Lucie A. Vincent

Global observed changes in daily climate extremes of temperature and precipitation

A suite of climate change indices derived from daily temperature and precipitation data, with a primary focus on extreme events, were computed and analyzed. By setting an exact formula for each index and using specially designed software, analyses done in different countries have been combined seamlessly. This has enabled the presentation of the most up-to-date and comprehensive global picture of trends in extreme temperature and precipitation indices using results from a number of workshops held in data-sparse regions and high-quality station data supplied by numerous scientists world wide. Seasonal and annual indices for the period 1951-2003 were gridded. Trends in the gridded fields were computed and tested for statistical significance. Results showed widespread significant changes in temperature extremes associated with warming, especially for those indices derived from daily minimum temperature. Over 70% of the global land area sampled showed a significant decrease in the annual occurrence of cold nights and a significant increase in the annual occurrence of warm nights. Some regions experienced a more than doubling of these indices. This implies a positive shift in the distribution of daily minimum temperature throughout the globe. Daily maximum temperature indices showed similar changes but with smaller magnitudes. Precipitation changes showed a widespread and significant increase, but the changes are much less spatially coherent compared with temperature change. Probability distributions of indices derived from approximately 200 temperature and 600 precipitation stations, with near-complete data for 1901-2003 and covering a very large region of the Northern Hemisphere midlatitudes (and parts of Australia for precipitation) were analyzed for the periods 1901-1950, 1951-1978 and 1979-2003. Results indicate a significant warming throughout the 20th century. Differences in temperature indices distributions are particularly pronounced between the most recent two periods and for those indices related to minimum temperature. An analysis of those indices for which seasonal time series are available shows that these changes occur for all seasons although they are generally least pronounced for September to November. Precipitation indices show a tendency toward wetter conditions throughout the 20th century.

Temperature and precipitation trends in Canada during the 20th century

Abstract Trends in Canadian temperature and precipitation during the 20th century are analyzed using recently updated and adjusted station data. Six elements, maximum, minimum and mean temperatures along with diurnal temperature range (DTR), precipitation totals and ratio of snowfall to total precipitation are investigated. Anomalies from the 1961–1990 reference period were first obtained at individual stations, and were then used to generate gridded datasets for subsequent trend analyses. Trends were computed for 1900–1998 for southern Canada (south of 60°N), and separately for 1950–1998 for the entire country, due to insufficient data in the high arctic prior to the 1950s. From 1900–1998, the annual mean temperature has increased between 0.5 and 1.5°C in the south. The warming is greater in minimum temperature than in maximum temperature in the first half of the century, resulting in a decrease of DTR. The greatest warming occurred in the west, with statistically significant increases mostly seen during spring and summer periods. Annual precipitation has also increased from 5% to 35% in southern Canada over the same period. In general, the ratio of snowfall to total precipitation has been increasing due mostly to the increase in winter precipitation which generally falls as snow and an increase of ratio in autumn. Negative trends were identified in some southern regions during spring. From 1950–1998, the pattern of temperature change is distinct: warming in the south and west and cooling in the northeast, with similar magnitudes in both maximum and minimum temperatures. This pattern is mostly evident in winter and spring. Across Canada, precipitation has increased by 5% to 35%, with significant negative trends found in southern regions during winter. Overall, the ratio of snowfall to total precipitation has increased, with significant negative trends occurring mostly in southern Canada during spring. Indices of abnormal climate conditions are also examined. These indices were defined as areas of Canada for 1950–1998, or southern Canada for 1900–1998, with temperature or precipitation anomalies above the 66th or below the 34th percentiles in their relevant time series. These confirmed the above findings and showed that climate has been becoming gradually wetter and warmer in southern Canada throughout the entire century, and in all of Canada during the latter half of the century.

HOMOGENEITY ADJUSTMENTS OF IN SITU ATMOSPHERIC CLIMATE DATA: A REVIEW

Long-term in situ observations are widely used in a variety of climate analyses. Unfortunately, most decade- to century-scale time series of atmospheric data have been adversely impacted by inhomogeneities caused by, for example, changes in instrumentation, station moves, changes in the local environment such as urbanization, or the introduction of different observing practices like a new formula for calculating mean daily temperature or different observation times. If these inhomogeneities are not accounted for properly, the results of climate analyses using these data can be erroneous. Over the last decade, many climatologists have put a great deal of effort into developing techniques to identify inhomogeneities and adjust climatic time series to compensate for the biases produced by the inhomogeneities. It is important for users of homogeneity-adjusted data to understand how the data were adjusted and what impacts these adjustments are likely to make on their analyses. And it is important for developers of homogeneity-adjusted data sets to compare readily the different techniques most commonly used today. Therefore, this paper reviews the methods and techniques developed for homogeneity adjustments and describes many different approaches and philosophies involved in adjusting in situ climate data.

Trends in Total and Extreme South American Rainfall in 1960–2000 and Links with Sea Surface Temperature

A weeklong workshop in Brazil in August 2004 provided the opportunity for 28 scientists from southern South America to examine daily rainfall observations to determine changes in both total and extreme rainfall. Twelve annual indices of daily rainfall were calculated over the period 1960 to 2000, examining changes to both the entire distribution as well as the extremes. Maps of trends in the 12 rainfall indices showed large regions of coherent change, with many stations showing statistically significant changes in some of the indices. The pattern of trends for the extremes was generally the same as that for total annual rainfall, with a change to wetter conditions in Ecuador and northern Peru and the region of southern Brazil, Paraguay, Uruguay, and northern and central Argentina. A decrease was observed in southern Peru and southern Chile, with the latter showing significant decreases in many indices. A canonical correlation analysis between each of the indices and sea surface temperatures (SSTs) revealed two large-scale patterns that have contributed to the observed trends in the rainfall indices. A coupled pattern with ENSO-like SST loadings and rainfall loadings showing similarities with the pattern of the observed trend reveals that the change to a generally more negative Southern Oscillation index (SOI) has had an important effect on regional rainfall trends. A significant decrease in many of the rainfall indices at several stations in southern Chile and Argentina can be explained by a canonical pattern reflecting a weakening of the continental trough leading to a southward shift in storm tracks. This latter signal is a change that has been seen at similar latitudes in other parts of the Southern Hemisphere. A similar analysis was carried out for eastern Brazil using gridded indices calculated from 354 stations from the Global Historical Climatology Network (GHCN) database. The observed trend toward wetter conditions in the southwest and drier conditions in the northeast could again be explained by changes in ENSO.

Observed Trends in Indices of Daily Temperature Extremes in South America 1960–2000

Abstract A workshop on enhancing climate change indices in South America was held in Maceio, Brazil, in August 2004. Scientists from eight southern countries brought daily climatological data from their region for a meticulous assessment of data quality and homogeneity, and for the preparation of climate change indices that can be used for analyses of changes in climate extremes. This study presents an examination of the trends over 1960–2000 in the indices of daily temperature extremes. The results indicate no consistent changes in the indices based on daily maximum temperature while significant trends were found in the indices based on daily minimum temperature. Significant increasing trends in the percentage of warm nights and decreasing trends in the percentage of cold nights were observed at many stations. It seems that this warming is mostly due to more warm nights and fewer cold nights during the summer (December–February) and fall (March–May). The stations with significant trends appear to be loca...

Characteristics of Daily and Extreme Temperatures over Canada

Recent studies have shown that, since 1900, mean annual temperature over southern Canada has increased by an average of 0.98C, with the largest warming during winter and early spring. Every season was associated with greater increases in minimum temperature as opposed to maximum, thus resulting in a significant decrease in the daily temperature range (DTR). The second half of the twentieth century was associated with significant winter and spring warming in the south and west, and cooling in the northeast. However, no significant changes in DTR were observed during this period. This investigation goes beyond the annual/seasonal scales by examining trends and variability in daily minimum and maximum temperature with particular emphasis on extremes. Using recently updated, homogenized daily data, spatial and temporal characteristics of daily and extreme temperaturerelated variables are analyzed on a seasonal basis for the periods of 1900‐98 (southern Canada), and 1950‐98 (the entire country). From 1900 to 1998, the majority of southern Canada shows significantly increasing trends to the lower and higher percentiles of the daily minimum and maximum temperature distribution. The findings translate into fewer days with extreme low temperature during winter, spring, and summer and more days with extreme high temperature during winter and spring. No consistent trends are found for the higher percentiles of summer daily maximum temperature, indicating little change to the number of extreme hot summer days. Over the southwest, increases are larger to the left-hand side of the daily minimum and maximum temperature distribution, resulting in significant decreases to the intraseasonal standard deviation of daily temperature. The 1950‐98 results are somewhat different from the entire century, especially, during winter and spring. This result includes significant increases to the low and high percentiles over the west, and decreases over the east. This analysis reveals that the largest individual daily temperature trends (both minimum and maximum) occur during winter and early spring, when substantial warming is observed. For summer, increases are only associated with daily minimum temperature. Autumn displays varying results, with some late season cooling, mainly over western regions. The observed warming trends have a substantial effect on several economically sensitive indices. This effect includes significant increases in the number of growing and cooling degree days and significant decreases in heating degree days. In addition, the length of the frost-free period is significantly longer over most of the country.

A Technique for the Identification of Inhomogeneities in Canadian Temperature Series

A new technique has been developed for the identification of inhomogeneities in Canadian temperature series. The objective is to identify two types of inhomogeneities—nonclimatic steps and trends—in the series of a candidate station in the absence of prior knowledge of the time of site changes and to properly estimate their position in time and their magnitude. This new technique is based on the application of four linear regression models in order to determine whether the tested series is homogeneous, if there is a nonclimatic trend, a step, or trends before and/or after a step. The dependent variable is the series of the candidate station and the independent variables are the series of some neighboring stations. Additional independent variables are used to describe and measure steps and trends existing in the tested series but not in the neighboring series. After the application of each model, the residuals are analyzed in order to determine the fit of the model. If there is significant autocorrelation in the residuals, nonidentified inhomogeneities are suspected in the tested series and a different model is applied to the datasets. A model is finally accepted when the residuals are considered to be random variables. The description of the technique is presented along with some evaluation of its ability to identify inhomogeneities. Results are illustrated through the provision of an example of its application to archived temperature datasets.

Changes in Daily and Extreme Temperature and Precipitation Indices for Canada over the Twentieth Century

Abstract This study examines the trends and variations in several indices of daily and extreme temperature and precipitation in Canada for the periods 1950–2003 and 1900–2003 respectively. The indices are based on homogenized daily temperature and adjusted daily precipitation measurements which are special datasets that include adjustments for site relocation, changes in observing programs and corrections for known instrument changes or measurement program deficiencies. For 1950–2003, the analysis of the temperature indices indicates the occurrence of fewer cold nights, cold days and frost days, and conversely more warm nights, warm days and summer days across the country. The results are generally similar for 1900–2003 but they also include a decrease in the diurnal temperature range in southern Canada and a decrease in the standard deviation of the daily mean temperatures for many stations in western Canada. The analysis of the precipitation indices for 1950–2003 reveals more days with precipitation, a decrease in daily intensity and a decrease in the maximum number of consecutive dry days. The annual total snowfall significantly decreased in the south and increased in the north and north‐east during the second half of the twentieth century. The results are generally similar for 1900–2003. The national series for the century shows an increase in annual snowfall from 1900 to the 1970s followed by a considerable decrease until the 1980s which also corresponds to a pronounced downward trend in the frequency of frost days. No consistent changes were found in most of the indices of extreme precipitation for both periods.

Homogenization of Daily Temperatures over Canada

Abstract A method to homogenize daily maximum and minimum temperatures over Canada is presented. The procedure is based on previously defined monthly adjustments derived from step changes identified in annual Canadian temperature series. Daily temperatures are adjusted by incorporating a linear interpolation scheme that preserves these monthly adjustments. The temperature trends and variations present in the homogenized monthly and annual datasets are therefore preserved. Comparisons between unadjusted and adjusted daily temperatures at collocated sites show that the greatest impact of the adjustments is on the annual mean of the daily maximum and minimum temperatures with little effect on the standard deviation. The frequency and distribution of the extremes are much closer to those provided by the target observations after adjustments. Furthermore, the adjusted daily temperatures produced by this procedure greatly improve the spatial pattern of the observed twentieth century extreme temperature trends a...

Canadian historical and homogeneous temperature datasets for climate change analyses

The Canadian Historical Temperature Database (CHTD) was developed to meet the need for detailed individual station datasets and to produce an improved historical climate change database. It contains datasets of monthly mean maximum and minimum temperatures for 210 Canadian stations. Stations were selected on the basis of length of record, data completeness, and spatial distribution across the country. Records from separate stations were sometimes joined to temporally extend their series backward. Missing data gaps were estimated using highly correlated neighbour stations. Relative homogeneity was assessed using a Canadian developed technique based on regression models. Nonclimatic steps resulting from station or site relocations were identified and quantified. Data adjustments were performed for large steps (greater than 0.6°C) with or without metadata and for medium steps (0.4–0.6°C) with support. A bias in minimum temperatures was also identified and adjusted at principal stations located in eastern Canada. The bias results from a change in observing procedure in 1961 throughout the country, and it typically produces a decreasing step of about 0.6–0.8°C in the annual series in the eastern part of the country. Although also detectable in western Canada, it appears to be small there and no bias adjustments were performed in the western part of the country. Large temporal and spatial differences in data availability exist between areas north and south of 60°N latitude making national analyses possible for only the latest 50 years of data. Spatial presentation of the linear trends before and after adjustments shows overall improvement to the regional and national trends in terms of spatial consistency. The CHTD contains the best available monthly temperature data in Canada and these datasets are now available to the climate change research community. Copyright