Acoustic thermometry data compared with two ocean models: the importance of Rossby waves and ENSO in modifying the ocean interior

An interpretation is made of interannual changes in acoustic travel time between Oahu and seven receivers at distances of 3000–4000 km. Measurements were made in late 1983, and over two 5-month intervals between 1987 and 1989. Previous publications demonstrated that these changes stem from variations in temperature. Two hydrodynamic ocean models are used to identify plausible oceanic features that could cause these variations. They are from the Naval Research Laboratory and the Florida State University at (1/8)° and (1/6)° resolution, respectively, and are forced with different interannual wind sets for more than a decade. Modelled El Niño's and La Niña's generate poleward travelling Kelvin waves on the eastern boundary of the Pacific. These excite Rossby waves that propagate westward at mid-latitudes. Rossby waves are the dominant model features which affect the modelled acoustic travel times, and hence section-averaged temperatures in the eastern North Pacific. These waves yield travel times whose standard deviations and rates of changes are similar to the measurements. In the observations, some sections separated by less than 500 km exhibit trends in heat content with opposite signs. Similar variability can be explained with modelled Rossby waves. Model wavelengths less than 500 km, eddies, and seasonal cycles induced by seasonal winds yield travel times that are two orders of magnitude too small to account for the data.     

Interannual and interdecadal variability of ocean temperature along the equatorial Pacific in conjunction with ENSO

In this paper, the leading modes of ocean temperature anomalies (OTA) along the equatorial Pacific Ocean are analyzed and their connection with El Niño-Southern Oscillation (ENSO) and interdecadal variation is investigated. The first two leading modes of OTA are connected with the different phases of the canonical ENSO and display asymmetric features of ENSO evolution. The third leading mode depicts a tripole pattern with opposite variation of OTA above the thermocline in the central Pacific to that along the thermocline in the eastern and western Pacific. This mode is found to be associated with so-called ENSO-Modoki. Insignificant correlations of this mode with the first two leading modes suggest that ENSO-Modoki may be a mode that is independent to the canonical ENSO and also has longer time scales compared with the canonical ENSO. The fourth mode reflects a warming (cooling) tendency above (below) the thermocline since 2000. Both the first and second modes have a large contribution to the interdecadal change in thermocline during 1979–2012. Also, the analysis also documents that both ENSO and OTA shifted into higher frequency since 2000 compared with that during 1979–1999. Interestingly, the ENSO-Modoki related OTA mode does not have any trend or significant interdecadal shift during 1979–2012. In addition, it is shown that first four EOF modes seem robust before and after 1999/2000, suggesting that the interdecadal shift of the climate system in the tropical Pacific is mainly a frequency shift and the changes in spatial pattern are relatively small, although the mean states over two periods experienced some significant changes.     

On the effect of ENSO precipitation anomalies in a global ocean GCM

An ocean general circulation model is used to study the influence of positive precipitation anomalies associated with El Nino and La Nina events. In this idealized model, the precipitation over the appropriate part of the equatorial Indo-Pacific region is doubled for one year. At the surface, salinity anomalies of up to –0.9 parts per thousand result from this anomalous precipitation. Perturbation surface currents ranging from 10–100% of the climatological values are induced in the tropical Indian and Pacific Oceans. A return flow is found beneath the thermocline with upwelling (downwelling) in (outside) the region of enhanced precipitation. The net effect of the precipitation anomalies is to generate a zonal overturning cell which transports fresher surface water away from the forcing region and replaces it with cooler, more saline water from below.     

Seasonal predictability of ENSO teleconnections: the role of the remote ocean response

In this modelling study, the teleconnections of ENSO are studied using an atmospheric general circulation model (AGCM), HadAM3. The influence of sea surface temperature anomalies (SSTAs) remote from the tropical Pacific but teleconnected with ENSO is investigated. Composite cycles of El Niño and La Niña SSTs are created and imposed on HadAM3. These SSTs are imposed in different areas, with climatological SSTs elsewhere, in order to find the influences of SSTs in different regions. It is found that most of the reproducible response to ENSO is forced directly from the tropical Pacific before the peak of the event. However, during the peak and decay of ENSO, remote SSTs become increasingly influential throughout the tropics (at the 98% significance level). This could lead to extended ENSO-related predictability due to the memory of the remote oceans. The Indian Ocean and Maritime Continent SSTs are found to be particularly influential. Indian Ocean SSTAs dampen the teleconnections from the tropical Pacific and force the atmosphere above the tropical Atlantic. More generally, when a tropical SSTA is imposed, atmospheric anomalies are forced locally with anomalies of the opposite sign to the west. Some of the reproducible response to ENSO in the tropical Atlantic is forced, not directly from the tropical Pacific but from the Indian ocean, which in turn is forced by the tropical Pacific. Subsequently, delayed SSTAs in the tropical Atlantic damp the local response and force the atmosphere above the tropical Pacific in the opposite manner.     

Simple analytic solutions of the linear delayed-action oscillator equation relevant to ENSO theory

The El Ni?o-Southern Oscillation (ENSO) is a major driver of climate variability in many parts of the world. Impressive progress has been made in the last 25?years in consolidating the scientific and mathematical basis to our understanding of ENSO. This includes the development and analysis of a hierarchy of models??including simple analogue models??to simulate and understand ENSO physics. The delayed-action oscillator (DAO) equation has been a particularly important analogue model in the historical development of our understanding of ENSO physics, and numerical solutions of this equation have been explored in detail in previous studies. Given this importance, it is surprising that no exact analytic solutions to the equation have been provided previously in the ENSO literature. This situation is rectified here by deriving and presenting analytic solutions to the linear DAO equation $ \frac{{dT}}{{dt}} = aT - bT\left( {t - \tau } \right) $ for parameter values relevant to ENSO. Here, T is an index for ENSO variability at time t; a, b, and ?? (the delay time >0) are real parameters. A comparison between observations and (linear) theory suggests that ENSO behaves as a damped oscillator with a period of 3.8?years and a damping time-scale of 0.9?years. The parameter $ \gamma = b\tau {e^{ - a\tau }} $ is found to be crucial in understanding the behavior of the solution and the lowest frequency mode. For example, if ???>?1/e the solution is oscillatory. Exact analytic solutions to the DAO equation which are phase-locked to the annual cycle??as is the case for ENSO??are also obtained. The overall (annual average) stability of a phase-locked system and its intrinsic periodicities differ from the corresponding properties of the system with parameters set to their annual averages (i.e., the corresponding solution which is not phase-locked). Phase-locking therefore alters the growth rate and period of the lowest frequency mode.     

中国热带海-气相互作用与ENSO动力学及预测研究进展

国际上针对海洋-大气系统的观测、理论和模拟方面已经开展了广泛而深入的研究,为短期气候预测水平的不断提升奠定了坚实基础,这其中中国学者做出了许多重要贡献。文中简要回顾了中国学者70年来在热带海-气相互作用与ENSO动力学及预测方面的研究进展。其中,热带海-气相互作用部分主要涉及4个方面的内容:热带太平洋气候特征与ENSO现象、热带印度洋海温主要模态及其与太平洋相互作用、热带大西洋海温主要模态及与海盆的相互作用、中高纬度海-气系统对ENSO的影响;ENSO动力学包括7个方面的内容:基本理论的相关研究、ENSO相关的诊断与模拟研究、两类ENSO相关研究、ENSO触发机制相关研究、ENSO与其他现象的相互作用、外部强迫与大气遥相关、气候变化与ENSO响应;ENSO预测主要包括2个方面的内容:动力-统计ENSO预测方法、ENSO预测系统与应用。最后,还讨论了上述相关方面亟待解决的问题。     

On the validity of layered models of ocean dynamics and thermodynamics with reduced vertical resolution

With the purpose of studying the upper part of the ocean, the shallow water equations (in a `reduced gravity' setting) have been extended in the last decades by allowing for horizontal and temporal variations of the buoyancy field ϑ, while keeping it as well as the velocity field u as depth-independent. In spite of the widespread use of this `slab' model, there has been neither a discussion on the range of validity of the system nor an explanation of points such as the existence of peculiar zero-frequency normal modes, the nature of the instability of a uniform u flow, and the lack of explicit vertical shear associated with horizontal density gradients. These questions are addressed here through the development of a subinertial model with more vertical resolution, i.e., one where the buoyancy ϑ varies linearly with depth. This model describes satisfactorily the problem of baroclinic instability with a free boundary, even for short perturbations and large interface slopes. An enhancement of the instability is found when the planetary β effect is compensated with the topographic one, due to the slope of the free boundary, allowing for a `resonance' of the equivalent barotropic and first baroclinic modes. Other low-frequency models, for which buoyancy stratification does not play a dynamical role, are invalid for short perturbations and have spurious terms in their energy-like integral of motion.     

ENSO循环的两个不同位相期印度洋海表温度异常的特征分析

通过对ENSO循环的两个不同位相中印度洋地区海表温度变化特征的分析,指出印度洋地区的海温变化与赤道东太平洋地区的海温变化有较好的相关关系,是ENSO循环的重要组成部分。对应于赤道东太平洋暖位相期,印度洋地区的海温分布为东冷西暖;与此相反,在赤道东太平洋冷位相期,印度洋地区的海温分布为东暖西冷。进-步的分析还发现,印度洋东、西部地区海温变化纬向差异最明显的区域位于印度洋赤道以南0～25°S附近,且这种差异具有明显的年季变化特征,在整个夏季风期间差异较大,而冬季风期间较小,其中冷位相期间的纬向差异比暖位相期间的纬向差异大。代表印度洋纬向差异的IDM(偶极指数)变化与赤道东太平洋地区的海温变化有很好的正相关关系。     

Effectiveness of the Bjerknes stability index in representing ocean dynamics

The El Niño-Southern Oscillation (ENSO) is a naturally occurring coupled phenomenon originating in the tropical Pacific Ocean that relies on ocean–atmosphere feedbacks. The Bjerknes stability index (BJ index), derived from the mixed-layer heat budget, aims to quantify the ENSO feedback process in order to explore the linear stability properties of ENSO. More recently, the BJ index has been used for model intercomparisons, particularly for the CMIP3 and CMIP5 models. This study investigates the effectiveness of the BJ index in representing the key ENSO ocean feedbacks—namely the thermocline, zonal advective, and Ekman feedbacks—by evaluating the amplitudes and phases of the BJ index terms against the corresponding heat budget terms from which they were derived. The output from Australian Community Climate and Earth System Simulator Ocean Model (a global ocean/sea ice flux-forced model) is used to calculate the heat budget in the equatorial Pacific. Through the model evaluation process, the robustness of the BJ index terms are tested. We find that the BJ index overestimates the relative importance of the thermocline feedback to the zonal advective feedback when compared with the corresponding terms from the heat budget equation. The assumption of linearity between variables in the BJ index formulation is the primary reason for these differences. Our results imply that a model intercomparison relying on the BJ index to explain ENSO behavior is not necessarily an accurate quantification of dynamical differences between models that are inherently nonlinear. For these reasons, the BJ index may not fully explain underpinning changes in ENSO under global warming scenarios.     

Analysis of the Slab Ocean El Nino atmospheric feedbacks in observed and simulated ENSO dynamics

In a recent study it was illustrated that the El Nino Southern Oscillation (ENSO) mode can exist in the absence of any ocean dynamics. This oscillating mode exists just due to the interaction between atmospheric heat fluxes and ocean heat capacity. The primary purpose of this study is to further explore these atmospheric Slab Ocean ENSO dynamics and therefore the role of positive atmospheric feedbacks in model simulations and observations. The positive solar radiation feedback to sea surface temperature (SST), due to reduced cloud cover for anomalous warm SSTs, is the main positive feedback in the Slab Ocean El Nino dynamics. The strength of this positive cloud feedback is strongly related to the strength of the equatorial cold tongue. The combination of positive latent and sensible heat fluxes to the west and negative ones to the east of positive anomalies leads to the westward propagation of the SST anomalies, which allows for oscillating behavior with a preferred period of 6–7 years. Several indications are found that parts of these dynamics are indeed observed and simulated in other atmospheric or coupled general circulation models (AGCMs or CGCMs). The CMIP3 AGCM-slab ensemble of 13 different AGCM simulations shows unstable ocean–atmosphere interactions along the equatorial Pacific related to stronger cold tongues. In observations and in the CMIP3 and CMIP5 CGCM model ensemble the strength and sign of the cloud feedback is a function of the strength of the cold tongue. In summary, this indicates that the Slab Ocean El Nino dynamics are indeed a characteristic of the equatorial Pacific climate that is only dominant or significantly contributing to the ENSO dynamics if the SST cold tongue is sufficiently strong. In the observations this is only the case during strong La Nina conditions. The presence of the Slab Ocean ENSO atmospheric feedbacks in observations and CGCM model simulations implies that the family of physical ENSO modes does have another member, which is entirely driven by atmospheric processes and does not need to have the same spatial pattern nor the same time scales as the main ENSO dynamics.     

Error evolution in the dynamics of an ocean general circulation model

The problem of error propagation is considered for spatially uncorrelated errors of the barotropic stream function in an oceanic general circulation model (OGCM). Such errors typically occur when altimetric data from satellites are assimilated into ocean models. It is shown that the error decays at first due to the dissipation of the smallest scales in the error field. The error then grows exponentially before it saturates at the value corresponding to the difference between independent realizations. A simple analytic formula for the error behavior is derived; it matches the numerical results documented for the present primitive-equation ocean model, and other models in the literature.     

四个耦合模式ENSO后报试验的“春季预报障碍”

用CliPAS计划中3个气候模式和中国科学院大气物理研究所耦合模式FGOALS-g短期气候异常回报试验结果,将动力和统计方法相结合,考察了1982—2003年厄尔尼诺/拉尼娜事件发展期和衰减期海表温度春季可预报性障碍现象。结果表明,所考察的耦合模式对ENSO事件预报的误差发展存在明显的季节依赖性,最大误差增长通常发生在春季,发生显著的可预报性障碍现象。进一步分析发现厄尔尼诺事件和拉尼娜事件在发展期的季节预报障碍现象比衰减期明显,以厄尔尼诺事件发展期春季可预报性障碍现象最为显著,拉尼娜事件衰减期季节预报障碍现象不显著。研究还发现,预报误差的增长在ENSO事件冷暖位相具有显著的非对称性,发展期暖位相预报误差强于冷位相,而衰减期冷位相的预报误差比暖位相大。通过回归分析,诊断了海-气相互作用的强度,发现耦合系统在春季最不稳定,使预报误差最易在春季发展,从而导致可预报性障碍。     

PDO和ENSO与大连6-9月降水关系分析

利用1905—2006年的太平洋年代际振荡(简称PDO)、ENSO和大连6—9月降水资料,分析三者之间的关系。结果表明:在PDO暖位相期,大连6—9月降水总体比常年偏少;PDO冷位相期,大连6—9月降水总体上比常年偏多;PDO与大连6—9月降水存在准周期对应关系,从PDO冷位相到暖位相,对应的大连6—9月降水距平8 a滑动平均曲线总体呈下降趋势。ENSO对大连6—9月降水的影响明显受PDO的调制,在PDO冷位相期,ENSO年大连6—9月降水总体上比常年偏多,而在PDO暖位相期,ENSO年大连6—9月降水总体上比常年偏少;不同强度和不同冷暖性质的ENSO,在不同PDO位相期内对大连6—9月降水的影响也各不相同。     

A further study on an extended nonlinear ocean—atmosphere coupled hydrodynamic characteristic system and the abrupt feature of ENSO events

In this paper, the decade data of meteorological satellite and surface meteorological observation of China have been analysed. The relationship between cloud and radiation has been studied. A set of empirical formulae of the ralationships between the albedo and cloud amount, the outgoing longwave radiation and cloud amount in Chinese different districts and different seasons has been deduced. They express simply the response of both planet reflectivity and earth-atmosphere outgoing longwave radiation to the change of cloud amount. So that the sensitivity of net radiation of the earth-atmosphere system to the change of cloud amount and the ratio of cloud reflective effect to greenhouse effect can be estimated. In this paper, the radiative process of the earth-atmosphere system, cloud and radiative balance and its effect on climate have been synthetically studied. ¹The project is supported by National Natural Sciences Fundation of China (NNSFC).     

Societies and nature in the Sahel: ecological diversity and social dynamics

This paper, based on the results of a comprehensive interdisciplinary research programme focused on five countries of the Western Sahel, suggests a framework for analysing the complex and constantly changing dynamics of the relations that Sahelian societies maintain with their environment. Firstly, tools for understanding local variability are required. Demographic variables and the diversity of the modes of land use are combined to show that the type and intensity of exploitation of natural resources varies dramatically from one locality to another. There are significant differences between Sahelian social systems and cultures, and these influence their relations with the ‘nature’ they exploit and transform. Secondly, understanding social, economical and technical changes in the Sahel requires that we recognise that powerful and conflicting processes of transformation are taking place (in contrast to the image of “tradition” widely associated with Sahelian societies). These changes are found to be the result of an interaction between the state, rural producers, urban speculators, international development agencies, and other actors. Practical lessons emerge from the analysis. Access and negotiation over natural resources must be facilitated between many actors who compete for them, and local social and environmental problems must be seen in the context of broader patterns of influence and change. The State will retain its importance in the region. “Participation” in development must not only involve local “populations”, but also the “developers” of all types, in contributing to and understanding the human dimensions of environmental change in the Sahel.     

The role of ocean thermodynamics and dynamics in Asian summer monsoon changes during the mid-Holocene

Studies of climate change 6,000 years before present using atmospheric general circulation models (AGCMs) suggest the enhancement and northward shift of the summer Asian and African monsoons in the Northern Hemisphere. Although enhancement of the African monsoonal precipitation by ocean coupling is a common and robust feature, contradictions exist between analyses of the role of the ocean in the strength of the Asian monsoon. We investigated the role of the ocean in the Asian monsoon and sought to clarify which oceanic mechanisms played an important role using three ocean coupling schemes: MIROC, an atmosphere–ocean coupled general circulation model [C]; an AGCM extracted from MIROC coupled with a mixed-layer ocean model [M]; and the same AGCM, but with prescribed sea surface temperatures [A]. The effect of “ocean dynamics” is quantified through differences between experiments [C] and [M]. The effect of “ocean thermodynamics” is quantified through differences between experiments [M] and [A]. The precipitation change for the African and Asian monsoon area suggested that the ocean thermodynamics played an important role. In particular, the enhancement of the Asian monsoonal precipitation was most vigorous in the AGCM simulations, but mitigated in early summer in ocean coupled cases, which were not significantly different from each other. The ocean feedbacks were not significant for the precipitation change in late summer. On the other hand, in Africa, ocean thermodynamics contributed to the further enhancement of the precipitation from spring to autumn, and the ocean dynamics had a modest impact in enhancing precipitation in late summer.