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• Rizzi, Jonathan; Nilsen, Irene Brox; Stagge, James Howard; Gisnås, Kjersti Gleditsch & Tallaksen, Lena M. (2018). Five decades of warming: impacts on snow cover in Norway. Hydrology Research.  ISSN 1998-9563.  49(3), s 670- 688 . doi: 10.2166/nh.2017.051 Vis sammendrag

  Northern latitudes are experiencing faster warming than other regions in the world, which is partly explained by the snow albedo feedback. In Norway, mean temperatures have been increasing since the 1990s, with 2014 being the warmest year on record, 2.2 °C above normal (1961–1990). At the same time, a concurrent reduction in the land area covered by snow has been reported. In this study, we present a detailed spatial and temporal (monthly and seasonal) analysis of trends and changes in snow indices based on a high resolution (1 km) gridded hydro-meteorological dataset for Norway (seNorge). During the period 1961–2010, snow cover extent (SCE) was found to decrease, notably at the end of the snow season, with a corresponding decrease in snow water equivalent except at high elevations. SCE for all Norway decreased by more than 20,000 km2 (6% of the land area) between the periods 1961–1990 and 1981–2010, mainly north of 63° N. Overall, air temperature increased in all seasons, with the highest increase in spring (particularly in April) and winter. Mean monthly air temperatures were significantly correlated with the monthly SCE, suggesting a positive land–atmosphere feedback enhancing warming in winter and spring.

• Nilsen, Irene Brox; Stagge, James Howard & Tallaksen, Lena M. (2017). A probabilistic approach for attributing temperature changes to synoptic type frequency. International Journal of Climatology.  ISSN 0899-8418.  37(6), s 2990- 3002 . doi: 10.1002/joc.4894 Vis sammendrag

  To understand the cause of regional temperature change, it is common to separate the temperature change signal into changes in atmospheric synoptic circulation and other factors, so-called within-type changes. In this study, we suggest a novel probabilistic approach that allows detection of months and regions where temperature changes can mainly be attributed to changes in synoptic circulation and where within-type changes also play a role. By combining resampling with a Monte Carlo test, we assess the likelihood that the observed warming can be explained by synoptic circulation changes alone. This method is applicable for any variable, and in any region of the world. We applied it to an example case using gridded WATCH Forcing Data ERA-Interim (WFDEI) temperature data and synoptic types derived from the SynopVis Grosswetterlagen catalogue (1981–2010). For this European example, the most widespread warming was found in summer, with up to 60% of the land area experiencing signicant warming during August, notably in Eastern and Northern Europe. In spring and autumn, this area was reduced to 10–30%. In December and January, only about 5% of the land area experienced signicant warming, most pronounced in northern Scandinavia. The probabilistic approach revealed that changes in synoptic circulation c ould not account for all the observed (WFDEI) warming, with the exception of regions in southeastern Europe in February and Western Europe in May. Signicant warming in other months and regions, such as the large-scale warming in April, June, July, August, and November, must also be caused by other factors. Within-type changes were conrmed for the Black Sea region in November, where the magnitude of a widespread temperature trend was strongest. This European example contributes to an improved understanding of the causes of recent temperature change by assessing the relative role of synoptic circulation changes and within-type changes on regional-scale warming.

• Nilsen, Irene Brox; Fleig, Anne K.; Tallaksen, Lena M. & Hisdal, Hege (2014). Recent trends in monthly temperature and precipitation patterns in Europe. IAHS-AISH publication.  ISSN 0144-7815.  363, s 132- 137

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• Nilsen, Irene Brox; Tallaksen, Lena M. & Stordal, Frode (2017). Potential feedbacks between snow cover, soil moisture and surface energy fluxes in Southern Norway. Vis sammendrag

  At high latitudes, the snow season has become shorter during the past decades because snowmelt is highly sensitive to a warmer climate. Snowmelt influences the energy balance by changing the albedo and the partitioning between latent and sensible heat fluxes. It further influences the water balance by changing the runoff and soil moisture. In a previous study, we identified southern Norway as a region where significant temperature changes in summer could potentially be explained by land-atmosphere interactions. In this study we hypothesise that changes in snow cover would influence the summer surface fluxes in the succeeding weeks or months. The exceptionally warm summer of 2014 was chosen as a test bed. In Norway, evapotranspiration is not soil moisture limited, but energy limited, under normal conditions. During warm summers, however, such as in 2014, evapotranspiration can be restricted by the available soil moisture. Using the Weather Research and Forecasting (WRF) model we replace the initial ground conditions for 2014 with conditions representative of a snow-poor spring and a snow-rich spring. WRF was coupled to Noah-MP at 3 km horizontal resolution in the inner domain, and the simulations covered mid-May through September 2014. Boundary conditions used to force WRF were taken from the Era-Interim reanalysis. Snow, runoff, soil moisture and soil temperature observational data were provided by the Norwegian Water Resources and Energy Directorate for validation. The validation shows generally good agreement with observations. Preliminary results show that the reduced snowpack, hereafter “sim1” increased the air temperature by up to 5 K and the surface temperature by up to 10 K in areas affected by snow changes. The increased snowpack, hereafter “sim2”, decreased the air and surface temperature by the same amount. These are weekly mean values for the first eight simulation weeks from mid May. Because of the higher net energy available (∼ 100 Wm-2) in sim 1, both the evapotranspiration and sensible heat fluxes increased. In sim 2, they decreased because of lower net energy. The ground heat flux decreased in sim1 (and increased in sim2). Large increases were seen in runoff, both surface and underground runoff, during the first weeks of sim2 (from mid May), but the timing of snowmelt was only slightly affected. This study contributes to a greater understanding of land-atmosphere interactions in a wet, temperate climate, in particular the role of snow cover (and snowmelt) and its feedback to the atmosphere.

• Girod, Luc Maurice Ramuntcho & Nilsen, Irene Brox (2016). ClimaByte, an Educational Youtube Channel About Climate Sciences. [Video ]. Vis sammendrag

  ClimaByte is an educational YouTube Channel that will explore numerous subjects related to climate sciences, geosciences and the tools associated with them. This channel is created by young scientists at the University of Oslo, Norway.

• Nilsen, Irene Brox; Stagge, James Howard & Tallaksen, Lena M. (2016). A probabilistic approach to attribute warming to changes in atmospheric circulation. Vis sammendrag

  Europe has been warming over the past decades, especially in southern Europe in summer and northern Europe in winter. To understand the causes of regional warming, it is common to separate the temperature change signal into changes in atmospheric circulation (or dynamic causes) and other factors, so-called within-type changes (or thermodynamic causes). For example, increasing temperatures due to greenhouse gases may alter the position and strength of the polar jet stream, thus causing a change in the atmospheric circulation signal. On the other hand, warming may be entirely independent of circulation, occurring as a general increase in surface temperature. With the aim to detect regions and time of the year in Europe in which recent warming can either be explained by changes in atmospheric circulation or by within-type changes, we suggest a novel probabilistic approach to calculate the circulation-induced trend, the part of the temperature trend that is induced by changes in atmospheric circulation. Through the use of resampling, in combination with a Monte Carlo test, we assessed the likelihood that the observed temperature trend can be explained entirely by changes in atmospheric circulation frequency. The temperature data originate from the gridded 0.5◦ Watch Forcing Data Era-Interim (WFDEI), and cover the period 1981–2010. The SynopVis Grosswetterlagen catalogue of circulation types was used to detect circulation-induced trends in the same time period. We analysed trends on the monthly time scale to reveal short-term responses, such as those related to snow or greening. The most wide-spread observed warming was found in summer, with up to 60% of the European land area experiencing a significant warming trend during August, most notable in eastern and northern Europe. In spring and autumn, the percent area with significant temperature increases reduced to 10–30%. In December and January, only 5% of the land area experienced significant warming, most pronounced in northern Scandinavia. The probabilistic approach revealed that changes in atmospheric circulation could not account for all the observed warming. Regions where the observed trend likely can be explained entirely by changes in atmospheric circulation include western Europe in May, eastern Europe in August and Scandinavia in September. In most of the regions and months experiencing significant trends, however, warming must be caused by other factors as well; such within-type changes potentially driven by feedbacks between the land surface and atmosphere. For two cases where the magnitude of the temperature trend was strongest, northern Scandinavia in December and in the Black Sea region in November, circulation types warmed over time. This confirms the role of within-type change, i.e., that circulation types changed their properties over time. This study provides an important contribution toward improved understanding of the causes of synoptic-scale temperature change in Europe, more specifically, the relative role of circulation-induced changes and within-type changes.

• Nilsen, Irene Brox; Tallaksen, Lena M. & Stagge, James Howard (2015). A probabilistic method of calculating circulation-induced trends. Vis sammendrag

  The water cycle in Europe has changed substantially over the past three decades. Increasing runoff is observed during winter and at northern latitudes in particular. Spring and summer months, as well as southern latitudes, are facing drier conditions. To understand what is driving large-scale changes in runoff, we look into changes in precipitation and temperature and link these to changes in atmospheric circulation. Previous studies have used the method of trend ratios (Cahynová and Huth, 2009) to attribute precipitation and temperature trends to changes in the frequency of circulation types. A trend ratio is the ratio of hypothetical trend, i.e., the trend that would result due to changes in circulation type frequency only, to the observed trend. However, the method of trend ratios has two drawbacks. First, if the observed trend is small, division by a very low value results in a meaningless trend ratio and thus requires a cut-off value to keep the trend ratio within meaningful boundaries. Second, the method does not allow a comparison of the observed trend to the spread of possible outcomes, because the method of hypothetical trends is based on a deterministic model. We propose a new, more robust method for detecting the importance of circulation-induced changes in explaining the observed trends, which has the benefit of being a non-parametric statistical test that assesses the entire range of hypothetical trends. Instead of creating a hypothetical series by replacing the observation on a given day with the long-term climatic mean of a certain month and circulation type (as in the existing trend ratio method), the new approach replaces the observation on a given day with a random sample from the distribution of the variable for the given month and circulation type. The method introduces the possibility to assign a rejection rate, thus allowing statistical significance to be assessed. We apply the method on time series of precipitation and temperature from the gridded 0.5 degree WFDEI dataset, covering Europe (40-65N, 10W-30E). The SynopVis Grosswetterlagen catalogue of circulation types for the time period 1981-2010, the same period as the climatic data, is used. The new approach is used to map in which regions and months changes in atmospheric circulation is the dominating factor controlling changes in precipitation and temperature in Europe.

• Nilsen, Irene Brox (2014). Does NOAH-LSM perform well for extremes, regarding heat fluxes?. Vis sammendrag

  As the climate and land surface in certain regions of Europe is drying or becoming wetter, certain feedback mechanisms may contribute to enhancing or reducing the effects on climate. For instance, soil moisture–precipitation feedbacks may act as a driving force to dry southern parts of Europe and to moisten northern parts. Understanding the partitioning of net radiation into turbulent fluxes into sensible and latent heat is important to represent these processes. In this project, NOAH will be run using ERA-Interim forcing data for four years covering both extremely wet and dry periods for regions in southern as well as northern Europe. In the first part, maps of the Bowen ratio will be plotted to establish the distribution of turbulent fluxes for wet and dry conditions. Second, the results will be compared with precipitation and temperature trends found in my PhD study to identify if possible concurrent changes in heat fluxes can explain the trends. The project contributes to understanding the driving forces controlling water availability which is important for water shortage mitigation.

• Nilsen, Irene Brox (2014). Hva gjør en klimaforsker?.
• Nilsen, Irene Brox (2014). Landoverflatemodellering og varmeflukser under hydrologiske ekstremer.
• Nilsen, Irene Brox & Tallaksen, Lena M. (2014). Temperature changes in Norway: Recent increases in WFDEI validated against interpolated observations. Vis sammendrag

  Increased global temperatures have been observed during the past decades, and trends are expected to be greatest at high latitudes because of the polar amplification. Polar amplification may be caused by feedbacks between the atmosphere and the surface, such as the ice–albedo feedback: when increased temperatures cause increased snow melting, areas with a lower albedo are exposed and more radiation is absorbed. Thus, the temperature increase is enhanced. In a previous study, we analysed precipitation and temperature trends for the period 1979–2009 using gridded, bias-corrected re-analysis data from the WATCH Forcing Data Era-Interim (WFDEI). The data used consisted of daily temperature and precipitation data from 1 January 1979 to 31 December 2009 (31 years) and covered 34◦ –72◦ N and 13◦ W–32◦ E with a spatial resolution of 0.5◦ × 0.5◦ . Temperature and precipitation trends covering all of Europe were mapped using the Theil-Sen slope as a robust trend estimator. We found the greatest warming in Europe in the northernmost parts of Norway during December and January. The temperature trends for certain months and regions were as high as 8 ◦ C. In this study, monthly temperature trends from WFDEI will be compared with and validated against a daily interpolated dataset for Norway (presented at www.seNorge.no). This dataset has a spatial resolution of 1 × 1 km and covers mainland Norway. Residual kriging is applied to the data to ensure stationarity and remove bias. The daily data will be aggregated to the monthly time scale and monthly temperature trends will be calculated using the Theil-Sen slope for the period 1979–2009. The results will be compared visually with the WFDEI trends covering Norway. In addition, the distribution of trends as well as the fraction of positive and negative trends in the two datasets will be compared.

• Nilsen, Irene Brox; Tallaksen, Lena M.; Hisdal, Hege & Fleig, Anne Kristina (2014). Regional precipitation and temperature trend patterns in Europe (presentasjon av Paper I: Recent trends in monthly temperature and precipitation patterns in Europe).
• Tallaksen, Lena M. & Nilsen, Irene Brox (2014). Norge er i gjennomsnitt blitt mer enn to grader varmere fra 1979 til 2009. Aftenposten Viten. Vis sammendrag

  Den største oppvarmingen finner vi om vinteren, med hele fem grader i januar. Har vi kunnskapen og beredskapen til å møte konsekvensene?

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