The variability of results from different automated methods of detection and tracking of extratropical cyclones is assessed in order to identify uncertainties related to the choice of method. Fifteen international teams applied their own algorithms to the same dataset—the period 1989–2009 of interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERAInterim) data. This experiment is part of the community project Intercomparison of Mid Latitude Storm Diagnostics (IMILAST; see www.proclim.ch/imilast/index.html). The spread of results for cyclone frequency, intensity, life cycle, and track location is presented to illustrate the impact of using different methods. Globally, methods agree well for geographical distribution in large oceanic regions, interannual variability of cyclone numbers, geographical patterns of strong trends, and distribution shape for many life cycle characteristics. In contrast, the largest disparities exist for the total numbers of cyclones, the detection of weak cyclones, and distribution in some densely populated regions. Consistency between methods is better for strong cyclones than for shallow ones. Two case studies of relatively large, intense cyclones reveal that the identification of the most intense part of the life cycle of these events is robust between methods, but considerable differences exist during the development and the dissolution phases.

Characteristics of Northern Hemisphere extratropical cyclone activity were compared for five concurrent reanalyses: the NCEP–U.S. Department of Energy (DOE) reanalysis (herein NCEP–DOE), the Japanese 25-year Reanalysis Project (JRA-25), the ECMWF Interim Re-Analysis (ERA-Interim), the National Aeronautics and Space Administration's Modern-Era Retrospective Analysis for Research and Applications (NASA-MERRA), and the NCEP Climate Forecast System Reanalysis (NCEP-CFSR), for the period 1979–2010 using a single cyclone tracking algorithm. The total number of cyclones, ranging from 1400 to more than 1800 yr−1, was found to depend strongly on the spatial resolution of the respective reanalysis. The largest cyclone population was identified using NASA-MERRA data, which also showed the highest occurrence of very deep cyclones. Of the reanalyses, two (NCEP–DOE and ERA-Interim) are associated with statistically significant positive trends in the total number of cyclones from 1% to 2% decade−1. These trends result from moderate and shallow cyclones contributing to approximately 90% of the total cyclone count on average. The number of very deep cyclones (<960 hPa) in the North Atlantic increased in most reanalyses until 1990 and then declined during the last decade. In the North Pacific, the number of these events reached a peak in 2000 and then decreased during the last decade. The winter pattern is characterized by robust trends in cyclone numbers, with an enhancement of the North Atlantic storm track and a weakening of the North Pacific subtropical storm track. In the summer, there is a robust intensification of the Mediterranean storm track and a decrease in counts over the North Atlantic. Interannual variability and decadal-scale variations of the cyclone counts are highly correlated among the reanalyses, with the greatest agreement in moderate and deep cyclones.

The beginning of the 21st century was marked by a number of severe summer floods in Central Europe associated with extreme precipitation (e.g., Elbe 2002, Oder 2010 and Danube 2013). Extratropical storms, known as Vb-cyclones, cause summer extreme precipitation events over Central Europe and can thus lead to such floodings. Vb-cyclones develop over the Mediterranean Sea, which itself strongly warmed during recent decades. Here we investigate the influence of increased Mediterranean Sea surface temperature (SST) on extreme precipitation events in Central Europe. To this end, we carry out atmosphere model simulations forced by average Mediterranean SSTs during 1970–1999 and 2000–2012. Extreme precipitation events occurring on average every 20 summers in the warmer-SST-simulation (2000–2012) amplify along the Vb-cyclone track compared to those in the colder-SST-simulation (1970–1999), on average by 17% in Central Europe. The largest increase is located southeast of maximum precipitation for both simulated heavy events and historical Vb-events. The responsible physical mechanism is increased evaporation from and enhanced atmospheric moisture content over the Mediterranean Sea. The excess in precipitable water is transported from the Mediterranean Sea to Central Europe causing stronger precipitation extremes over that region. Our findings suggest that Mediterranean Sea surface warming amplifies Central European precipitation extremes.

Arctic cyclone activity is analyzed in 11 year (2000–2010), 3‐hourly output from the Arctic System Reanalysis (ASR) interim version. Compared to the global modern era reanalyses (European Centre for Medium‐Range Weather Forecasts Reanalysis (ERA)‐Interim, Modern Era Retrospective Analysis for Research and Applications, and National Centers for Environmental Prediction‐Climate Forecast System Reanalysis), ASR shows a considerably higher number of cyclones over the Arctic with the largest differences over the high‐latitude continental areas (up to 40% in summer and 30% in winter). Over the Arctic Ocean during both seasons ASR captures well the cyclone maximum in the Eastern Arctic which has 30% less cyclones in summer and is hardly detectable in winter in ERA‐Interim. High resolution of the ASR model coupled with more comprehensive data assimilation allows for more accurate (compared to the global reanalyses) description of the life cycle of the most intense Arctic cyclones, for which ASR shows lower central pressure (4 hPa on average), faster deepening, and stronger winds on average.

This work is focused on the seasonal and interannual variability of the freshened surface layer (FSL) in the Kara Sea during ice‐free periods. The majority of annual freshwater runoff inflows to the Kara Sea during the freshet period in June–July. As a result, a large low‐salinity and strongly stratified FSL is formed in the central Kara Sea at the beginning of the ice‐free period. Initially, this layer is fresh with salinities of less than 15. Then, in August–October, river discharge steadily decreases, the fresh (<15) and strongly stratified river plume remains limited to the inner part of the FSL adjacent to the Ob and Yenisei gulfs, which receives reduced freshwater discharge. The area of the fresh plume steadily decreases with decreasing of river discharge; in October, it remains only within the Ob and Yenisei gulfs. Area and position of the outer part of the FSL, on the opposite, remain stable until the end of ice‐free period. Salinities in the outer part steadily increase to 20–25, vertical salinity gradients at the boundary with the subjacent sea decrease, albeit remain prominent. Area and position of the FSL also have low interannual variability, because the volume of river runoff during freshet period in June–July is stable on interannual time scale. As a result, downward freshwater transport and the formation of the seasonal halocline occur in August–October at the stable area located in the central Kara Sea.