太平洋地区海气系统年代际变率研究的若干进展

系统回顾了近年来太平洋地区海气系统年代际变率的研究进展,涉及大气环流、海洋状况及海气相互作用的年代际变化,以及产生这种年代际变率的主要机制,并简单讨论了目前该领域研究中存在的主要问题及研究前景.     

国外关于年代际气候变率的研究

文中指出在气候变率与可预报性研究计划中,列出年代际气候变率及可预报性研究子计划充分显示出这个问题的重要性,回顾并总结了近年来年代际气候变率的研究成果,特别对全球气候变暖、海气相互作用、温盐环流、自然气候变率和ＥＮＳＯ的年代际变率的研究几个重点问题进行了讨论,最后对年代际气候变率研究今后的发展方向提出了看法。     

北大西洋年际变率的海气耦合模式模拟Ⅰ：局地海气相互作用

检验了一个全球海气耦合模式对北大西洋年际气候变率的模拟,讨论了导致这种年际变率型的物理机制,并分析了其对年代际变率的可能影响。北大西洋冬季SST的主导变率模态,在经向上表现为三核型,自北而南出现“- -”的带状距平型;最大距平中心位于副极地大洋、中纬度大洋的西部以及热带海域,耦合模式较为真实地再现了这一特征。与三核型SST异常相对应的大气环流型表现为北大西洋涛动,具有显著的正压结构。上述异常型主要发生在年际尺度,具有3—4年的谱峰;在次年代际尺度上,也存在谱峰。分析表明,模式中三核型SST异常的产生,主要来自大气的强迫,NAO增强,中纬度大洋上的西风减弱,海洋感热和潜热通量损失减少,中纬度大洋得到的净热通量增加,导致SST出现正距平;在包括Labrador海在内的副极地大洋,NAO增强、冰岛低压加深,气旋性环流增强,来自高纬度的冷空气吹过洋面,海气温差加大,大洋的感热通量损失增加,SST降低。热带地区东风的增强,也是导致那里SST降低的重要机制。三核型SST异常对大气的反馈作用较弱,文中没有证据表明它能够影响到北大西洋地区的年代际气候变率。     

近百年热带太平洋海温年际及年代际 时间变率特征的诊断研究

年际及年代际时间变率是当代气候研究的重要问题之一，通过对近百年热带太平洋海温资料做子波分析发现,20世纪50年代以来的海温升高及频繁发生的ENSO事件伴随着海温年代际时间尺度背景场的明显改变，同时还研究了其年代际及年际时间尺度时间变率特征。     

太平洋年代际振荡的研究进展

作者以太平洋年代际振荡(PDO)现象为重点,系统地回顾了太平洋年代际变率观测、模拟和理论研究的国内外进展.在PDO时空结构方面,总结了PDO的基本观测事实,在PDO成因方面,从海洋大气相互作用观点出发比较了三类PDO形成机制的理论或假说,讨论了现有理论或假说中存在的若干问题,并提出了未来研究的方向和需要解决的关键科学问题.作者也简要介绍了东亚及中国气候年代际变化的特征及其和太平洋海表温度异常的联系,并讨论了东亚大气环流异常在PDO形成中的可能作用.     

热带太平洋经向风异常年际变化与海温异常的关系

运用奇异谱分析（SSA）方法对热带太平洋海温和经向风进行周期分析，结果表明太平洋海气系统存在准4年、准2年和年代际尺度的变化周期，其中准4年的变率最大。不同区域的风场，在不同时间尺度上与海温异常联系的紧密程度不同，位相的演变也有差异。文中还讨论了在准4年与准2年分量之间因风场的不同而导致的海温异常的差异。     

全球海气耦合模式系统(NIM/COAMS)Ⅱ.年际变化的模拟

利用文献[1]建立的全球海气耦合模式系统（NIM／COAMS），对模式的年际变化模拟能力进行了检验。50a积分显示，模式模拟出了大气和海洋界面的主要年际变率，能真实地模拟热带太平洋ENSO循环的主要特征，较好地再现了ENSO循环的过程，循环周期在3—5a之间，与实际观测值一致，同时模式也较好地反映了大气和海洋的耦合特征，对年际变化有较强的模拟能力，这与FRAC耦合方案设计有关，该方法能避免气候场的牵制作用，增强模式对年际变化的模拟能力。     

近百年来热带太平洋海温年际及年代际时间变率特征的诊断研究

年际及年代际时间变率是当代气候研究的重要问题之一 ,通过对近百年热带太平洋海温资料做子波分析发现 ,2 0世纪 50年代以来的海温升高及频繁发生的 ENSO事件伴随着海温年代际时间尺度背景场的明显改变 ,同时还研究了其年代际及年际时间尺度时间变率特征。     

Influence of solar wind energy flux on the interannual variability of ENSO in the subsequent year

作为地球系统的主要能量来源,进入地磁系统的太阳风能量通量(E_(in))一直难以估算。因此,E_(in)对气候的影响也没有得到广泛的研究。基于三维磁流体动力模拟估算的E_(in),本文指出,太阳风能量通量不仅存在准11年周期的年代际变率,同时还存在2-4年周期的年际变率。与以往主要关注太阳活动在年代际尺度上的气候效应的研究不同,本文揭示出太阳风能量通量与次年ENSO年际变率存在显著的联系。     

Progress and future prospects of decadal prediction and data assimilation:A review

年代际预测,也称为“近期气候预测”,旨在预测未来1-10年内的气候变化,是气候预测和气候变化研究领域的一个新关注点.它位于季节至年际预测和长期气候变化预测之间,结合了初值问题和外部强迫问题的两个方面.年代际预测的核心技术在于用于模式初始化的同化方法的准确性和效率,其目标是为模式提供准确的初始条件,其中包含观测到的气候系统内部变率.年代际预测的初始化通常涉及在耦合框架内同化海洋观测,其中观测到的信号通过耦合过程传递到其他分量,如大气和海冰.然而,最近的研究越来越关注在海洋-大气耦合模式中探索耦合数据同化(CDA),有人认为CDA有潜力显著提高年代际预测技巧.本文综合评述了该领域的三个方面的研究现状:初始化方法,年代际气候预测的可预测性和预测技巧,以及年代际预测的未来发展和挑战.     

THE DECADAL CLIMATE VARIABILITY AND THE ANOMALOUS ENSO DEVELOPMENTS IN 1990S*

By using the wavelet transform method,the ENSO (2-7 a) signal and the decadal variability (8-20 a) are filtered out from the long-term SST data sets in order to investigate characteristics of the decadal variability and its impact on the ENSO.It is found that there are two different kinds of decadal SSTA modes-horseshoe and horse saddle patterns in the tropical Pacific.The horseshoe pattern represents that the decadal SSTA variability in the central Pacific is in phase with that in the eastern Pacific.The horse saddle pattern is named that they are out of phase.The former constituted the decadal variability before 1990s and the latter mainly prevailed during 1990s.As the response of atmosphere to the ocean,two decadal wind patterns appear in association with the SST decadal modes.One is characterized by anomalous development of the zonal wind,the other by anomalous development of the meridional wind.These two kinds of modes can also be regarded as different phases of the decadal oscillation.Further studies have shown that the influences of the two kinds of modes on the ENSO are different.The horse saddle mode has a stronger impact on the ENSO than the horseshoe mode.A possible mechanism for the influence of the decadal variability on the ENSO signal is presented.The central part of the thermocline along the equatorial Pacific moves up or down simultaneously with its eastern part while the decadal variability bears the horseshoe pattern.But the two segments of the thermocline in the central and eastern Pacific act oppositely while the decadal variability shows the horse saddle pattern.In this case it has an-influence on the individual ENSO'events by the superposition of the decadal variability.     

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.     

Intensification of decadal and multi-decadal sea level variability in the western tropical Pacific during recent decades

Previous studies have linked the rapid sea level rise (SLR) in the western tropical Pacific (WTP) since the early 1990s to the Pacific decadal climate modes, notably the Pacific Decadal Oscillation in the north Pacific or Interdecadal Pacific Oscillation (IPO) considering its basin wide signature. Here, the authors investigate the changing patterns of decadal (10–20 years) and multidecadal (>20 years) sea level variability (global mean SLR removed) in the Pacific associated with the IPO, by analyzing satellite and in situ observations, together with reconstructed and reanalysis products, and performing ocean and atmosphere model experiments. Robust intensification is detected for both decadal and multidecadal sea level variability in the WTP since the early 1990s. The IPO intensity, however, did not increase and thus cannot explain the faster SLR. The observed, accelerated WTP SLR results from the combined effects of Indian Ocean and WTP warming and central-eastern tropical Pacific cooling associated with the IPO cold transition. The warm Indian Ocean acts in concert with the warm WTP and cold central-eastern tropical Pacific to drive intensified easterlies and negative Ekman pumping velocity in western-central tropical Pacific, thereby enhancing the western tropical Pacific SLR. On decadal timescales, the intensified sea level variability since the late 1980s or early 1990s results from the “out of phase” relationship of sea surface temperature anomalies between the Indian and central-eastern tropical Pacific since 1985, which produces “in phase” effects on the WTP sea level variability.     

North African climate variability. Part 1: Tropical thermocline coupling

Summary Tropical ocean thermocline variability is studied using gridded data assimilated by an ocean model in the period 1950–2000. The dominant patterns and variability are identified using EOF analysis applied to E–W depth slices of sea temperatures averaged over the tropics. After removing the annual cycle, an east–west ‘see-saw’ with an interannual to decadal rhythm is the leading mode in each of the tropical basins. In the case of the leading mode in the Pacific, the thermocline oscillation forms a dipole structure, but in the (east) Atlantic and (southwest) Indian Ocean there is a single center of action. The interaction of the ocean thermocline and atmospheric Walker circulations is studied through cross-modulus analysis of wavelet-filtered EOF time scores. Our study demonstrates how tropical ocean thermocline variability contributes to zonal circulation anomalies in the atmosphere. The equatorial Pacific thermocline oscillation explains 62 and 53% of the variability of the Pacific and Atlantic zonal overturning circulations, the latter driving convective polarity between North Africa and South America. The Pacific sea-saw leads the Atlantic zonal circulation by a few months.     

Pacific interdecadal variability driven by tropical–extratropical interactions

Interactions between the tropical and subtropical northern Pacific at decadal time scales are examined using uncoupled oceanic and atmospheric simulations. An atmospheric model is forced with observed Pacific sea surface temperatures (SST) decadal anomalies, computed as the difference between the 2000–2009 and the 1990–1999 period. The resulting pattern has negative SST anomalies at the equator, with a global pattern reminiscent of the Pacific decadal oscillation. The tropical SST anomalies are responsible for driving a weakening of the Hadley cell and atmospheric meridional heat transport. The atmosphere is then shown to produce a significant response in the subtropics, with wind-stress-curl anomalies having the opposite sign from the climatological mean, consistent with a weakening of the oceanic subtropical gyre (STG). A global ocean model is then forced with the decadal anomalies from the atmospheric model. In the North Pacific, the shallow subtropical cell (STC) spins down and the meridional heat transport is reduced, resulting in positive tropical SST anomalies. The final tropical response is reached after the first 10 years of the experiment, consistent with the Rossby-wave adjustment time for both the STG and the STC. The STC provides the connection between subtropical wind stress anomalies and tropical SSTs. In fact, targeted simulations show the importance of off-equatorial wind stress anomalies in driving the oceanic response, whereas anomalous tropical winds have no role in the SST signal reversal. We further explore the connection between STG, STC and tropical SST with the help of an idealized model. We argue that, in our models, tropical SST decadal variability stems from the forcing of the Pacific subtropical gyre through the atmospheric response to ENSO. The resulting Ekman pumping anomaly alters the STC and oceanic heat transport, providing a negative feedback on the SST. We thus suggest that extratropical atmospheric responses to tropical forcing have feedbacks onto the ocean dynamics that lead to a time-delayed response of the tropical oceans, giving rise to a possible mechanism for multidecadal ocean-atmosphere coupled variability.     

The response of the North Pacific Decadal Variability to strong tropical volcanic eruptions

In this study, the effects of volcanic forcing on North Pacific climate variability, on interannual to decadal time scales, are examined using climate model simulations covering the last 600?years. The model used is the Bergen Climate Model, a fully coupled atmosphere–ocean general circulation model. It is found that natural external forcings, such as tropical strong volcanic eruptions (SVEs) and variations in total solar irradiance, play an important role in regulating North Pacific Decadal Variability (NPDV). In response to tropical SVEs the lower stratospheric pole–to–equator temperature gradient is enhanced. The North polar vortex is strengthened, which forces a significant positive Arctic Oscillation. At the same time, dipole zonal wind anomalies associated with strong polar vortex propagate downward from the lower stratosphere. Through positive feedbacks in the troposphere, the surface westerly winds across the central North Pacific are significantly weakened, and positive sea level pressure anomalies are formed in the North Pacific. This anomalous surface circulation results in changes in the net heat fluxes and the oceanic advection across the North Pacific. As a result of this, warm water converges in the subtropical western North Pacific, where the surface waters in addition are heated by significantly reduced latent and sensible heat fluxes from the ocean. In the eastern and high–latitude North Pacific the ocean loses more heat, and large–scale decreases in sea surface temperatures are found. The overall response of this chain of events is that the North Pacific enters a negative phase of the Pacific decadal oscillation (PDO), and this negative phase of the PDO is maintained for several years. It is thus concluded that the volcanic forcing plays a key role in the phasing of the PDO. The model results furthermore highlight the important role of troposphere–stratosphere coupling, tropical–extratropical teleconnections and extratropical ocean–atmosphere interactions for describing NPDV.     

Can oceanic reanalyses be used to assess recent anthropogenic changes and low-frequency internal variability of upper ocean temperature?

A multivariate analysis of the upper ocean thermal structure is used to examine the recent long-term changes and decadal variability in the upper ocean heat content as represented by model-based ocean reanalyses and a model-independent objective analysis. The three variables used are the mean temperature above the 14°C isotherm, its depth and a fixed depth mean temperature (250?m mean temperature). The mean temperature above the 14°C isotherm is a convenient, albeit simple, way to isolate thermodynamical changes by filtering out dynamical changes related to thermocline vertical displacements. The global upper ocean observations and reanalyses exhibit very similar warming trends (0.045°C per decade) over the period 1965–2005, superimposed with marked decadal variability in the 1970s and 1980s. The spatial patterns of the regression between indices (representative of anthropogenic changes and known modes of internal decadal variability), and the three variables associated with the ocean heat content are used as fingerprint to separate out the different contributions. The choice of variables provides information about the local heat absorption, vertical distribution and horizontal redistribution of heat, this latter being suggestive of changes in ocean circulation. The discrepancy between the objective analysis and the reanalyses, as well as the spread among the different reanalyses, are used as a simple estimate of ocean state uncertainties. Two robust findings result from this analysis: (1) the signature of anthropogenic changes is qualitatively different from those of the internal decadal variability associated to the Pacific Interdecadal Oscillation and the Atlantic Meridional Oscillation, and (2) the anthropogenic changes in ocean heat content do not only consist of local heat absorption, but are likely related with changes in the ocean circulation, with a clear shallowing of the tropical thermocline in the Pacific and Indian oceans.     

Mechanisms of tropical Pacific interannual-to-decadal variability in the ARPEGE/ORCA global coupled model

Spatial and temporal structures of interannual-to-decadal variability in the tropical Pacific Ocean are investigated using results from a global atmosphere–ocean coupled general circulation model. The model produces quite realistic mean state characteristics, despite a sea surface temperature cold bias and a thermocline that is shallower than observations in the western Pacific. The periodicity and spatial patterns of the modelled El Niño Southern Oscillations (ENSO) compare well with those observed over the last 100 years, although the quasi-biennial timescale is dominant. Lag-regression analysis between the mean zonal wind stress and the 20°C isotherm depth suggests that the recently proposed recharge-oscillator paradigm is operating in the model. Decadal thermocline variability is characterized by enhanced variance over the western tropical South Pacific (~7°S). The associated subsurface temperature variability is primarily due to adiabatic displacements of the thermocline as a whole, arising from Ekman pumping anomalies located in the central Pacific, south of the equator. Related wind anomalies appear to be caused by SST anomalies in the eastern equatorial Pacific. This quasi-decadal variability has a timescale between 8 years and 20 years. The relationship between this decadal tropical mode and the low-frequency modulation of ENSO variance is also discussed. Results question the commonly accepted hypothesis that the low-frequency modulation of ENSO is due to decadal changes of the mean state characteristics.