Trends in extreme precipitation and return values in Norway 1900-2004 (original) (raw)
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Trend of extreme precipitation in Sweden and Norway during 1961-2004
The question whether climate is about to become more extreme has motivated numerous studies in different parts of the world both based on model simulation as well as on observations. A frequently used approach is to derive a set of climate indices based on daily temperature and precipitation data in order to quantify various statistics related to the mean climate, as well as moderate and severe extremes. These indices can then be used to study various aspects of their spatial distribution and temporal variability such as trends. The overall aim of this report is to study the spatial patterns and long term trends of precipitation indices in Sweden and Norway on the annual and seasonal basis for the years 1961 to 2004. These indices are based on daily data from 471 stations. Among these indices are simple measures characterizing basic precipitation statistics like the frequency of rain days and the mean precipitation amount of rainy days, as well as various measures for moderate to severe extremes like percentile-based thresholds, maximum precipitation over a certain number of days, maximum number of consecutive wet days and number of very wet days (≥40 mm). Earlier studies applying these or slightly different indices did include stations from Nordic countries, however, none of them dealt more specifically with the conditions in Scandinavia. The spatial distribution of these indices shows considerably variability across Sweden and Norway, both for annual indices and in different seasons. Especially the Norwegian West Coast emerges as a region with the highest mean amounts and shows often the highest extremes as well. Over Sweden, spatial variability is less pronounced and rainfall means and extremes are generally more moderate. Over the course of a year, the annual spatial pattern is somewhat modulated with rising extreme in inland regions and decreased extremes in coastal areas close to the West Coast. Analyzing the trends of the various indices for the period 1961-2004 shows that magnitude and sign of the trends vary depending on index, region and season. A clear majority of stations show increasing trends, though the fraction having statistically significant trends is small. In Norway, positive trends are most common during winter, while at Swedish stations, positive trends are most frequent in spring and summer. Autumn has the highest number of stations in both countries with negative trends. The findings are generally in line with results from other studies concluding that regions at middle and higher latitudes are becoming wetter and extremes are becoming more frequent and more intense.
Future changes in atmospheric rivers and extreme precipitation in Norway
Climate Dynamics
Flooding events associated with extreme precipitation have had large impacts in Norway. It is well known that these heavy precipitation events affecting Norway (and other parts of Europe) are strongly associated with atmospheric rivers (ARs). We assess trends in Norwegian AR characteristics, and the influence of AR variability on extreme precipitation in Norway. We first evaluate the ability of a high-resolution global climate model (EC-Earth) to simulate ARs, compared to ERA-Interim. We evaluate the EC-Earth simulated relationship between ARs and extreme precipitation in western Norway, compared to the observed relationship. We find that EC-Earth is able to simulate well the statistics of AR events and the related precipitation. The intensity and frequency of ARs making landfall in Norway both increase by the end of the century and we find a shift in seasonality of AR events in the future period. In two regions on the west coast, the majority of winter precipitation maxima are asso...
Scandinavian floods: From past observations to future trends
Global and Planetary Change, 2014
Although most climate models agree on a general increase in future precipitation in the Northern Hemisphere due to higher temperatures, no consensus has yet been reached on how this warming will perturb flooding rates. Here we examine the potential co-variability between winter precipitation (Pw) and floods on millennial time scales. This is accomplished by analyzing reconstructed Pwfromfive records in Scandinavia, which is, compared to data from two high-resolution flood records from southern Norway. These Holocene records reveal a positive correlation (Rsq = 0.41, p N 0.01) between the number of floods and Pw on centennial time scales over the last 6000 years. Future projections for Pwover central Scandinavia for the next 100 years suggest a continued increase in Pw that approximates maximum Holocene precipitation values. Despite an anticipated increase in Pw, the paleodata, nevertheless, suggest that we are likely to witness a decrease in future floods 50–100 years from now because the accompanying warming will cancel that net effect of a wetter regime.
Projecting future local precipitation and its extremes for Sweden
2014
A procedure to obtain future local precipitation characteristics focused on extreme conditions has been developed based on a weather generator. The method involves six major steps: (1) the weather generator was calibrated using observed daily precipitation at 220 Swedish stations during 1961–2004; (2) present and future daily precipitation characteristics for the Swedish stations from two global climate models, namely ECHAM5 and HadCM3, were used to calculate weather generator parameters for the present and future climates at global climate model spatial scales; (3) the ratio of the weather generator parameters for the present climate simulated by the global climate models to those calculated for each station falling into the global climate model grid box were computed for all the stations; (4) these ratios were also assumed to be valid in the future climate, that way the future parameters for each station for the global climate model projected future climate could be calculated; (5) using the estimated future parameters of the weather generator, the future daily precipitation at each station could be simulated by the weather generator; (6) the simulated daily precipitation was used to compute eight indices describing mean and extreme precipitation climates. The future mean and extreme precipitation characteristics at the stations under the Second Report on Emission Scenarios A2 scenario were obtained and presented. An overall increasing trend for frequency and intensity of the indices are identified for the majority of the stations studied. The developed downscaling methodology is relatively simple but useful in deriving local precipitation changes, including changes in the precipitation extremes.
This paper presents the background, objectives, and preliminary outcomes from the first year of activities of the Polish–Norwegian project CHIHE (Climate Change Impact on Hydrological Extremes). The project aims to estimate the influence of climate changes on extreme river flows (low and high) and to evaluate the impact on the frequency of occurrence of hydrological extremes. Eight " twinned " catchments in Po-land and Norway serve as case studies. We present the procedures of the catchment selection applied in Norway and Poland and a database consisting of near-natural ten Polish and eight Norwegian catchments constructed for the purpose of climate impact assessment. Climate projections for selected catchments are described and compared with observations of temperature and precipitation available for the reference period. Future changes based on those projections are analysed and assessed for two periods, the near future (2021-2050) and the far-future (2071-2100). The results indicate increases in precipitation and temperature in the periods and regions studied both in Poland and Norway.
Climate impact on floods: changes in high flows in Sweden in the past and the future (1911–2100)
There is an ongoing discussion whether floods occur more frequently today than in the past, and whether they will increase in number and magnitude in the future. To explore this issue in Sweden, we merged observed time series for the past century from 69 gauging sites throughout the country (450 000 km2) with high-resolution dynamic model projections of the upcoming century. The results show that the changes in annual maximum daily flows in Sweden oscillate between dry and wet periods but exhibit no significant trend over the past 100 years. Temperature was found to be the strongest climate driver of changes in river high flows, which are related primarily to snowmelt in Sweden. Annual daily high flows may decrease by on average 1% per decade in the future, mainly due to lower peaks from snowmelt in the spring (2% per decade) as a result of higher temperatures and a shorter snow season. In contrast, autumn flows may increase by C3% per decade due to more intense rainfall. This indicates a shift in floodgenerating processes in the future, with greater influence of rain-fed floods. Changes in climate may have a more significant impact on some specific rivers than on the average for the whole country. Our results suggest that the temporal pattern in future daily high flow in some catchments will shift in time, with spring floods in the northern–central part of Sweden occurring about 1 month earlier than today. High flows in the southern part of the country may become more frequent. Moreover, the current boundary between snow-driven floods in northern–central Sweden and rain-driven floods in the south may move toward higher latitudes due to less snow accumulation in the south and at low altitudes. The findings also indicate a tendency in observations toward the modeled projections for timing of daily high flows over the last 25 years. Uncertainties related to both the observed data and the complex model chain of climate impact assessments in hydrology are discussed.