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In this paper we analyse the joint distribution of extreme temperature and heavy precipitation events in southern South America during 1961-2000, and the predominant atmospheric circulation associated with the occurrence of compound extreme events. We show that the probability of occurrence of intense precipitation (daily rainfall higher than the 75th percentile) significantly increases during or following a warm night (minimum temperature higher than the 90th percentile), but decreases during a cold night (minimum temperature lower than the 10th percentile) during the warm season. Heavy precipitation events are associated with the simultaneous occurrence of warm days (maximum temperature higher than the 90th percentile) or following such an event in eastern Argentina, but they rarely occur before. In contrast, cold days (maximum temperature lower than the 10th percentile) happen more often after an intense rain. Compound events are usually associated with 1 or 2 typical circulation patterns in each subregion. For example, warm days and heavy precipitation tends to occur more often when a trough over the Pacific Ocean and a cold front over the continent lead to warm and wet air advected to the east of the region of study. We also analysed the skill of 7 regional climate models from the CLARIS LPB project to simulate the statistical relationship between temperature and precipitation extremes in 1990-2000. Overall, models were able to simulate an increase in the probability of occurrence of extreme rainfall during warm nights and cold days, and an inhibition of precipitation during cold nights. However, models tend to fail to capture the spatial distribution of the compound extreme events in southeastern South America. Fil: Tencer, Barbara. University Of Victoria; Canadá. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Bettolli, Maria Laura. 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: 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

International audience; European agricultural production is influenced by the space-time distribution of regional climate. Because regional distributions of temperature and precipitation in Europe are affected by changes in the wintertime atmospheric circulation, this paper aims at identifying the relationships between the wintertime Euro-Atlantic variability and wheat yield for the Member States ofthe European Union. An empirical orthogonal function (EOF) decomposition of the 500 hPa geopotential height fields is used to describe the wintertime climate variability, associating the leading 4 components of the EOF decomposition into known climatic patterns (such as North Atlantic Oscillation or Eastern Atlantic patterns). Using statistical methods such as ANOVA, linear regression and ‘leave-one-out’ cross-validation, those patterns are related to time series of wheat yield anomalies. It is shown that, depending on the country, there is a link between wheat yield and modes of winter climate variability, and this link differs from the relationship between temperature and precipitation with the modes. Looking ahead to the improvement of seasonal climate forecasts, it is expected that such meteorological patterns may be predicted with some accuracy, which in turn could improve cropyield forecasts.

Regional climate models nested in global climate models are now being used in Aus- tralia to produce high resolution climate change scenarios for use in impact assessments. This paper describes the development of such scenarios for the State of Victoria in Southeastern Australia using the CSIRO Division of Atmospheric Research Limited Area Model (DARLAM) at a horizontal resolu- tion of 60 km nested in the slab ocean version of the CSIRO global climate model (GCM). Rainfall and temperature over Victoria are substantially better simulated in DARLAM than they are in the host GCM, although some poor features of the simulation of the seasonal cycle of rainfall have not been improved with the increase in resolution. There is evidence to suggest that some of these errors are due to errors in the simulation of atmospheric circulation provided by the host GCM. Under 2 × CO2 conditions, DARLAM simulates patterns of rainfall and temperature change which differ significantly from those provided by the host GCM and which appear more physically plausible. In particular, winter rainfall in southern Victoria increases in the DARLAM simulation, but decreases in the host GCM. Although the DARLAM scenario could be considered the best single scenario available for Victoria at the time of its production, it is noted that different, but equally plausible, scenarios could be obtained by nesting DARLAM in other GCMs, or by increasing the simulation sample size. Our assessment is that the rainfall changes simulated by DARLAM in summer may be most sensitive to such changes. In general, it is concluded that there is great potential to increase the realism of climate change scenarios in the Victorian region through careful use of nested regional modelling.

Atmospheric circulations, and related climate conditions, exert a strong influence on the transport and ultimate deposition of atmospheric pollutants. One potential outcome of climate change is alteration of atmospheric circulation patterns. The first stage in the development of a downscaling methodology to assess the influence of these possible future atmospheric circulation changes on trans- port and deposition of atmospheric pollutants over Europe is described. Firstly, the main modes of regional-scale circulation over a domain covering the North Atlantic and Europe are determined using principal components analysis of sea level pressure patterns. To determine whether the principal com- ponents represent circulation types found in reality, and show realistic relationships with surface temperature and precipitation amount, they are compared to the Lamb Weather Types, Central Eng- land Temperature, and England and Wales Rainfall. There are statistically significant relationships between some of the principal components and aqueous and ambient pollutant concentrations at 5 selected stations from the European Monitoring and Evaluation Programme monitoring network. Although, for any one combination of principal component and pollutant variable, the level of expla- nation can be quite small, notwithstanding some levels of variance explained for individual combina- tions being up to 69% (45% for the winter season, described in detail here). Because the pollution cli- mate at any one location is a function of many combinations of circulation/pollution relationships, despite the relatively small magnitude of variance explained for individual combinations in this study, the results confirm the utility of atmospheric circulation principal components in deriving downscaling relationships for surface pollution behaviour (via eventual multiple regressions, not reported in this paper), as well as giving further insight into the climatic influences on air and precipitation chemistry over Europe.

Generalized circulation ~ndices, as descriptors of atmospheric circulation features, have been used to analyze the relationships between circulation and regional climates. Currently, data for such circulation indices are limited. Records for some indices extend back into the 19th century, but most are based on data that exist for less than 100 yr. Tree rings have been successfully used to extend records of circulation indices, most notably the Southern Oscillation Index. Researchers are continuing to improve reconstructions and to explore the potential of reconstructing other indices. In contrast to more general indices, it is possible to select and/or construct indices that describe circulation features important to a specific region. These regionally tailored circulation indices, along with more general indices that describe features also influencing the climate of a region, can be reconstructed from tree rings. Reconstructions can provide information about variability in atmospheric circulation and about the interactions between influential circulation features in past centuries. In this paper, an example of such a study is given in which a set of circulation indices, compiled for the southwestern U.S., is reconstructed from tree rings. This preliminary study demonstrates the potential value of regionally tailored circulation indices and the reconstruction of these circulation indices for gaining an understand~ng of how circulation patterns may influence regional climates, on scales of decades to centuries.