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Matilde Rusticucci
and
Bárbara Tencer

Abstract

Extreme temperature events are one of the most studied extreme events since their occurrence has a huge impact on society. In this study, the frequency of occurrence of absolute extreme temperature events in Argentina is analyzed. Four annual extremes are defined based on minimum and maximum daily data: the highest maximum (minimum) temperature of the year, and the lowest maximum (minimum) temperature of the year. Applying the extreme value theory (EVT), a generalized extreme value (GEV) distribution is fitted to these extreme indices and return values are calculated for the period 1956–2003. Its spatial distribution indicates that, for warm extremes, maximum temperature (Tx) is expected to be greater than 32°C at least once every 100 yr throughout the country (reaching values even higher than 46°C in the central region), while minimum temperature (Tn) is expected to exceed 16°C (reaching 30°C in the central and northern regions). Cold annual extremes show larger gradients across the country, with Tx being lower than 8°C at least once every 100 yr, and Tn lower than 0°C every 2 yr, with values even less than −10°C in the southwestern part of the country.

However, the frequency of occurrence of climatic extremes has changed throughout the globe during the twentieth century. Changes in return values of annual temperature extremes due to the 1976–77 climatic shift at six long-term datasets are then analyzed. The lowest Tx of the year is the variable in which the 1976–77 shift is less noticeable. At all the stations studied there is a decrease in the probability of occurrence of the highest Tx if the study is based on more recent records, while the frequency of occurrence of the highest Tn increases at some stations and decreases at others. This implies that in the “present climate” (after 1977) there is a greater frequency of occurrence of high values of Tn at Observatorio Central Buenos Aires and Río Gallegos together with a lower frequency of occurrence of high values of Tx, leading to a decrease in the annual temperature range.

The most noticeable change in return values due to the 1976–77 shift is seen in Patagonia where the 10-yr return value for the highest Tn increases from 13.7°C before 1976 to 18.6°C after 1977. That is, values of the highest Tn that occurred at least once every 10 yr in the “past climate” (before 1976) now happened more than once every 2 yr.

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Bárbara Tencer
,
Andrew Weaver
, and
Francis Zwiers

Abstract

The occurrence of individual extremes such as temperature and precipitation extremes can have a great impact on the environment. Agriculture, energy demands, and human health, among other activities, can be affected by extremely high or low temperatures and by extremely dry or wet conditions. The simultaneous or proximate occurrence of both types of extremes could lead to even more profound consequences, however. For example, a dry period can have more negative consequences on agriculture if it is concomitant with or followed by a period of extremely high temperatures. This study analyzes the joint occurrence of very wet conditions and high/low temperature events at stations in Canada. More than one-half of the stations showed a significant positive relationship at the daily time scale between warm nights (daily minimum temperature greater than the 90th percentile) or warm days (daily maximum temperature above the 90th percentile) and heavy-precipitation events (daily precipitation exceeding the 75th percentile), with the greater frequencies found for the east and southwest coasts during autumn and winter. Cold days (daily maximum temperature below the 10th percentile) occur together with intense precipitation more frequently during spring and summer. Simulations by regional climate models show good agreement with observations in the seasonal and spatial variability of the joint distribution, especially when an ensemble of simulations was used.

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Bárbara Tencer
,
Matilde Rusticucci
,
Phil Jones
, and
David Lister

This study presents a southeastern South American gridded dataset of daily minimum and maximum surface temperatures for 1961–2000. The data used for the gridding are observed daily data from meteorological stations in Argentina, Brazil, Paraguay, and Uruguay from the database of the European Community's Sixth Framework Programme A Europe–South America Network for Climate Change Assessment and Impact Studies in La Plata Basin (EU FP6 CLARIS LPB), with some additional data series. This gridded dataset is new for the region, not only for its spatial and temporal extension, but also for its temporal resolution. The region for which the gridded dataset has been developed is 20°–40°S, 45°–70°W, with a resolution of 0.5° latitude × 0.5° longitude. Since the methodology used produces an estimation of gridbox averages, the developed dataset is very useful for the validation of regional climate models. The comparison of gridded and observed data provides an evaluation of the usefulness of the interpolated data. According to monthly-mean values and daily variability, the methodology of interpolation developed during the EU FP6 ENSEMBLE-based predictions of climate changes and their impacts (ENSEMBLES) project for its application in Europe is also suitable for southeastern South America. Root-mean-square errors for the whole region are 1.77°C for minimum temperature and 1.13°C for maximum temperature. These errors are comparable to values obtained for Europe with the same methodology.

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