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The study of psychological barriers and the attribution of social meaning is a complex and subjective process, but essential for the development of strategies to adapt to climate change and land-use management. The objective of this paper is to point out and discuss aspects that comprise climate vulnerability, especially regarding the climate perceptions understood as an element for the development of community adaptation strategies. The first part presents two study cases and the second part presents propositions for developing climate adaptation strategies based on the results of these cases. The paper focuses on two aspects: the perception of climate dynamics and the relationship between vulnerability and local climate dynamics. In order to establish the relationship between climate dynamics and the level of vulnerability perceived, a qualitative approach was adopted. Field data collection consisted of observing local civil association meetings and conducting semi-structured interviews with local actors. Although both communities studied are strongly affected by climate events, their perception of climate influences is essentially different. The difference in perception arises mainly from the different context in which climate events acquire meaning, rather than any ability of assigning meaning to possible climate change. To understand the dynamics of establishing an adaptation strategy, the motivational problems would be a stimulus, a causative agent of reflections on the conditions of vulnerability to climate change and reflection about their limit-situations. The final propositions present “Adaptation learning process pre-steps framework” that emerges from the description of contrasts in perceptions/cognitions observed between the two study cases. It is a model for the visualization of different and better living conditions, the unknown-viable, which enable social actors to become aware of capacity of acting and governance.

Climate change impacts are already happening through the world, and it is now clear that there is the need for an adaptive response from global institutions down to the local level. Reducing vulnerability to cope with climate variability might be more challenging in tropical countries than in North America or Europe. The ten papers of this special issue were presented during the Adaptclim conference that was held by the Sinergia Project, the CLARIS LPB project, and the GeoData Institute in Asuncion, Paraguay, in 2010. All papers, except one regarding the Brahmaputra Basin in South Asia, present studies from South America. These studies are first contextualized geographically and then are related one to another by a simplified vulnerability concept linking climate stress to sensitivity and adaptive capacity of natural and human systems. One half of the papers focus on actual or future climate change and the present-day causes of the vulnerability of natural and agrosystems. Droughts are and will be the main source of stress for agriculture in South America. Increasing fragmentation of forest of the center of this continent is aggravating their vulnerability to dry spells. Another half of the studies of this special issue deal with the adaptive capacity human populations to system perturbations produced or enhanced by climate change. The studies point out inclusion of traditional knowledge and involvement of local actors in their own vulnerability assessment to increase adaptive capacity. These elements of climate justice, giving voice to those less responsible for carbon emissions but bearing their most severe consequences, allow the particular needs of a community to be considered and can direct adaptation policy toward preserving or rebuilding their specific capabilities under threat from climate change. The special issue also made clear that a basin analysis of the climate change problem could provide information, results, and methods more readily of use for the local population and decision makers.

The CLARIS-LPB project, funded from 2008 to 2012 by the 7th Framework Programme of the European Commission, aimed at projecting regional climate change impacts in the La Plata Basin (LPB) in southeastern South America focusing on the periods 2010-2040 and 2070-2100. Such a coordinated projection of regional climate change scenarios was key in successfully designing adaptation strategies for land-use, agriculture, rural development, hydro - power production, river transportation, water resources and wetland ecosystems. Topics covered in this special include characteristics of climate variability over the region, together with trends, extreme events and uncertainties for modelled regional climate projections. Fil: Boulanger, Jean Philippe. Locean; Francia Fil: Carril, Andrea Fabiana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Sanchez, Enrique. Universidad de Castilla-La Mancha; España

A weather pattern clustering method is applied and calibrated to Argentinean daily weather stations in order to predict daily precipitation data. The clustering technique is based on k-means and is applied to a set of 17 atmospheric variables from the ERA-40 reanalysis covering the period 1979-1999. The set of atmospheric variables represent the different components of the atmosphere (dynamical, thermal and moisture). Different sensitivity tests are applied to optimize (1) the number of observations (weather patterns) per cluster, (2) the spatial domain size of the weather pattern around the station and (3) the number of members of the ensembles. All the sensitivity tests are compared using the ROC (Relative Operating Characteristic) Skill Score (RSS) derived from the ROC curve used to assess the performance of a predictive system. First, we found the number of observations per cluster to be optimum for values larger than 39. Second, the spatial domain size (similar to 4 degrees x 4 degrees) was found to be closer to a local scale than to a synoptic scale, certainly due to a dominant role of the moisture components in the optimization of the transfer function. Indeed, when reducing the set of variables to the subset of dynamical variables, the predictive skill of the method is significantly reduced, but at the same time the domain size must be increased. A potential improvement of the method may therefore be to consider different domains for dynamical and non-dynamical variables. Third, the number of members per ensembles of simulations was estimated to be always two to three times larger than the mean number of observations per cluster (meaning that at least all the observed weather patterns are selected by one member). The skill of the statistical method to predict daily precipitation is found to be relatively homogeneous all over the country for different thresholds of precipitation.

The present investigation aims at identifying the needs of the Argentinean agro-business sector for climate research and information. Our approach is focused on any type of climate information from past observations to seasonal prediction and climate change projections. We tried, in particular, to evaluate the possible existence of specific research themes that would be of major interest for these agriculture end-users. We interviewed representatives of seventeen companies from three major agrobusiness sectors: (1) regional/national entities such as agricultural associations, cooperatives and commercial trade boards; (2) insurance companies in the Argentinean agro-insurance market; and (3) large national and international companies representing cereal producers, agro-chemistry and agro-seeds. While all the interviewees recognized the strong influence of climate on their activities, they all pointed out that, at the time of making decision, they considered the political and economic risks rather than the climate one. In many aspects, climate is often considered as a fatality against which it is difficult to be protected. An interesting result is the confidence of the private sectors in the climate information provided by public sources (INTA, SMN, Universities) although they contract private consultants for frequent reports and tendencies. Finally, we explain how such sectors could make a better use of climate information (seasonal prediction, climate change scenarios) that could be integrated in their business projections. In particular, the development of new financial contracts (climate derivatives) open in countries with a relatively poor insurance history new ways to protect the different agricultural sectors, from the producer to large companies.

The present paper describes the quality-control component of an automatic procedure (APACH: A Procedure for Automated Quality Control and Homogenization of Weather Station Data) developed to control quality and homogenize the historical daily temperature and precipitation data from meteorological stations. The quality-control method is based on a set of decision-tree algorithms analyzing separately precipitation and minimum and maximum temperature. All our tests are non-parametric and therefore are potentially useful in regions or countries presenting different climates as those observed in Argentina. The method is applied to the 1959-2005 historical daily database of the Argentine National Weather Service. Our results are coherent with the history of the Weather Service and more specifically with the history of implementation of systematized quality control processes. In temperature, our method detects a larger number of suspect values before 1967 (when there was no quality control) and after 1997 (when only real-time quality control had been applied). In precipitation, the detection of error in extreme precipitations is complex, but our method clearly detected a strong decrease in the number of potential outliers after 1976 when the National Weather Service was militarized, and the network was strongly reduced, focusing more on airport weather stations. Also in precipitation, we analyze in detail the long dry sequences and are able to identify potential long erroneous sequences. This is important for the use of the data for hydrological or agricultural impact studies. Finally, all the data are flagged with codes representing the path followed by the record in our decision-tree algorithms. While each code is associated to one of the categories (“Useful”, “Need-Check”, “Doubtful” or “Suspect”), the final user is free to redefine such category-assignment.

National audience

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation.

The present manuscript analyzes monthly equatorial Pacific indices by using a specific neural algorithm, the so-called “Self-Organizing Maps” (SOMs). The main result is a change found in the nature of the transitions between cold to warm and warm to cold extreme events from 1950 to present, around the late 1970s. SOM is an unsupervised clustering technique which allows one to reduce high-dimensional data space (in this case, three indices over 636 months) in terms of a smaller set of three-dimensional reference vectors (100) characterizing pertinent situations. These reference vectors, which are displayed on a two-dimension map, are closely related by a topological relationship leading us to discriminate La Nina conditions from the opposite El Nino conditions. In a second step, a Hierarchical Agglomerative Clustering (HAC) method is used to further group the reference vectors into a small number of clusters (12) whose spatial and temporal characteristics can be analyzed and interpreted in terms of physical parameters. Schematically, these 12 clusters can be divided into two “warm” clusters, six “neutral” or “transition” clusters and four “cold” clusters. In each particular group (warm, neutral, cold), the clusters mainly differ from each other by the amplitude of the anomalies, their spatial patterns and their temporal variability. Some clusters are found to be strongly linked to the boreal spring period, while others have barely any records during that season. Other clusters are associated with records mainly observed either prior to or after 1980. This suggests that the method is able to identify changes in the variability of the tropical Pacific basin observed on decadal time scales (1976 climate shift in our case). Each monthly record can be summarized by the cluster to which it belongs. The temporal evolution of this value during extreme ENSO events shows similar patterns (persistence in specific clusters and transition between groups of clusters) associated with comparable El Nino or La Nina events. The methodology described in the present study (SOM plus HAC) is suggested to be useful both for seasonal ENSO predictability and for the detection of decadal changes in ENSO behavior.

The Parana-Plata basin is the second largest hydrological basin in South America and is of great importance for the countries of the region (Argentina, Bolivia, Brazil, Paraguay and Uruguay). The present study focuses on the long-term trends in basin-scale precipitation with special emphasis on the role of distribution changes in extreme large-scale precipitation events and on the characteristics and evolution of ENSO teleconnections over the last 50 years. First, we defined a Parana-Plata basin total precipitation index (PTPI) as the precipitations spatially averaged over the hydrological basin. On interannual time scales, such an index is mainly representative of anomalous monsoon precipitations in the northern part of the basin and large convective precipitation anomalies in the center of the basin (Paraguay-southern Brazil-Uruguay-northern Argentina) typical of the canonical ENSO teleconnection pattern. Our major findings clearly highlight a positive trend in yearly averaged PTPI mainly from the late 1960s to the early 1980s with a strong dependence from month-to-month. The largest precipitation increase is observed from November to May in southern Brazil and Argentina. A close examination of PTPI distributions during the two halves of the period 1950–2001 shows that the changes in the mean state from 1950–1975 to 1976–2001 result from significant changes in each calendar month mean state and in the tails of the PTPI anomaly distributions in May with lesser and weaker large-scale dry events and stronger large-scale wet events. Further studies will be needed to assess whether the observed trend in large-scale extreme precipitation conditions can be related to natural climate variability or anthropogenic activities and whether it is associated to changes in local/regional extreme events. The stronger wet conditions in different months seem to be associated to changes in ENSO characteristics (amplitude, propagation, spatial structure, ...) since the 1982–1983 El Nino. Indeed, spatial ENSO teleconnections (stronger in November and April–May) have greatly evolved from 1950–1975 to 1976–2001. Moreover, we demonstrate that there is a strong modulation and displacement of the teleconnection patterns from one event to another, impeding the definition of robust statistical relationship between ENSO and precipitation in the Parana-Plata basin (except maybe over a very limited area near the common border between Paraguay, Argentina and Brazil). Finally, the non-antisymmetrical patterns of precipitation between El Nino and La Nina conditions and the non-linear relationship between precipitation and either Nino3.4 or Nino1+2 sea surface temperature indices show that linear statistical forecast systems are actually of very limited use for impact predictions on society on a local or regional scale.

International audience; ADVICLIM project aims at observing climate at local scales in different European vineyards, representing the climate diversity in European wine regions; simulating climate and climate change in order to produce a fine scale assessment of the climate change impacts, thereafter simulating scenario of adaptation for viticulture. In order to demonstrate the importance of local management, the project will develop technologies which can be further applied to other agricultural territories in EU and worldwide; i.e. a network of agro-climatic measurements and a web platform that professionals will be able to access to visualize simulations of adaptation to climate change according to cultural practices as well as associated carbon footprints. It will combine field work, different modeling methods and information transfer to the wine industry and further agricultural organisms with the aims to providing information to help reflection on mitigation and adaptation strategies to climate change based upon local management of the environment and cultural practices as well as guidance to decision-makers in the viticultural sector

The present study offers a description of long equatorial wave propagation and reflection in the Indian Ocean as observed by the TOPEX/POSEIDON satellite. The equatorial longwave amplitudes are computed from January 1993 until December 1998, focusing on Kelvin (K) and first Rossby (R1) modes. The K wave is observed to propagate with a baroclinic phase speed close to 2 m/s while the R1 has a phase speed of 0.7 m/s. Wave reflections at both eastern and western boundaries are clearly observed during the entire period. At the eastern boundary K reflects into R1 with a reflection coefficient of 1.04 (reflection efficiency of 85% of that of an infinite meridional wall), and at the western boundary R1 reflects into K with a coefficient of 0.50 on average although a large seasonal and interannual variability of the reflection efficiency of the African coast is observed. Reflected K waves strongly influence the R1 variability near the western boundary as far as 60°E. Reflected R waves in turn, influence the K variability near the eastern boundary as far as 80°E. Although long equatorial waves are mainly forced by the changes in the winds during the inter-monsoon periods, reflection have a strong influence on the ocean's variability over a large part of the basin on both seasonal and interannual time scales. Focusing on the seasonal cycle of long equatorial waves (computed over 1993–1996), it is found that the constructive/destructive interaction between the wind forcing and wave reflection is responsible for the K (or R1) seasonal cycle to show a semi-annual (or annual) period near the western (or eastern) boundary. In 1994 and 1997–98, strong interannual easterlies coincident with El Nino onsets in the Pacific occurred in the Indian Ocean. Long equatorial wave propagation and reflection play a major role in creating an east–west sea level gradient along the equator, suggesting that wave dynamics and wave reflection may play a significant role in the Indian dipole mode suggested by several authors.

Zonal advection by long equatorial waves has been shown to be an important process in the evolution of sea surface temperature in the central Pacific on ENSO time scales. The present study aims at investigating how well an oceanic model whose dynamics are based on long equatorial waves can simulate the large-scale surface zonal current variability. Thus an ocean linear model which can be run with two or three layers is validated against several sets of observations in the Pacific ocean (TOPEX/POSEIDON sea level, TAO zonal currents, surface current climatology). The surface layer (mixed-layer) has a constant depth. Therefore the layer model is equivalent to considering a shear layer solution and either one or two baroclinic modes. It allows evaluation of the impact of adding a second baroclinic mode on the simulation of surface currents. This evaluation is done for different friction parametrizations: a weak linear Rayleigh friction (24 months−1), a strong linear Rayleigh friction (6 months−1), and a new parametrization using quadratic friction in the momentum equation only. It is shown in all simulations using various Rayleigh friction parametrizations that the addition of a second baroclinic mode always improves the simulation of both the sea level and the surface currents, especially in the central western Pacific. In that region, there is a reduction of the propagating long Rossby waves whose amplitude is much too large when only one baroclinic mode is used. Despite this reduction, the use of a weak friction (24 months−1) always yields results which compare only poorly to observations confirming results from previous studies. The use of strong friction (6 months−1) improves the model simulation, but surface current variability still remains too large. Finally, the use of quadratic friction as proposed in the present study considerably improves the simulation of zonal currents and its comparison to all data sets. This result gives more confidence in the choice of such a simple model to further explore the role of zonal advection by long equatorial waves on ENSO time scales.

Sea surface temperature, sea level, and pseudo wind stress anomaly data from late 1996 to early 1998 are assimilated into an intermediate coupled model of the Tropical Pacific. Model data consistency is examined. Impact of the assimilation on forecast is evaluated. The ocean component of the coupled model consists of a shallow water model with two baroclinic modes, an Ekman shear layer, and a mixed layer temperature equation. The atmospheric model is a statistical one (based on dominant covariance of historical surface temperature and pseudo wind stress anomaly data). The adjoint method is used to fit the coupled model to 6 months of data by optimally adjusting the initial state and model parameters. A forecast is performed using the end state of an assimilation experiment as initial conditions and using parameters estimated during the assimilation period. Thus the model state during the assimilation and that during the forecast belong to the same model trajectory in different periods. Such an initialization procedure is useful in avoiding initial shock during forecast due to inconsistency of an initial state with the coupled model physics. As a result of optimal adjustments of initial state and parameters, the model is able to reproduce observed interannual variability of sea surface temperature and sea level reasonably well. The averaged residual model data misfits over various 6 month periods are 0.5°C and 5 cm, respectively. The model has a limited skill in reproducing much of the off-equatorial wind anomalies. The residual model data misfit in pseudo wind stress anomaly is larger than 10 m 2 s -2 . Forecasts initialized from the assimilation product are overall more realistic than those simply initialized from wind-forced ocean states. Consistent improvement due to optimal initialization is found for sea surface temperature and sea level anomalies in the central-eastern Pacific and zonal pseudo wind stress anomaly in the central Pacific, both in terms of root-mean-squared deviation from and correlation with the data. The adjustments of parameters in addition to initial state in a coupled context is found to be important to improving the model data consistency during the assimilation and the forecast. In particular, the estimated drag and damping coefficients properly regulate the relative strength of forcing and damping of the ocean state so as to fit the three types of observations during the assimilation (initialization) period, which facilitates the development of a large-amplitude warming event during the forecast. The study demonstrates the utility of oceanic and atmospheric data to estimate initial state and model parameters in a coupled context, which is useful to the evaluation, improvement, and initialization of El Nino-Southern Oscillation forecast models.

This paper introduces the use of sets of multiple networks (bundled networks) to manage the variability due to different initialization parameters. This method makes it statistically impossible for the networks to be trapped in the same local minimum, and therefore allows better control of the confidence of the prediction eventually given. The spread of the forecasts given by these different networks can be used for prediction reliability purposes. An illustration of this usage is given with the El Nino phenomenon.

The role of the meridional wind anomalies on the El Nino-Southern Oscillation as simulated by a simple coupled tropical Pacific ocean-atmosphere model is investigated. It is found that these anomalies play a key role in maintaining finite amplitude interannual variability in the coupled simulations. When the meridional wind stress anomalies are not allowed to feed back to the ocean, the simulated oscillations are damped out within a few years. This happens irrespective of initial conditions. During a warm (cold) event, the simulated meridional wind stress anomalies drive convergent (divergent) surface currents in the equatorial east Pacific, inducing downwelling (upwelling) anomalies that act to strongly reinforce the SST and wind anomalies. This is in contrast to the model zonal wind stress anomalies, which due to the tendency to reverse sign in the eastern Pacific, induce upwelling (downwelling) anomalies in the east, and SST patterns with weaker net positive feedback. Observed wind stress anomalies present some similarity with those simulated by the coupled model in the near-equatorial region. The zonal component reverses sign between the central and the eastern Pacific. There, the meridional component induces a vertical current anomaly of opposite sign and similar magnitude to that induced by the zonal one. These features suggest the importance of meridional wind stress anomalies in the real climate system.

The TOPEX/POSEIDON sea level data lead to new opportunities to investigate some theoretical mechanisms suggested to be involved in the El Nino-Southern Oscillation phenomenon in the tropical Pacific ocean. In particular, we are interested in studying the western boundary reflection, a process crucial for the delayed action oscillator theory, by using the TOPEX/POSEIDON data from November 1992 to May 1995. We first projected the sea level data onto Kelvin and first-mode Ross waves. Then we estimated the contribution of wind forcing to these waves by using a single baroclinic mode simple wave model forced by the ERS-1 wind data. Wave propagation was clearly observed with amplitudes well explained by the wind forcing in the ocean interior. Evidence of wave reflection was detected at both the western and eastern boundaries of the tropical Pacific ocean. At the eastern boundary, Kelvin waves were seen to reflect as first-mode Rossby waves during the entire period. The reflection efficiency (in terms of wave amplitude) of the South American coasts was estimated to be 80% of that of an infinite meridional wall. At the western boundary, reflection was observed in April-August 1993, in January-June 1994, and, later, in December 1994 to February 1995. Although the general roles of these reflection events in the variability observed in the equatorial Pacific ocean are not clear, the data suggest that the reflections in January-June 1994 have played a role in the onset of the warm conditions observed in late 1994 to early 1995. Indeed, during the January-June 1994 period, as strong downwelling first-mode Rossby waves reflected into downwelling Kelvin waves, easterly wind and cold sea surface temperature anomalies located near the date line weakened and eventually reversed in June-July 1994. The presence of the warm anomalies near the date line then favored convection and westerly wind anomalies that triggered strong downwelling Kelvin waves propagating throughout the basin simultaneously with the beginning of the 1994-1995 warm conditions.

International audience; The ability of four regional climate models to reproduce the present-day South American climate is examined with emphasis on La Plata Basin. Models were integrated for the period 1991-2000 with initial and lateral boundary conditions from ERA-40 Reanalysis. The ensemble sea level pressure, maximum and minimum temperatures and precipitation are evaluated in terms of seasonal means and extreme indices based on a percentile approach. Dispersion among the individual models and uncertainties when comparing the ensemble mean with different climatologies are also discussed. The ensemble mean is warmer than the observations in South Eastern South America (SESA), especially for minimum winter temperatures with errors increasing in magnitude towards the tails of the distributions. The ensemble mean reproduces the broad spatial pattern of precipitation, but overestimates the convective precipitation in the tropics and the orographic precipitation along the Andes and over the Brazilian Highlands, and underestimates the precipitation near the monsoon core region. The models overestimate the number of wet days and underestimate the daily intensity of rainfall for both seasons suggesting a premature triggering of convection. The skill of models to simulate the intensity of convective precipitation in summer in SESA and the variability associated with heavy precipitation events (the upper quartile daily precipitation) is far from satisfactory. Owing to the sparseness of the observing network, ensemble and observations uncertainties in seasonal means are comparable for some regions and seasons.

International audience; The present article is a contribution to the CLARIS WorkPackage "Climate and Agriculture", and aims at testing whether it is possible to predict yields and optimal sowing dates using seasonal climate information at three sites (Pergamino, Marcos Juarez and Anguil) which are representative of different climate and soil conditions in Argentina. Considering that we focus on the use of climate information only, and that official long time yield series are not always reliable and often influenced by both climate and technology changes, we decided to build a dataset with yields simulated by the DSSAT (Decision Support System for Agrotechnology Transfer) crop model, already calibrated in the selected three sites and for the two crops of interest (maize and soybean). We simulated yields for three different sowing dates for each crop in each of the three sites. Also considering that seasonal forecasts have a higher skill when using the 3-month average precipitation and temperature forecasts, and that regional climate change scenarios present less uncertainty at similar temporal scales, we decided to focus our analysis on the use of quarterly precipitation and temperature averages, measured at the three sites during the crop cycle. This type of information is used as input (predictand) for non-linear statistical methods (Multivariate Adaptive Regression Splines, MARS; and classification trees) in order to predict yields and their dependency to the chosen sowing date. MARS models show that the most valuable information to predict yield amplitude is the 3-month average precipitation around flowering. Classification trees are used to estimate whether climate information can be used to infer an optimal sowing date in order to optimize yields. In order to simplify the problem, we set a default sowing date (the most representative for the crop and the site) and compare the yield amplitudes between such a default date and possible alternative dates sometimes used by farmers. Above normal average temperatures at the beginning and the end of the crop cycle lead to respectively later and earlier optimal sowing. Using this classification, yields can be potentially improved by changing sowing date for maize but it is more limited for soybean. More generally, the sites and crops which have more variable yields are also the ones for which the proposed methodology is the most efficient. However, a full evaluation of the accuracy of seasonal forecasts should be the next step before confirming the reliability of this methodology under real conditions. © Springer Science + Business Media B.V. 2009.

The goal of the CLARIS project was to build an integrated European–South American network dedicated to promote common research strategies to observe and predict climate changes and their consequent socio-economic impacts taking into account the climate and societal peculiarities of South America. Reaching that goal placed the present network as a privileged advisor to contribute to the design of adaptation strategies in a region strongly affected by and dependent on climate variability (e.g. agriculture, health, hydro-electricity). Building the CLARIS network required fulfilling the following three objectives: (1) The first objective of CLARIS was to set up and favour the technical transfer and expertise in earth system and regional climate modelling between Europe and South America together with the providing of a list of climate data (observed and simulated) required for model validations; (2) The second objective of CLARIS was to facilitate the exchange of observed and simulated climate data between the climate research groups and to create a South American high-quality climate database for studies in extreme events and long-term climate trends; (3) Finally, the third objective of CLARIS was to strengthen the communication between climate researchers and stakeholders, and to demonstrate the feasibility of using climate information in the decision-making process. Fil: Boulanger, Jean-Philippe. Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques; Francia Fil: Brasseur, G.. National Center for Atmospheric Research; Estados Unidos Fil: Carril, Andrea Fabiana. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina Fil: de Castro, Manuel. Universidad de Castilla-La Mancha; España Fil: Degallier, Nicolas. Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques; Francia Fil: Ereño, Carlos. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Le Treut, H.. Institut Pierre Simon Laplace; Francia Fil: Marengo, José Antonio. Centro de Previsao de Tempo e Estudos Climáticos. Instituto Nacional de Pesquisas Espaciais; Brasil Fil: Menendez, Claudio Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina Fil: Nuñez, Mario Nestor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina Fil: Penalba, Olga Clorinda. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Rolla, Alfredo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmosfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmosfera; Argentina Fil: Rusticucci, Matilde Monica. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias de la Atmósfera y los Océanos; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Terra, Rafael. Universidad de la Republica. Facultad de Ingeniería; Uruguay

We investigate the performance of one stretched-grid atmospheric global model, five different regional climate models and a statistical downscaling technique in simulating 3 months (January 1971, November 1986, July 1996) characterized by anomalous climate conditions in the southern La Plata Basin. Models were driven by reanalysis (ERA-40). The analysis has emphasized on the simulation of the precipitation over land and has provided a quantification of the biases of and scatter between the different regional simulations. Most but not all dynamical models underpredict precipitation amounts in south eastern South America during the three periods. Results suggest that models have regime dependence, performing better for some conditions than others. The models' ensemble and the statistical technique succeed in reproducing the overall observed frequency of daily precipitation for all periods. But most models tend to underestimate the frequency of dry days and overestimate the amount of light rainfall days. The number of events with strong or heavy precipitation tends to be under simulated by the models. Fil: Menendez, Claudio Guillermo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: de Castro, M.. Universidad de Castilla-La Mancha; España Fil: Boulanger, J. P.. Universite de Bretagne Occidentale. Departamento Umr 6538; Francia. Centre National de la Recherche Scientifique; Francia Fil: D'Onofrio, A.. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina Fil: Sanchez, E.. Universidad de Castilla-La Mancha; España Fil: Sörensson, Anna. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Blazquez, Josefina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Elizalde, A.. Universidad de Castilla-La Mancha; España Fil: Jacob, D.. Max Planck Institute for Meteorology; Alemania Fil: Le Treut, H.. Centre National de la Recherche Scientifique; Francia Fil: Li, Z. X.. Centre National de la Recherche Scientifique; Francia Fil: Nuñez, Mario Nestor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Pessacg, Natalia Liz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Pfeiffer, S.. Max Planck Institute for Meteorology; Alemania Fil: Rojas, M.. Universidad de Chile; Chile Fil: Rolla, Alfredo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Samuelsson, P.. Swedish Meteorological and Hydrological Institute; Suecia Fil: Solman, Silvina Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Centro de Investigaciones del Mar y la Atmósfera. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Centro de Investigaciones del Mar y la Atmósfera; Argentina Fil: Teichmann, C.. Max Planck Institute for Meteorology; Alemania

OBJECTIVE: To estimate the basic reproduction number (R0) of dengue fever including both imported and autochthonous cases. METHODS: The study was conducted based on epidemiological data of the 2003 dengue epidemic in Brasília, Brazil. The basic reproduction number is estimated from the epidemic curve, fitting linearly the increase of initial cases. Aiming at simulating an epidemic with both autochthonous and imported cases, a "susceptible-infectious-resistant" compartmental model was designed, in which the imported cases were considered as an external forcing. The ratio between R0 of imported versus autochthonous cases was used as an estimator of real R0. RESULTS: The comparison of both reproduction numbers (only autochthonous versus all cases) showed that considering all cases as autochthonous yielded a R0 above one, although the real R0 was below one. The same results were seen when the method was applied on simulated epidemics with fixed R0. This method was also compared to some previous proposed methods by other authors and showed that the latter underestimated R0 values. CONCLUSIONS: It was shown that the inclusion of both imported and autochthonous cases is crucial for the modeling of the epidemic dynamics, and thus provides critical information for decision makers in charge of prevention and control of this disease. OBJETIVO: Estimar o número básico de reprodução da dengue (R0), com base nos casos importados, além dos casos autóctones. MÉTODOS: O estudo foi feito sobre dados epidemiológicos da epidemia de dengue em Brasília, 2003. O número básico de reprodução é determinado a partir da curva epidêmica, ajustando uma reta ao crescimento inicial do número de casos. Para simular uma epidemia com casos autóctones e importados, foi criado um modelo compartimentado do tipo "suscetíveis-infectados-resistentes". O R0 real foi estimado pela fração entre R0 dos casos autóctones e dos importados. RESULTADOS: A comparação de ambos valores de reprodução (apenas autóctones versus todos os casos) mostrou que considerando todos casos como autóctones, o valor de R0 foi superior a um, enquanto o R0 real era inferior a um. O mesmo resultado foi obtido com o conjunto de dados simulando uma epidemia com R0 fixo. O método foi também comparado a outros, observando-se que estes últimos subestimaram os valores do R0. CONCLUSÕES: A inclusão de tanto casos autóctones como os importados é essencial para modelar a dinâmica da epidemia, possibilitando informação crítica aos tomadores de decisão, responsáveis pelo controle da doença. OBJETIVO: Estimar el número de reproducción básica (R0) de la fiebre del dengue incluyendo casos importados y autóctonos. MÉTODOS: El estudio fue realizado basándose en datos epidemiológicos de la epidemia del dengue ocurrida en Brasilia, Districto Federal de Brasil, en el 2003. El número de reproducción básica es estimado de la curva epidémica, fijando el incremento lineal de los casos iniciales. Señalando casos importados y autóctonos en una simulación epidémica, fue diseñado un compartimiento "infeccioso-susceptible-resistente", en el cual los casos importados fueron considerados una fuerza externa. La tasa entre R0 de casos importados versus casos autóctonos fue usado como una estimación real de R0. RESULTADOS: La comparación de ambos números de reproducción (sólo autóctonos versus todos los casos) mostró que considerando todos los casos como autóctonos produjo un R0 por encima de uno, a pesar de que el valor real de R0 era menor que uno. Los mismos resultados fueron obtenidos cuando se aplicó el método a epidemias simuladas con valor fijo de R0. Este método fue también comparado con métodos propuestos anteriormente por otros autores y mostró los mismos valores subestimados de R0. CONCLUSIONES: Fue demostrado que la inclusión de casos importados y autóctonos es crucial para el modelaje de dinámicas epidémicas, y que provee información crítica para aquellos que participan en la toma de decisiones en la prevención y control de esta enfermedad.

In this paper, we assess and compare to observations the spatial characteristics of the twentieth Century ENSO SST variability simulated by 23 models of the IPCC-AR4/CMIP3 database. The analysis is confined to the SST anomalies along the equatorial Pacific and is based on the use of a non-linear neural classification algorithm, the Self-Organizing Maps. Systematic biases include a larger than observed proportion for modelled ENSO maximum variability occurring in the Western Pacific. No clear relationship is found between this bias and the characteristics of the modelled mean state bias in the equatorial Pacific. This bias is mainly related to a misrepresentation of both El Ni (n) over tildeo and La Ni (n) over tildea termination phases for most of the models. In contrast, the onset phase is quite well simulated. Modelled El Ni (n) over tildeo and La Ni (n) over tildea peak phases display an asymmetric bias. Whereas the main bias of the modelled El Ni (n) over tildeo peak is to exhibit a maximum in the western Pacific, the simulated La Ni (n) over tildea bias mainly occurs in the central Pacific. In addition, some models are able to capture the observed El Ni (n) over tildeo peak characteristics while none of them realistically simulate La Ni (n) over tildea peaks. It also arises that the models closest to the observations score unevenly in reproducing the different phases, preventing an accurate classification of the models quality to reproduce the overall ENSO-like variability.

The objective of this paper is to reflect and discuss how the use of crop models by aware practitioners might trigger learning of how to think and act differently about the adaptation process of farming systems to climate dynamics. The development of adaptation strategies is discussed from the perspective of contrasting metaphors, since the metaphors in use have distinctive practical implications regarding how crop models might be used for adaptation purposes. Further, in this paper it is pointed out that adaptation should be understood as the result of a learning process and therefore the use of crop models for adaptation purposes must be transformed. Instead of seeing them only as tools to secure yield of cropping systems under a changing climate they must be conceived as components of learning systems for adaptation of whole farming systems.

A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2 degrees S - 6 degrees N, 160 degrees - 90 degrees W, where instabilities are most active. TIWinduced horizontal advection leads to a warming of 0.84 degrees C month(-1), which is of the same order as the 0.77 degrees C month(-1) warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by -0.59 degrees C month(-1) and -1.06 degrees C month(-1), respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution.

The objective was to evaluate a new method of deriving the reproductive number for vector-borne diseases from the early epidemic curves for vector-borne diseases with incubations in the vectors and in the hosts. We applied the model to several dengue epidemics in different climatic regions of Brazil: Brasilia Belem Fortaleza Boa Vista. The new method leads to higher estimates of the reproductive number than previous models. At present Aedes aegypti densities the meeting of more compatible strains of viruses and mosquitoes may lead to re-emergence of urban yellow fever epidemics. (authors)

In this study, the mechanisms leading to the El Niño peak and demise are explored through a coupled general circulation model ensemble approach evaluated against observations. The results here suggest that the timing of the peak and demise for intense El Niño events is highly predictable as the evolution of the coupled system is strongly driven by a southward shift of the intense equatorial Pacific westerly anomalies during boreal winter. In fact, this systematic late-year shift drives an intense eastern Pacific thermocline shallowing, constraining a rapid El Niño demise in the following months. This wind shift results from a southward displacement in winter of the central Pacific warmest SSTs in response to the seasonal evolution of solar insolation. In contrast, the intensity of this seasonal feedback mechanism and its impact on the coupled system are significantly weaker in moderate El Niño events, resulting in a less pronounced thermocline shallowing. This shallowing transfers the coupled system into an unstable state in spring but is not sufficient to systematically constrain the equatorial Pacific evolution toward a rapid El Niño termination. However, for some moderate events, the occurrence of intense easterly wind anomalies in the eastern Pacific during that period initiate a rapid surge of cold SSTs leading to La Niña conditions. In other cases, weaker trade winds combined with a slightly deeper thermocline allow the coupled system to maintain a broad warm phase evolving through the entire spring and summer and a delayed El Niño demise, an evolution that is similar to the prolonged 1986/87 El Niño event. La Niña events also show a similar tendency to peak in boreal winter, with characteristics and mechanisms mainly symmetric to those described for moderate El Niño cases.

http://www.asiabiotech.com/readmore/vol09/0922/dengue.html

[1] The present study focuses on the existence of equatorial Rossby waves at periods shorter than 180 days and on their potential reflection into equatorial Kelvin waves. Toward that objective, TOPEX/Poseidon sea level data are used from 1992 to 2002. No Rossby waves are evidenced at periods shorter than 100 days. At 120-day period, Rossby waves of first meridional mode dominate in the western part of the basin. Their reflection efficiency at the western boundary in energy is found to be around 40–45% of theoretical estimates for low-frequency waves reflecting at a perfect wall. It is suggested that the 120-day reflected Rossby waves contribute to a large extent to the generation of 120-day Kelvin waves and that they superimpose on the directly wind-forced Kelvin waves, hence reinforcing this signal.

International audience; The present study aims at studying the role played by high-frequency wind variability, wave reflection and easterly wind anomalies in the western Pacific in the onset, growth and termination phases of the 1997–1998 El Niño using the Trident intermediate coupled model and observations. While the anomalous strength of the trade winds in 1996 favored the initiation of a warm event in 1997 (via western Pacific boundary Rossby wave reflection), the actual timing of the onset and the amplitude of the event resulted from the large March 1997 wind event. Once initiated, high-frequency westerly winds strongly contributed to the rapid growth of the warm event and to the displacement of the eastern edge of the warm-pool. Moreover, both easterly and westerly high-frequency wind variability in 1997–1998 contributed to the amplitude of the event, set the evolution of the warm event and potentially influenced the equatorial Pacific conditions at least one year after the El Niño event. In addition, eastern boundary reflection also significantly contributed to the amplitude and duration of the warm event, whereas its termination was a combination of various factors: reflection of upwelling Rossby waves at the western boundary and large easterly wind anomalies observed in the western Pacific from November 1997 to early 1998. These factors were sufficient to terminate the event and to switch temperature anomalies from warm to cold. To conclude, understanding the coupling between the high- and low-frequency wind variability, i.e., studying ENSO as a multi-scale phenomenon, will certainly lead to a better comprehension of the diversity of its behavior and potentially to an improvement of its predictability.

Two ten-members ensemble experiments using a coupled ocean-atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event (WWE) when the tropical Pacific has initial conditions favourable to the development of a warm event. In the reference ensemble (CREF), no wind perturbation is introduced, whereas a strong westerly wind event anomaly is introduced in boreal winter over the western Pacific in the perturbed ensemble (CWWE). Our results demonstrate that an intense WWE is capable of establishing the conditions under which a strong El Nino event can occur. First, it generates a strong downwelling Kelvin wave that generates a positive sea surface temperature (SST) anomaly in the central-eastern Pacific amplified through a coupled ocean-atmosphere interaction. This anomaly can be as large as 2.5degreesC 60 days after the WWE. Secondly, this WWE also initiates an eastward displacement of the warm-pool that promotes the occurrence of subsequent WWEs in the following months. These events reinforce the initial warming through the generation of additional Kelvin waves and generate intense surface jets at the eastern edge of the warm-pool that act to further shift warm waters eastward. The use of a ten-members ensemble however reveals substantial differences in the coupled response to a WWE. Whereas four members of CWWE ensemble develop into intense El Nino warming as described above, four others display a moderate warming and two remains in neutral conditions. This diversity between the members appears to be due to the internal atmospheric variability during and following the inserted WWE. In the four moderate warm cases, the warm-pool is initially shifted eastward following the inserted WWE, but the subsequent weak WWE activity ( when compared to the strong warming cases) prevents to further shift the warm-pool eastwards. The seasonal strengthening of trade winds in June - July can therefore act to shift warm waters back into the western Pacific, reducing the central-eastern Pacific warming. This strong sensitivity of the coupled response to WWEs may therefore limit the predictability of El Nino events, as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead.

[1] The role of salinity during the onset and growth of the 1997 Indian Ocean Dipole Mode event is explored with an ocean general circulation model thoroughly validated to observations. In fall 1997, anomalous easterlies drive an Indian Ocean equatorial circulation similar to that regularly observed in the Pacific Ocean: a South Equatorial Current (SEC) near the equator, an Equatorial Under Current (EUC) subsurface, and intense upwelling in the eastern part of the basin. The SEC transports westward the eastern Indian Ocean fresh pool creating, along the equator, a shallow salinity stratification favoring the creation of a barrier layer. In our experiment, the shallow top of the thermocline limits the barrier layer thickness to about 10 m; nevertheless, salinity has a significant impact on sea surface temperature (SST). Indeed, the shallow salinity stratification along the equator traps the wind forcing in a thin surface mixed-layer. The reduction of wind momentum penetration decreases the deceleration of the EUC, and the greater amplitude of wind momentum input in surface layers strengthens the SEC. This intensification of the equatorial zonal circulation increases the Sumatra upwelling and its associated meridional circulation. This strengthening of the whole equatorial circulation shifts upward the thermohaline structure and reinforces the cold SST anomaly off Sumatra by about 20%. Overall, the effect of salinity on the 1997 Indian Ocean Dipole is to reinforce the oceanic anomalies favoring a strengthening of the air-sea interactions.

[1] The impact of salinity and the barrier layer on eastern Indian Ocean dynamics is investigated using the OPA ocean general circulation model for 1979–1993. The study focuses on processes involved in the eastern Indian Ocean circulation during the period of the fall Wyrtki Jet. An exploration of interactions between salinity and ocean dynamics is made using a set of salinity sensitivity experiments. The inclusion of salinity favors a stronger Wyrtki Jet that extends further eastward in two steps. First, a sporadic barrier layer along the jet increases the jet speed by trapping wind momentum in a thinner mixed layer. Second, zonal advection transports the current maximum eastward and participates in the creation of a strong current front at the eastern edge of the jet during the following month. Off Sumatra, the sea surface salinity gradient, bordering the Indian Ocean fresh pool, strengthens when the Wyrtki Jet reaches this region. The pressure gradient associated with the enhanced salinity front then becomes larger than the eastward wind stress forcing in the zonal momentum equation. This initiates a westward surface current opposed to the wind stress. The later penetration of the Wyrtki Jet into the Indian warm and fresh pool modifies the sea level and favors geostrophic westward currents around 4°N. A combination of these mechanisms additionally favors a northward shift of the Wyrtki Jet. All these phenomena affect the zonal displacement of the Indian fresh pool and create SST anomalies of 0.5°C, in a region of high ocean-atmosphere coupling.

In this study, the processes affecting sea surface temperature variability over the 1992–98 period, encompassing the very strong 1997–98 El Niño event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1–2 and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s−1 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 10°N–10°S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992–98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s−1 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing.\ud \ud This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993–96 period and the 1997–98 El Niño. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central–western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997–98 El Niño. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Niño can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

International audience; The present study aims to give a quantitative description of the role played by long equatorial waves on sea surface temperature anomalies during the 1993-1998 TOPEX/POSEIDON period and more specifically during the 1997-1998 El Niño using a new Pacific Ocean model called Trident. In a companion paper, the Trident dynamical component was described and validated with observations. Briefly, the model exhibits skill in simulating not only sea level but also surface zonal current variability in the equatorial wave guide. Here the model thermodynamics are described and validated. The thermodynamics of Trident consist of one single equation for interannual sea surface temperature anomalies. Compared to other similar models, the introduction in the temperature equation of a term equivalent to a vertical mixing term improves significantly the temperature simulations in the eastern Pacific. Thus the model comparison to interannual sea level, zonal current and sea surface temperature anomalies is fairly good over the entire equatorial Pacific Ocean. The model is found to be sensitive to both subsurface variability (central and eastern Pacific) and to zonal current advection, especially near the dateline where the data suggest that advection is a first order process for determining sea surface temperature changes. The role of long equatorial wave reflection observed in TOPEX/POSEIDON data is studied by either cancelling the eastern boundary or western boundary reflection. First, at the eastern boundary, although the reflected Rossby waves were found to act against the warming during the onset phase of the 1997-1998 El Niño through zonal advection, the major impact of these reflected Rossby waves was to reinforce the deepening of the thermocline in the eastern Pacific initiated by the impinging Kelvin waves. Therefore, all things considered, the Rossby waves strongly contributed to the warming in 1997-1998 east of 120°W. Secondly, at the western boundary, Rossby wave reflection acted as suggested by the delayed action oscillator theory: the reflected Kelvin waves caused shoalling of the thermocline in the central and eastern Pacific where they weakened the warm or cold conditions observed in the Pacific Ocean. During the 1997-1998 event specifically, they contributed to about a third of the upwelling Kelvin wave amplitude propagating in the central Pacific. The other two thirds of the amplitude were found to be actually wind-forced, strongly suggesting that during that event easterly wind anomalies in the western Pacific played a significant role in the termination of the recent 1997-1998 El Niño event and its switch to the La Niña period.

In March 1997, a strong westerly wind event (WWE) occurred in the western equatorial Pacific prior to the 1997-1998 E1 Nifio event. It produced downwelling Kelvin waves that interacted non linearly with the surface temperature, salinity and zonal current fronts located at the eastern edge of the warm-fresh pool (EEWP). This non-linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response, and advected the EEWP at an unexpected rate (-l m/s) to which the ocean-atmosphere coupled system may have been responding rapidly to trigger E1 Nino conditions.

International audience; The TOPEX/POSEIDON sea level data have been shown to observe the sea surface height variability accurately over the globe and, more particularly, in the tropical Pacific, core region of the El Niño/Southern Oscillation. Various studies have already provided descriptions of long equatorial wave propagation as well as estimates of their reflection at both the eastern and western boundaries using TOPEX/POSEIDON data over short time periods. In this present work, we are first interested in computing long equatorial wave amplitudes over a much longer period (October 1992-May 1998) focusing on the Kelvin and first three largest Rossby waves. Secondly, we are examining qualitatively potential theories of ENSO in the light of observations of the strong 1997-1998 El Niño event. At the eastern boundary, Kelvin waves are observed to reflect into first (third) mode Rossby waves with a reflection efficiency of 75% (65%) of that of an infinite meridional wall. In the western Pacific Ocean, the reflection of the first Rossby wave does explain more than 90% of the Kelvin wave variance. The second Rossby wave is found not to be correlated to the Kelvin wave amplitude, and the third Rossby wave is highly correlated to the Kelvin signal although its contribution is weak. The investigation during the 1997-1998 event of two theories potentially important for ENSO (the delayed action oscillator mechanism and the recent theory of Picaut et al., 1997) led us to the following conclusions. The westerly wind anomalies observed in the western Pacific in December 1996 and March 1997 forced downwelling Kelvin waves which advected the eastern edge of the warm pool eastward and deepened the thermocline in the east Pacific. Both zonal advection and vertical processes are suggested to act constructively during the onset of positive anomalies in the central and eastern Pacific. From May to September 1997, downwelling Kelvin wave wind-forced in the central Pacific and reflected downwelling Rossby waves (reinforced by local easterly wind anomalies) acted against each other, and the 28°C isotherm was found to move slightly westward. In October 1997, strong westerly wind anomalies forced a strong downwelling Kelvin wave potentially responsible for the strong warming in the east Pacific. The 28°C isotherm reached the eastern boundary. At that time, the equatorial sea surface temperatures are zonally homogeneous. The strong downwelling Rossby waves reflected at the boundary could not act to terminate the warm event through zonal advection as suggested by Picaut et al. However the reflected upwelling Kelvin waves coming from the western boundary weakened the downwelling Kelvin signal in the central and east Pacific leading to a decrease of the sea surface temperature anomalies. Following this decrease, westerly wind anomalies in the central Pacific weakened, and upwelling Kelvin waves reinforced by easterly wind anomalies in the western Pacific propagated toward the eastern Pacific. As a conclusion, both theories must be considered simultaneously to understand the variability observed during the 1997-1998 El Niño event. However observations strongly suggest the delayed action oscillator mechanism to be the major process at work during the weakening of the warm 1997-1998 ENSO.

A high-resolution oceanic general circulation model (OGCM) of the three tropical oceans is used to investigate long equatorial wave activity in the Pacific Ocean during the 1985–94 TOGA period. The ARPEGE atmospheric general circulation model simulated zonal wind stress forcing and the OPA OGCM simulated dynamic height are interpreted using techniques previously applied to data. Long equatorial waves of the first baroclinic mode (Kelvin and first-mode Rossby waves) are detected propagating in the model outputs during the entire period. A seasonal cycle and interannual anomalies are computed for each long equatorial wave. In the east Pacific basin, long equatorial wave coefficients are dominated by seasonal variations, while west of the date line they display strong interannual anomalies. Interannual long-wave anomalies are then compared to wave coefficients simulated by a simple wind-forced model. The results presented here indicate the major role played by wind forcing on interannual timescales ...

[1] By modifying mixed layer thickness, the barrier layer (BL) may have a significant impact on air–sea interactions. In the tropical Indian Ocean, thermohaline observations are lacking over most of the basin, despite a few BL observations in the Bay of Bengal and off Sumatra. A 15-yearlong model simulation is used to investigate the seasonal variations and formation mechanisms of the BL. In the eastern equatorial area, the model thermohaline structure is in close agreement with observations and simulates a realistic seasonal variability. A robust and thick BL (of more than 40 m) is observed seasonally with a maximum horizontal extent (of about 1500 km) in November. This BL corresponds to the merging of two BLs that undergo different formation mechanisms. The first is formed as the Wyrtki Jet propagates eastward, deepening the thermocline while zonally advecting a salinity subsurface maximum. At the same time, northward wind stresses induce a northward advection of a thin fresh water layer toward the equator. This combination decouples salinity from temperature, and a BL is formed. Later, as the jet penetrates further toward the fresh pool, a second BL appears off Sumatra. This one is controlled by two different mechanisms. A westward surface current trapped in the thin mixed layer off Sumatra advects fresh water from the coastal precipitation area. Then, significant upwelling associated with the westward current further raises the halocline. In situ observations confirm the realism of these simulated formation mechanisms. From year to year, the timing, thickness, and shape of the BLs follow the strong interannual variability of the Wyrtki Jet. Despite these variations, the formation mechanisms remain unchanged.

An Ocean General Circulation Model is used to investigate the oceanic response to the March 1997 Westerly Wind Event that is suggested to have played an important role in the onset of the 1997–1998 El Nino. Our results point out three distinct impacts. First a strong wind-forced downwelling Kelvin wave propagates eastward generating sea surface temperature anomalies up to 1°C and large subsurface temperature and zonal current anomalies, mainly located in the core of the thermocline. Second the northward and westward extension of this wind event is responsible for a surface advection of cold waters from 130°E–5°N to the equator. Third it generates large zonal surface currents at the eastern edge of the warm and fresh pool by a nonlinear interaction between the wind-forced surface jet and the local thermohaline front. Salinity through both its contribution to the local zonal pressure gradient at the front and the barrier layer effect is crucial in the occurrence of this nonlinear mechanism. The fast displacement of the front (2000 km in a month) together with the cooling in the western Pacific is likely to be responsible for the eastward displacement of atmospheric deep convection and eastward winds observed in April–June 1997 and thus to have played a major role in initiating the El Nino of the century.

Fields of TOPEX-derived sea level anomalies are validated against and compared with fields of Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) dynamic height anomalies for the first 470 days of the TOPEX/POSEIDON mission. At periods longer than 35 days, TOPEX sea level anomalies compare extremely well with TOGA-TAO dynamic height anomalies throughout the whole basin, with a mean correlation of 0.79 and mean rms differences of 2.6 cm. Zonal geostrophic current anomalies are then derived from both data sets and validated against in situ current meter measurements at the equator. TOPEX-derived geostrophic current anomalies at the equator are in good agreement with in situ currents at 0°–165°E, 0°–170°W, 0°–140°W, and 0°–110°W. TOGA-TAO-derived current anomalies compare well with in situ currents at 0°–110°W, 0°–140°W, and 0°–170°W, but poor comparisons at 0°–165°E are thought to be linked to the absence of instantaneous salinity measurements in the dynamic height calculation. Despite localized discrepancies, basin-wide TOGA-TAO geostrophic currents compare reasonably well with TOPEX-derived geostrophic currents. The discrepancies in the comparison between TOPEX and TAO are shown to be mostly associated with gaps in TOGA-TAO space-time sampling and subsequent gridding procedures.

The TOPEX/POSEIDON satellite, together with the Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) array, provides oceanographic and atmospheric observations which allow a detailed study of the equatorial Pacific variability. During the November 1992 to December 1993 El Nino period, sea level, dynamic height, wind stress, sea surface temperature, and surface zonal current data derived from TOPEX/POSEIDON and TOGA-TAO measurements were used to describe the Pacific ocean-atmosphere system and to understand the role played by long equatorial waves. A potentially important mechanism of the El Nino-Southern Oscillation (ENSO), commonly referred to as the delayed action oscillator, involves Kelvin and long Rossby waves and their reflections at the Pacific western boundary. In order to investigate if this process was at work during the period under study, a method for projecting TOPEX sea level, TOGA-TAO dynamic height, and zonal wind stress onto meridional wave structures was designed both in unbounded and bounded regions. The Kelvin and first three Rossby waves of the first baroclinic mode are propagating at theoretical wave speeds in all data sets. Zonal wind stress projections show that oceanic propagating wave features are strongly linked to wind variability. Reflections are then examined at both boundaries. At the eastern boundary most of the signal reflected from incoming Kelvin waves is either counteracted by unfavorable wind forcing or strongly reinforced and therefore does not seem to play a significant role for generating the major Rossby wave signals during the period under study. In the western Pacific, wind forcing, rather than western boundary reflections, appears to be the main trigger for returning Kelvin waves from the western Pacific to the eastern Pacific. Simultaneously with the weakening of the extended 1991–1993 ENSO event, an upwelling Kelvin wave is observed propagating from the western Pacific in September 1993 to the eastern Pacific in November 1993. This scenario is consistent with some features of the delayed action oscillator mechanism, where an upwelling Kelvin wave is systematically seen returning from the western boundary to the east at the end of warm events. However, here, contrary to the delayed action oscillator, most of this returning Kelvin wave seems to be forced by a strong easterly anomaly located in the western Pacific, rather than by reflection of an upwelling first Rossby wave at the western boundary.

Equatorial wave dynamics are essential in most oceanic models for reproducing aspects of the El Nino-Southern Oscillation (ENSO) phenomenon. Observational evidence of first baroclinic equatorial Kelvin and first symmetric meridional Rossby waves is found in Geosat-derived sea level anomalies (SLA) and surface zonal current anomalies (ZCA) in the equatorial Pacific ocean during the 1986-1987 El Nino and the ensuing 1988-1989 La Nina. This was made possible after extensive quality control and specific processing of the recently improved Geosat geophysical data records pertaining to the 17-day Exact Repeat Mission. In particular, the processing was made so that the Geosat-derived ZCA best fit near-surface zonal currents from three equatorial moorings at 165 deg E, 140 deg W, and 110 deg W. The Geosat-derived SLA and ZCA are decomposed into first baroclinic equatorial Kelvin and gravest Rossby modes. The emphasis is then put on the chronology of the ZCA at the equator where the currents are the most energetic and where Kelvin and first symmetric Rossby waves explain most of the variance in ZCA, in similar proportion. The 1986-1987 El Nino is mostly characterized by a strong downwelling Kelvin wave in December 1986, a series of downwelling Kelvin waves in March-October 1987, and a strong upwelling Rossby wave in March-September 1987. These waves are consistent with wind forcing, and all give rise to notable eastward ZCA for almost an entire year. During El Nino-La Nina transition period there is the occurrence of two downwelling Rossby waves originating from the eastern Pacific at times of favorable wind forcing. These Rossby waves switch the basin-wide ZCA from eastward to westward, terminating the warm event. Then, the 1988-1989 La Nina shows the generation of a series of upwelling Kelvin waves which are clearly identified in March/April and September/October 1988. These upwelling Kelvin waves are apparently related to the enhencement of the trade wind in the western half of the basin. These waves maintain the basin-wide westward ZCA for more than a year, although the ZCA is slightly reduced ba front of upwelling Rossby waves in phase with the normal seasonal cycle. The Kelvin wave forcing and sign of kelvin wave contribution versus Rossby wave contribution to ZCA in the western Pacific are discussed, and we conclude that neither pleads in favor of the delayed action oscillator mechanism.

[1] Evaluating long equatorial wave reflections at the Pacific western boundary has been a concern for many past studies in order to discuss whether the delayed action oscillator mechanism may indeed be at work to terminate El Nino events. In the present study, we investigate the respective contributions of Rossby wave reflection and wind forcing in the generation of interannual long equatorial Kelvin waves in the western Pacific Ocean using satellite data over the 1993–2001 period. The reflection efficiency of the western Pacific boundary (east of 165°E) is found to be about 80–90% of a perfect wall at interannual timescales. This result would suggest that the delayed action oscillator mechanism is indeed at work during that period. However, the wind contribution increases significantly from 145°E to 175°E and actually represents more than 80% of the Kelvin wave variance at 175°E. As a conclusion, although Rossby wave reflection is efficient and sufficient to theoretically terminate an El Nino event, easterly wind anomalies in the western Pacific during the mature phase of the 1997–1998 event mostly contribute to the upwelling Kelvin signal involved in the termination of the event confirming previous studies mainly based on other methods. If such a mechanism were found to be at work during other El Nino events, then the negative feedback mechanism of the delayed action oscillator mechanism would be a combination of easterly wind anomalies and wave reflection with a significant role played by easterly wind anomalies. Our result may support some aspects of the western Pacific oscillator paradigm.

Observational and modeling aspects about Westerly Wind Events (WWEs) and their influence on the tropical Pacific ocean-atmosphere system are reviewed. WWEs are a large part of the intraseasonal zonal wind activity over the warm pool. They have typical amplitudes of 7 m s -1 , zonal width of 20° longitude and duration of about 8 days. Their root causes are often a combination of various factors including the Madden-Julian Oscillation, cold surges from mid-latitudes, tropical cyclones and other mesoscale phenomena. The relationship between WWEs and the ENSO cycle is complex, involving among others the equatorial characteristics of the WWEs, the oceanic background state and the internal atmospheric variability. Both observational and modeling studies demonstrate that WWEs tend to cool the far western Pacific, shift the warm pool eastward and warm the central-eastern Pacific through the generation of Kelvin waves. They are therefore important processes for the central and eastern Pacific warming during the onset and development phase of El Nino. The strong atmospheric feedbacks that are likely to be generated by the ocean response to WWEs even suggest that a single WWE is capable of establishing the conditions under which El Nino can occur. The important role played by WWEs in the evolution and amplitude of recent El Nino events may therefore strongly limit the predictability of El Nino.