏南时则有与之相反的结果。冬季黑潮延伸体区域海表温度锋强度和位置的变化均对北太平洋风暴轴具有显著的影响,其具体的物理机制还需要进一步的研究。      

冬季北太平洋风暴轴的年际变化及其与500hPa高度以及热带和北太平洋海温的联系

文中研究了冬季北太平洋风暴轴的年际异常及其与500hPa高度以及热带和北太平洋海温的联系。结果发现,各年冬季北太平洋风暴轴的中心强度和位置具有显著的年际差异。对15个冬季北太平洋风暴轴区域500hPa天气尺度滤波位势高度方差与热带和北太平洋海温的SVD分析表明,第一对空间典型分布反映了赤道中、东太平洋区域海温异常对风暴轴年际变化的影响,而第二对空间典型分布反映了黑潮区域海温异常对风暴轴年际变化的影响。进一步的合成分析显示,赤道中、东太平洋区域海温异常主要影响冬季北太平洋风暴轴的东西摆动和中、东端的强度变化,而黑潮区域海温异常则主要影响冬季北太平洋风暴轴中、西端的强度变化和南北位移。并且这种影响分别与500hPa高度场上的PNA遥相关型和WP遥相关型有密切联系。     

An analysis on the physical process of the influence of AO on ENSO

The influence of the spring AO on ENSO has been demonstrated in several recent studies. This analysis further explores the physical process of the influence of AO on ENSO using the NCEP/NCAR reanalysis data over the period 1958–2010. We focus on the formation of the westerly wind burst in the tropical western Pacific, and examine the evolution and formation of the atmospheric circulation, atmospheric heating, and SST anomalies in association with the spring AO variability. The spring AO variability is found to be independent from the East Asian winter monsoon activity. The spring AO associated circulation anomalies are supported by the interaction between synoptic-scale eddies and the mean-flow and its associated vorticity transportation. Surface wind changes may affect surface heat fluxes and the oceanic heat transport, resulting in the SST change. The AO associated warming in the equatorial SSTs results primarily from the ocean heat transport in the face of net surface heat flux damping. The tropical SST warming is accompanied by anomalous atmospheric heating in the subtropical north and south Pacific, which sustains the anomalous westerly wind in the equatorial western Pacific through a Gill-like atmospheric response from spring to summer. The anomalous westerly excites an eastward propagating and downwelling equatorial Kelvin wave, leading to SST warming in the tropical central-eastern Pacific in summer-fall. The tropical SST, atmospheric heating, and atmospheric circulation anomalies sustain and develop through the Bjerknes feedback mechanism, which eventually result in an El Niño-like warming in the tropical eastern Pacific in winter.     

The Impact of the Storm-Induced SST Cooling on Hurricane Intensity

The effects of storm-induced sea surface temperature (SST) cooling on hurricane intensity are investigated using a 5-day cloud-resolving simulation of Hurricane Bonnie (1998). Two sensitivity simulations are performed in which the storm-induced cooling is either ignored or shifted close to the modeled storm track. Results show marked sensitivity of the model-simulated storm intensity to the magnitude and relative position with respect to the hurricane track. It is shown that incorporation of the storm-induced cooling, with an average value of 1.3℃, causes a 25-hPa weakening of the hurricane, which is about 20 hPa per 1℃ change in SST. Shifting the SST cooling close to the storm track generates the weakest storm, accounting for about 47% reduction in the storm intensity. It is found that the storm intensity changes are well correlated with the air-sea temperature difference. The results have important implications for the use of coupled hurricane-ocean models for numerical prediction of tropical cyclones.     

A numerical study of a TOGA-COARE squall-line using a coupled mesoscale atmosphere-ocean model

1. Introduction Air-sea interaction plays an important role in theglobal seasonal to inter-annual climate variability,most notably, the El Ni?no and Southern Oscillation(ENSO) phenomenon (Webster and Lukas, 1992). Be-cause of its widespread impacts on …     

Twentieth century North Atlantic climate change. Part II: Understanding the effect of Indian Ocean warming

Ensembles of atmospheric general circulation model (AGCM) experiments are used in an effort to understand the boreal winter Northern Hemisphere (NH) extratropical climate response to the observed warming of tropical sea surface temperatures (SSTs) over the last half of the twentieth Century. Specifically, we inquire about the origins of unusual, if not unprecedented, changes in the wintertime North Atlantic and European climate that are well described by a linear trend in most indices of the North Atlantic Oscillation (NAO). The simulated NH atmospheric response to the linear trend component of tropic-wide SST change since 1950 projects strongly onto the positive polarity of the NAO and is a hemispheric pattern distinguished by decreased (increased) Arctic (middle latitude) sea level pressure. Progressive warming of the Indian Ocean is the principal contributor to this wintertime extratropical response, as shown through additional AGCM ensembles forced with only the SST trend in that sector. The Indian Ocean influence is further established through the reproducibility of results across three different models forced with identical, idealized patterns of the observed warming. Examination of the transient atmospheric adjustment to a sudden “switch-on” of an Indian Ocean SST anomaly reveals that the North Atlantic response is not consistent with linear theory and most likely involves synoptic eddy feedbacks associated with changes in the North Atlantic storm track. The tropical SST control exerted over twentieth century regional climate underlies the importance of determining the future course of tropical SST for regional climate change and its uncertainty. Better understanding of the extratropical responses to different, plausible trajectories of the tropical oceans is key to such efforts.     

Mid latitude Atlantic SST influence on European winter climate variability in the NCEP Reanalysis

 Forty one years (1958–1998) of NCEP reanalysis data are used to perform a set of statistical analyses, investigating the interactions between the sea surface temperature (SST), the storm track activity (STA) and the time mean atmospheric circulation in the North Atlantic-Europe (NAE) region. When the atmospheric region of study is restricted to Europe, the singular value decomposition (SVD) lead-lag analysis between seasonal 500 hPa geopotential height (Z500) and SST captures a significant covariance between a summer SST anomaly and a strong winter anticyclonic anomaly over Europe. The summer SST pattern is close to the first empirical orthogonal function (EOF) of SST for the consecutive months J-A-S-O-N. The same analysis, but extending the atmospheric area of interest to the entire NAE region, points out the same signal with a phase shift of one month. A more zonally oriented North-Atlantic-Oscillation-like (NAO) pattern is then found as the SST structure remains practically unchanged. This summer SST anomaly is found to persist through surface heat fluxes exchanges until winter, when it can finally have an impact on the atmospheric circulation. Composites are made from the former SST SVD scores, showing the winter STA and different transient and stationary eddies diagnostics associated with the extreme positive and negative events of the SST anomaly. These suggest that the SST anomaly induces an anomalous stationary wave in winter, creating an initially small anticyclonic anomaly over Europe. Anomalous transient eddies located over northern Europe then strengthen this anomaly and maintain it during winter, thus acting as a positive feedback. Received: 15 September 2000 / Accepted: 17 April 2001