Over the first half of 2020, Siberia experienced the warmest period from January to June since records began and on the 20th of June the weather station at Verkhoyansk reported 38 °C, the highest daily maximum temperature recorded north of the Arctic Circle. We present a multi-model, multi-method analysis on how anthropogenic climate change affected the probability of these events occurring using both observational datasets and a large collection of climate models, including state-of-the-art higher-resolution simulations designed for attribution and many from the latest generation of coupled ocean-atmosphere models, CMIP6. Conscious that the impacts of heatwaves can span large differences in spatial and temporal scales, we focus on two measures of the extreme Siberian heat of 2020: January to June mean temperatures over a large Siberian region and maximum daily temperatures in the vicinity of the town of Verkhoyansk. We show that human-induced climate change has dramatically increased the probability of occurrence and magnitude of extremes in both of these (with lower confidence for the probability for Verkhoyansk) and that without human influence the temperatures widely experienced in Siberia in the first half of 2020 would have been practically impossible.

Regional warming and modifications in precipitation regimes has large impacts on streamflow in Norway, where both rainfall and snowmelt are important runoff generating processes. Hydrological impacts of recent changes in climate are usually investigated by trend analyses applied on annual, seasonal, or monthly time series. None of these detect sub-seasonal changes and their underlying causes. This study investigated sub-seasonal changes in streamflow, rainfall, and snowmelt in 61 and 51 catchments respectively in Western (Vestlandet) and Eastern (Østlandet) Norway by applying the Mann-Kendall test and Theil-Sen estimator on 10-day moving averaged daily time series over a 30-year period . The relative contribution of rainfall versus snowmelt to daily streamflow and the changes therein have also been estimated to identify the changing relevance of these driving processes over the same period. Detected changes in 10-day moving averaged daily streamflow were finally attributed to changes in the most important hydro-meteorological drivers using multiple-regression models with increasing complexity. Earlier spring flow timing in both regions occur due to earlier snowmelt. Østlandet shows increased summer streamflow in catchments up to 1100 m a.s.l. and slightly increased winter streamflow in about 50% of the catchments. Trend patterns in Vestlandet are less coherent. The importance of rainfall has increased in both regions. Attribution of trends reveals that changes in rainfall and snowmelt can explain some streamflow changes where they are dominant processes (e.g., spring snowmelt in Østlandet and autumn rainfall in Vestlandet). Overall, the detected streamflow changes can be best explained by adding temperature trends as an additional predictor, indicating the relevance of additional driving processes such as increased glacier melt and evapotranspiration.

<p>Under the influence of climate change, high mountain areas like the European Alps are in a transient state where catchment conditions and processes that determine sediment dynamics are changing. Hydro-sedimentary events can account for a substantial proportion the annual sediment yield in alpine catchments, and are often associated with heavy rainfall and rainfall-triggered mass-movements. It is therefore of interest to study the driving conditions and processes of these events, especially due to the potential downstream impacts they can have to eco- and human systems.</p> <p>The dynamics, characteristics and, in particular, (suspended) sediment-discharge hysteresis are often used in conjunction with hydro-meteorological and catchment state variables to identify driving processes and conditions of events. However, is it possible to elucidate the determining conditions and processes or determine meaningful event classes based solely on metrics derived from the suspended sediment and discharge data of the event? </p> <p>Using two catalogs of manually and automatically detected hydro-sedimentary events from Rofental, Austria, we attempt to answer this question. We perform a cluster analysis with various approaches on event metrics (e.g. hysteresis class, suspended sediment yield, peak discharge, time since last event). To avoid biasing the results towards a specific number of event types, we explicitly use clustering algorithms which do not require the number of clusters (i.e. event types) to be specified. We then look for commonalities within the identified event clusters in terms of catchment conditions and processes during the event (e.g. high temperatures, snowmelt, intense rainfall, wet antecedent conditions, mass movement occurrences). Finally, we discuss the advantages and disadvantages of grouping events on their characteristics alone.</p>

<p>Mountainous and Nordic regions are experiencing more rapid temperature increases as compared to regions at lower altitudes and latitudes. This will impact the hydrology in these regions.  For Norway, there is increasing evidence for gradually increasing temperatures and recent changes in the amount, intensity, and frequency of precipitation as well as in the number of days with snow cover. The most pronounced differences regarding their hydro-meteorological regime can be found between Western and Eastern Norway (Vestlandet vs. Østlandet). Most catchments in these regions are characterized by mixed snowmelt/rainfall streamflow regimes with peak flows during spring (dominant in Østlandet) and autumn (dominant in Vestlandet). Changes in the hydro-meteorological drivers will have direct implications on the snow regime, and thus, also on streamflow via their direct effect on the relative importance of snowmelt vs. rainfall for streamflow generation.</p> <p>In this study, we analyze daily-resolved streamflow trends for 112 catchments in Western vs. Eastern Norway for the period 1983-2012 and compare them with daily-resolved trends in the hydro-meteorological drivers. We also estimate the relative contribution of snowmelt and rainfall on daily streamflow for each catchment and identify trends therein. This process-orientated approach at high temporal resolution allows for a better identification of (in)consistencies with changes in the hydro-meteorological drivers than simple seasonal comparisons. Lastly, we aim to attribute observed changes in daily streamflow to the most dominant hydro-meteorological drivers by applying seasonal multiple-regressions. The major findings of this study are as follows:</p> <ul> <li>The high-resolution trend analysis allows for in-depth seasonal-specific insights into the hydrological response of catchments with different hydrological regimes to changes in the hydro-meteorological drivers.</li> <li>Increasing (decreasing) contributions of rainfall (snowmelt) to streamflow generally agree with prior expectations. The trends, however, show differences in magnitude and timing, depending on the geographical location (Vestlandet vs. Østlandet) and altitude.</li> <li>The seasonal multiple regression approach suggests that daily streamflow changes can be explained best by adding temperature as an additional predictor to snowmelt and rainfall, which may indicate the changing relevance of evapotranspiration particularly during summer.</li> </ul>

Climate change in terms of regional warming and modifications in precipitation regimes has large impacts on streamflow in regions where both rainfall and snowmelt are important runoff generating processes like in Norway. Hydrological impacts of recent changes in climate are usually investigated by trend analyses applied on annual, seasonal, or monthly time series. However, neither of them can detect sub-seasonal changes and their underlying causes. Based on high-resolution trend analyses (i.e., applying the Mann-Kendall test on 10-day-moving-averaged daily time series), this study investigated sub-seasonal changes in daily streamflow, rainfall, and snowmelt in 61 and 51 catchments in Western vs. Eastern Norway (Vestlandet vs. Østlandet), respectively, over the period 1983-2012. The relative contribution of rainfall vs. snowmelt to daily streamflow and the changes therein have also been estimated to identify the changing relevance of these driving processes over the same period. Detected changes in daily streamflow were finally attributed to changes in the most important hydro-meteorological drivers using multiple-regression models with increasing complexity. Results reveal a coherent picture of earlier spring flow timing in both regions due to earlier snowmelt. Other streamflow trend patterns differ between both regions: Østlandet shows increased summer streamflow in catchments up to ~1100 m a.s.l. and slightly increased winter streamflow in about 50 % of the catchments, while trend patterns in Vestlandet are less coherent. The importance of rainfall for streamflow contribution has increased in both regions, and the trend attribution reveals that changes in rainfall and snowmelt can explain streamflow changes to some degree in periods and regions where they are dominant (snowmelt: spring and Østlandet; rainfall: autumn and Vestlandet). However, detected streamflow changes can be best explained by adding temperature as an additional predictor which indicates the relevance of additional driving processes for streamflow changes like increased glacier melt and evapotranspiration.