The impact of internal atmospheric variability on the North Pacific SST variability

The impact of internal atmospheric variability on North Pacific sea surface temperature (SST) variability is examined based on three coupled general circulation model simulations. The three simulations differ only in the level of atmospheric noise occuring over the ocean at the air-sea interface. The amplitude of atmospheric noise is controlled by use of the interactive ensemble technique. This technique simultaneously couples multiple realizations of a single atmospheric model to a single realization of an ocean model. The atmospheric component models all experience the same SST, but the ocean component is forced by the ensemble averaged fluxes thereby reducing the impact of internal atmospheric dynamics at the air-sea interface. The ensemble averaging is only applied at the air-sea interface so that the internal atmospheric dynamics (i.e., transients) of each atmospheric ensemble member is unaffected. This interactive ensemble technique significantly reduces the SST variance throughout the North Pacific. The reduction in SST variance is proportional to the number of ensemble members indicating that most of the variability can simply be explained as the response to atmospheric stochastic forcing. In addition, the impact of the internal atmospheric dynamics at the air-sea interface masks out much of the tropical-midlatitude SST teleconnections on interannual time scales. Once this interference is reduced (i.e., applying the interactive ensemble technique), tropical-midlatitude SST teleconnections are easily detected.     

The influence of sea-surface temperatures on Eastern North Pacific tropical cyclone activity

The influence of sea-surface temperatures on six measures of tropical cyclone activity in the Eastern North Pacific is examined using historical sea-surface temperature and tropical cyclone data spanning from 1971 to 2002. Relationships are evaluated using methods of trend analysis, extreme year analysis, and bivariate correlation. Results suggest that in order to understand the climatological factors affecting topical cyclone activity in the Eastern North Pacific, the main development region must be divided into two sub-regions of development to the east and west of 112°W longitude. Increasing trends of sea-surface temperature are not accompanied by increasing trends in tropical cyclone activity. In the western development region, sea-surface temperatures are significantly correlated with all measures of tropical cyclone activity during extreme years. In this region, sea-surface temperatures are on average below the threshold for tropical cyclone development. In the Eastern development region, the only significant correlation with sea-surface temperatures is for the more intense measures of hurricane activity. In this region, sea-surface temperatures are on average above the threshold for cyclone formation. This leads to the hypothesis that the proximity to the cyclone formation temperature threshold in the WDR enhances the sensitivity of tropical cyclone activity to SSTs. This may have application to other tropical cyclone basins such as the North Atlantic.     

The Influence of Tibetan Plateau on the Interannual Variability of Atmospheric Circulation over Tropical Pacific

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Optimal heat source for the interannual variability of the western North Pacific summer monsoon

西北太平洋夏季风是热带影响东亚气候的桥梁,具有很强的年际变率。西北太平洋夏季风异常能造成东亚夏季旱涝和高低温灾害,因此研究它的年际变率驱动因子具有重要意义。本文利用一个格林函数方法定量的评估了热带各个格点热源对西北太平洋夏季风的贡献。具体为将南北55纬度之间的区域划分成132个小格子,在每个格子加上一个椭圆形理想热源;接着利用一个线性理论大气环流模式计算每个热源对西北太平洋夏季风的贡献。研究表明导致西北太平洋夏季风增强的最理想加热场结构是位于热带印度洋和中纬度东亚地区上空的冷源和位于副热带西北太平洋地区的热源。     

Decadal predictability and prediction skill of sea surface temperatures in the South Pacific region

The South Pacific Ocean is a key driver of climate variability within the Southern Hemisphere at different time scales. Previous studies have characterized the main mode of interannual sea surface temperature (SST) variability in that region as a dipolar pattern of SST anomalies that cover subtropical and extratropical latitudes (the South Pacific Ocean Dipole, or SPOD), which is related to precipitation and temperature anomalies over several regions throughout the Southern Hemisphere. Using that relationship and the reported low predictive skill of precipitation anomalies over the Southern Hemisphere, this work explores the predictability and prediction skill of the SPOD in near-term climate hindcasts using a set of state-of-the-art forecast systems. Results show that predictability greatly benefits from initializing the hindcasts beyond the prescribed radiative forcing, and is modulated by known modes of climate variability, namely El Niño-Southern Oscillation and the Interdecadal Pacific Oscillation. Furthermore, the models are capable of simulating the spatial pattern of the observed SPOD even without initialization, which suggests that the key dynamical processes are properly represented. However, the hindcast of the actual phase of the mode is only achieved when the forecast systems are initialized, pointing at SPOD variability to not be radiatively forced but probably internally generated. The comparison with the performance of an empirical prediction based on persistence suggests that initialization may provide skillful information for SST anomalies, outperforming damping processes, up to 2–3 years into the future.     

Revisiting the intraseasonal,interannual and interdecadal variability of tropical cyclones in the western North Pacific

This paper reviews the recent progress and research on the variability of tropical cyclones(TCs) at different time scales. Specific focus is placed on how different types of external forcings or climatic oscillations contribute to TC variability in the western North Pacific(WNP). At the intraseasonal scale, recent advances on the distinctive impacts of the Madden–Julian Oscillation(MJO), the Quasi-biweekly Oscillation, and the asymmetric MJO modulation under different El Ni?o–Southern Oscillation(ENSO) states, as well as the influences of the Pacific–Japan teleconnection, are highlighted. Interannually, recent progress on the influences of the ENSO cycle, different flavors of ENSO, and impacts of Indian Ocean warming is presented. In addition, the uncertainty concerning interdecadal TC variations is discussed, along with the recently proposed modulation mechanisms related to the zonal sea surface temperature gradient, the North Pacific Gyre Oscillation, and the Pacific Decadal Oscillation(PDO). It is hoped that this study can deepen our understanding and provide information that the scientific community can use to improve the seasonal forecasting of TCs in the WNP.     

Multidecadal signals in the interannual variability of atmospheric angular momentum

To assess whether trends in the troposphere's circulation have occurred this century in concert with changes at the surface, we consider the low-frequency behavior in the global angular momentum of the atmosphere. Values of this index have been computed each month since 1870 from model experiments conducted by the UK Hadley Centre for Climate Prediction and Research, in which the model atmosphere is driven by prescribed boundary forcings, most especially sea surface temperature (SST) fields. The interannual variability of the model's atmospheric angular momentum compares favorably with that derived from the NCEP/NCAR reanalysis for the period of overlap (the second half of the twentieth century). Evidence is found for a notable increase in interannual variability since around 1970, but this recent high level of variability may not be significantly greater than that during 1900–1920. This latter result is strongly dependent on the character of the SST field used in an experiment, because low-frequency signals in atmospheric momentum are closely linked to those in the El Niño/Southern Oscillation. Large differences exist, however, among SST estimates in the tropical Pacific for the early part of the century.     

The role of ocean dynamics in producing decadal climate variability in the North Pacific

 Decadal time scale climate variability in the North Pacific has implications for climate both locally and over North America. A crucial question is the degree to which this variability arises from coupled ocean/atmosphere interactions over the North Pacific that involve ocean dynamics, as opposed to either purely thermodynamic effects of the oceanic mixed layer integrating in situ the stochastic atmospheric forcing, or the teleconnected response to tropical variability. The part of the variability that is coming from local coupled ocean/atmosphere interactions involving ocean dynamics is potentially predictable by an ocean/atmosphere general circulation model (O/A GCM), and such predictions could (depending on the achievable lead time) have distinct societal benefits. This question is examined using the results of fully coupled O/A GCMs, as well as targeted numerical experiments with stand-alone ocean and atmosphere models individually. It is found that coupled ocean/atmosphere interactions that involve ocean dynamics are important to determining the strength and frequency of a decadal-time scale peak in the spectra of several oceanic variables in the Kuroshio extension region off Japan. Local stochastic atmospheric heat flux forcing, integrated by the oceanic mixed layer into a red spectrum, provides a noise background from which the signal must be extracted. Although teleconnected ENSO responses influence the North Pacific in the 2–7 years/cycle frequency band, it is shown that some decadal-time scale processes in the North Pacific proceed without ENSO. Likewise, although the effects of stochastic atmospheric forcing on ocean dynamics are discernible, a feedback path from the ocean to the atmosphere is suggested by the results. Received: 23 January 2000 / Accepted: 10 January 2001     

影响夏季西北太平洋副热带高压年际变率的关键海区及影响机制

西北太平洋副热带高压(西太副高)是影响东亚夏季气候的主要环流系统,其年际变率受热带多个海区的海-气相互作用过程的调控。为明确影响夏季西太副高的关键海区及其影响机制,在总结最近十余年来相关研究进展的基础上,归纳出影响夏季西太副高年际变率的5个关键海区,包括赤道中东太平洋、热带印度洋、副热带西北太平洋、海洋大陆附近海区以及热带大西洋。阐述了这5个关键海区的海温异常影响西太副高年际变率的机制,并探讨了5个关键海区海温异常的形成机制。围绕夏季西太副高的年际变率,回顾了当前气候模式的模拟和预测研究的现状。最后,提出了本领域亟待解决的关键科学问题,展望未来可能的研究热点。     

北太平洋700毫巴月平均环流形势与海温的相互关系

北太平洋700 mb环流形势具有相当鲜明的季节性与地域性特征.北太平洋的海、气相互作用是这些特征以及东亚季风形成的一个原因.从海、气相互作用的角度出发,对比分析北太平洋700 mb环流形势和海表温度距平的分布,可以看出:有些情况下,大气对海洋的影响比较明显;在另一些情况下,海洋对700 mb环流形势的影响较为明显.它们之间的矛盾与矛盾的主要方面是不断地、经常地在转化以及相互影响着.它们的转化、演变对我国副热带高压及旱涝天气的变化有一定的关系。     

The impact of ENSO periodicity on North Pacific SST variability

The periodicity of ENSO in nature varies. Here we examine how changes in the frequency of ENSO impacts remote teleconnections in the North Pacific. The numerical experiments presented here are designed to simulate perfectly periodic ENSO in the tropical Pacific, and to enable the air–sea interaction in other regions (i.e., the North Pacific) via a simple mixed layer ocean model. The temporal evolution and spatial structure of the North Pacific SST teleconnection patterns are relatively insensitive to the frequency of ENSO, but the amplitude of the variability is sensitive. Specifically, the 2-year period ENSO experiment (P2) shows weak event-by-event consistency in the ENSO response mature pattern. This is because there is not enough time to damp the previously forced ENSO teleconnections (i.e., 1 year earlier). The 4-year period ENSO experiment (P4) has 1 year damping time before a successive ENSO event matures, so the structure of the response pattern is stably repeated. However, the event-by-event variance of anomaly magnitude, specifically responding to El Niño, is still larger than that in the 6-year ENSO experiment (P6), which has 2-year damping time between consecutive ENSO events. In addition, we tested whether the variability due to tropical remote forcing is linearly independent of the extratropical local variability. Statistical tests indicate that tropical remote forcing can constructively or destructively interfere with local variability in the North Pacific. Lastly, there is a non-linear rectification of the ENSO events that can be detected in the climatology.     

The role of synoptic systems in the interannual variability of Sahel rainfall

Summary Components of the June-September climate over the Sahel are investigated in simulations with the GCM of the NASA/Goddard Institute for Space Studies, forced by SST observed during 1987 and 1988. The study analyzes the role of the synoptic patterns in determining precipitation differences between the two seasons, with special attention given to African wave disturbances (AWD). Emphasis is placed on deducing the characteristics of individual systems which may be missed by spectral and/or composite analyses. Results are derived from time-longitude cross-sections and spatial distributions of daily and weekly averages of key climatological variables. Despite the overall rainier season, rainless AWD are more prevalent in the simulations corresponding to June–September 1988 forcing than for 1987. Daily precipitation is shown to be highly correlated with mid-tropospheric vorticity, near surface convergence and 200 mb divergence. August daily rainfall was some-what better correlated with implied large scale vertical motion for 1988 forcing, emphasizing the dominance of broad circulation influences during that summer. While significant rainfall variability is attributed to AWD, quasistationary mechanisms cannot be ignored. In these simulations, upper tropospheric divergence modulated by changes in the Tropical Easterly Jet serves to both intensify the rainfall triggered by AWD and to sustain broader rainfall patterns during events of massive uplift.With 16 Figures     

The global distribution of sources of atmospheric decadal variability and mechanisms over the tropical Pacific and southern North America

 Understanding natural atmospheric decadal variability is an important element of climate research, and here we investigate the geographic and seasonal diversity in the balance between its competing sources. Data are provided by an ensemble of multi-decadal atmospheric general circulation model experiments, forced by observed sea surface temperatures (SSTs), and verified against observations. First, the nature of internal atmospheric variability is studied. By assessing its spectral character, we refute the idea that internal modes may persist or oscillate on multi-annual time-scales, either through mechanisms purely internal to the atmosphere, or via coupling to the land surface; instead, they behave as a white noise process. Second, and more importantly, the role of oceanic forcing, relative to internal variability, is investigated by extending the ‘analysis of variance’ technique to the frequency domain. Significance testing and confidence intervals are also discussed. In the tropics, atmospheric decadal variability is usually dominated by oceanic forcing, although for some regions less so than at interannual time-scales. A moderate oceanic impact is also found for some extratropical regions in some seasons. Verification against observed mean sea-level pressure (MSLP) data suggests that many of these influences are realistic, although some model errors are also revealed. In other mid- and high-latitude regions, local simulated decadal variability is dominated by random processes, i.e. the integrated effects of chaotic weather systems. Third, we focus on the mechanisms of decadal variability in two specific regions (where the model is well behaved). Over the tropical Pacific, the relative impact of SSTs on decadal MSLP is strongly seasonal such that it peaks in September to November (SON). This is explained by noting that the model atmosphere is responsive to SSTs a little farther west in SON than it is in other seasons, and here it picks up relatively more decadal power from the ocean (the western Pacific being less dominated by ENSO time-scales), causing atmospheric ‘signal-to-noise ratios’ to be enhanced at decadal timescales in SON. Over southern North America, a strong SST impact is found in summer and autumn, resulting in an upward trend of MSLP over recent decades. We suggest this is caused by decadal SST variability in the Caribbean (and to some extent the tropical northeast Pacific in summer), which induces anomalous convective heating over these regions and hence the wider MSLP response. Received: 30 November 1998 / Accepted: 22 April 1999     

The impact of tropical western Pacific convection on the North Pacific atmospheric circulation during the boreal winter

In this study, we investigate the impact of atmospheric convection over the western tropical Pacific (100–145°E, 0–20°N) on the boreal winter North Pacific atmosphere flow by analyzing National Center for Environmental Prediction Reanalysis 1, Extended Reconstructed Sea Surface Temperature and Global Precipitation Climatology Project data. The western tropical Pacific convection is not only the main energy source driving the local Hadley and Walker circulations, but it also significantly influences North Pacific circulation, by modifying a mid-latitude Jet stream through the connection with the local Hadley circulation. On the one hand, this strong convection leads to a northward expansion of local Hadley cells simultaneous with a northward movement of the western North Pacific jet because of the close correlation between the Jet and Hadley circulation boundaries. On the other hand, this strong convection also intensifies tropical Pacific Walker circulation, which reduces the eastern Pacific sea surface temperature, resembling a La Nina state through the enhanced equatorial upwelling. The cooling of the eastern tropical Pacific has an inter-tropical convergence zone located further north; thus, the local Hadley circulation moves northward. As a result, the jet axis over the eastern North Pacific, which also corresponds to the boundary of the local Hadley circulation, moves to higher latitude. Consequently, this northward movement of the Jet axis over the North Pacific is reflected as a northwest–southeast dipole sea level pressure (SLP) pattern. The composite analysis of SLP over the North Pacific against the omega (Ω) (Pa/s) at 500 hPa over the western tropical Pacific actually reveals that this northwest-southeast dipole structure is attributed to the intensified tropical western Pacific convection, which pushes the Pacific Jet to the north. Finally we also analyzed south Pacific for the austral winter as did previously to North Pacific, and found that the results were consistent.     

The impact of atmospheric initialisation on seasonal prediction of tropical Pacific SST

The impact of realistic atmospheric initialisation on the seasonal prediction of tropical Pacific sea surface temperatures is explored with the Predictive Ocean–Atmosphere Model for Australia (POAMA) dynamical seasonal forecast system. Previous versions of POAMA used data from an Atmospheric Model Intercomparison Project (AMIP)-style simulation to initialise the atmosphere for the hindcast simulations. The initial conditions for the hindcasts did not, therefore, capture the true intra-seasonal atmospheric state. The most recent version of POAMA has a new Atmosphere and Land Initialisation scheme (ALI), which captures the observed intra-seasonal atmospheric state. We present the ALI scheme and then compare the forecast skill of two hindcast datasets, one with AMIP-type initialisation and one with realistic initial conditions from ALI, focussing on the prediction of El Niño. For eastern Pacific (Niño3) sea surface temperature anomalies (SSTAs), both experiments beat persistence and have useful SSTA prediction skill (anomaly correlations above 0.6) at all lead times (forecasts are 9 months duration). However, the experiment with realistic atmospheric initial conditions from ALI is an improvement over the AMIP-type initialisation experiment out to about 6 months lead time. The improvements in skill are related to improved initial atmospheric anomalies rather than an improved initial mean state (the forecast drift is worse in the ALI hindcast dataset). Since we are dealing with a coupled system, initial atmospheric errors (or differences between experiments) are amplified though coupled processes which can then lead to long lasting errors (or differences).     

An Approach for Improving Short-Term Prediction of Summer Rainfall over North China by Decomposing Interannual and Decadal Variability简

A statistical downscaling approach was developed to improve seasonal-to-interannual prediction of summer rainfall over North China by considering the effect of decadal variability based on observational datasets and dynamical model outputs.Both predictands and predictors were first decomposed into interannual and decadal components.Two predictive equations were then built separately for the two distinct timescales by using multivariate linear regressions based on independent sample validation.For the interannual timescale,850-hPa meridional wind and 500-hPa geopotential heights from multiple dynamical models＇ hindcasts and SSTs from observational datasets were used to construct predictors.For the decadal timescale,two well-known basin-scale SST decadal oscillation （the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation） indices were used as predictors.Then,the downscaled predictands were combined to represent the predicted/hindcasted total rainfall.The prediction was compared with the models＇ raw hindcasts and those from a similar approach but without timescale decomposition.In comparison to hindcasts from individual models or their multi-model ensemble mean,the skill of the present scheme was found to be significantly higher,with anomaly correlation coefficients increasing from nearly neutral to over 0.4 and with RMSE decreasing by up to 0.6 mm d-1.The improvements were also seen in the station-based temporal correlation of the predictions with observed rainfall,with the coefficients ranging from-0.1 to 0.87,obviously higher than the models＇ raw hindcasted rainfall results.Thus,the present approach exhibits a great advantage and may be appropriate for use in operational predictions.     

春季中国南方雨带年际变动与大气环流异常

利用1960—2008年中国693个站逐日降水资料和NCEP/NCAR日平均再分析资料,采用统计分析方法,分析了中国南方春季降水强度和位置的年际变率及其与大气环流的关系。结果表明:在年代际尺度上,江南春季降水在20世纪60年代中、后期偏少,70年代中期到80年代初偏多,90年代初开始减少;在年际尺度上,当春季西太平洋副热带高压和青藏高原东侧的低层低压系统加强,并且异常中心分别位于20°N以南和30°N以南时,异常西南风主要位于长江以南地区,在异常西南风逐渐减弱区出现明显的辐合,伴随着该地区低层空气质量辐合、对流层上升运动和水汽辐合加强,造成江南地区降水偏多,此时来自西太平洋的异常水汽到达南海后,没有在南海聚集,而是转向北输送到江南;当春季西太平洋副热带高压以及青藏高原东侧低压系统加强且异常中心位于30°N以北时,异常西南风盛行在中国东部大部分地区,此时低层异常空气质量辐合、对流层异常上升运动以及异常水汽通量辐合区都向北移到江淮地区,使江淮地区降水增加,而华南地区为异常空气质量辐散、异常下沉运动以及异常水汽通量辐散,伴随着降水减少,这时异常水汽主要来自西太平洋副热带地区。由于上述观测结果与通过改变东亚和周边海域海-陆热力差异的数值试验结果有很好的一致性,因此,这里观测到的降水和大气环流异常可以被东亚区域热力差异异常激发出来。