Seasonal Variation of Mass Transport Across the Tropopause

The annual cycle of the net mass transport across the extratropical tropopause is examined. Contributions from both the global-scale meridional circulation and the mass variation of the lowermost stratosphere are included. For the northern hemisphere the mass of the lowermost stratosphere has a distinct annual cycle, whereas for the southern hemisphere, the corresponding variation is weak. The net mass transport across the tropopause in the northern hemisphere has a maximum in late spring and a distinct minimum in autumn. This variation and its magnitude compare well with older estimates based on representative Sr-90 mixing ratios. For the southern hemisphere the seasonal cycle of the net mass transport is weaker and follows roughly the annual variation of the net mass flux across a nearby isentropic surface

Challenges posed by and approaches to the study of seasonal-to-decadal climate variability

The tasks of providing multi-decadal climate projections and seasonal plus sub-seasonal climate predictions are of significant societal interest and pose major scientific challenges. An outline is presented of the challenges posed by, and the approaches adopted to, tracing the possible evolution of the climate system on these various time-scales. First an overview is provided of the nature of the climate system's natural internal variations and the uncertainty arising from the complexity and non-linearity of the system. Thereafter consideration is given sequentially to the range of extant approaches adopted to study and derive multi-decadal climate projections, seasonal predictions, and significant sub-seasonal weather phenomena. For each of these three time-scales novel results are presented that indicate the nature (and limitations) of the models used to forecast the evolution, and illustrate the techniques adopted to reduce or cope with the forecast uncertainty. In particular, the contributions (i) appear to exemplify that in simple climate models uncertainties in radiative forcing outweigh uncertainties associated with ocean models, (ii) examine forecast skills for a state-of-the-art seasonal prediction system, and (iii) suggest that long-lived weather phenomena can help shape intra-seasonal climate variability. Finally, it is argued, that co-consideration of all these scales can enhance our understanding of the challenges associated with uncertainties in climate predictio

North Atlantic oscillation modulates total ozone winter trends

The North Atlantic Oscillation (NAO) is modulating the Earth's ozone shield such that the calculated anthropogenic total ozone decrease is enhanced over Europe whereas over the North Atlantic region it is reduced (for the last 30 years). Including the NAO in a statistical model suggests a more uniform chemical winter trend compared to the strong longitudinal variation reported earlier. At Arosa (Switzerland) the trend is reduced to −2.4% per decade compared to −3.2% and at Reykjavik (Iceland) it is enhanced to −3.8% compared to 0%. The revised trend is slightly below the predictions by 2D chemical models. Decadal ozone variability is linked to variations in the dynamical structure of the atmosphere, as reflected in the tropopause pressure. The latter varies in concert with the NAO index with a distinct geographical pattern

Towards operational impact forecasting of building damage from winter windstorms in Switzerland

National meteorological and hydrological services issue warnings for severe weather events, typically based on stakeholder-agreed fixed thresholds of meteorological parameters such as wind speeds or precipitation amounts. Yet societal decisions on preventive actions depend on the expected impacts of the weather event. In order to better inform such preventive actions, meteorological services are currently working towards including expected impacts into their warnings. We develop an open-source impact forecasting system for building damage due to winter windstorms in Switzerland. It combines a numerical ensemble weather prediction model with exposure and vulnerability data. This system forecasts expected building damage in Swiss Francs with a 2-day lead time on a 500-m grid or aggregated to administrative regions. We compare the forecasted building damage with insurance claims in the canton of Zurich. The uncertainty of the impact forecasts is large. For the majority of days with severe winter windstorm damage, the mean forecasted damage was in the right order of magnitude, with one missed event and one false alarm. For thunderstorms and foehn storms, the rate of missed events and false alarms is much higher, most likely related to the limited meteorological forecast skill. Such impact forecasts can inform decision makers on preventive actions, such as allocating emergency response and other assets. Additionally, impact forecasts could also help communicating the severity of the upcoming event to the general public as well as indirectly help meteorological forecasters with taking warning decisions.ISSN:1350-4827ISSN:1469-808

Natural climate variability and climate change in the North-Atlantic European region; chance for surprise?

Long-term variability in the North Atlantic Oscillation (NAO) and the Atlantic thermohaline ocean circulation (THC) are both shaping the European climate on time scales of decades and longer. Possible linear and non-linear changes in the characteristics of these natural climate modes due to global warming are an important source of uncertainty in long-term regional projections of future climate changes

Climate change and circulation types in the Alpine region

The frequency of circulation types over the Alpine region is explored using 20 different global and regional climate model chains. The projected changes in these circulation types are investigated for the 21st century using the SRES A1B scenario. The multi-model approach relies on the climate models from the ENSEMBLES project and circulation type classifications provided by the COST Action 733. For the latter, the two circulation type classifications GWT (Grosswetter-types) and CAP (Cluster Analysis of Principal components) are selected. GWT is applied to sea level pressure and geopotential height at 500 hPa whereas CAP is applied to sea level pressure. Overall, the ensemble of climate models captures the frequency of individual circulation types well, as shown by the comparison of circulation types from climate models and re-analysis data between 1980 and 2009. Discrepancies occur during winter (DJF) when westerlies are overestimated both at the sea level and at the 500 hPa geopotential height level. The model spread is largest during summer. The frequency of circulation types is simulated best during spring and autumn irrespective of the applied circulation type classification.The analysis of circulation types in the climate projections indicates that in winter easterlies are expected to decrease mostly at the benefit of westerlies until the end of the 21st century. In summer projected changes depend on the height level considered. At sea level westerlies are projected to decrease while easterlies increase markedly in their frequency. This change is not occurring on the 500 hPa geopotential height level

Natural climate variability and climate change in the North-Atlantic European region; chance for surprise?

Long-term variability in the North Atlantic Oscillation (NAO) and the Atlantic thermohaline ocean circulation (THC) are both shaping the European climate on time scales of decades and longer. Possible linear and non-linear changes in the characteristics of these natural climate modes due to global warming are an important source of uncertainty in long-term regional projections of future climate changes

Focus on climate projections for adaptation strategies

Most papers in this focus issue on ‘climate and climate impact projections for adaptation strategies’ are solicited by the guest editorial team and originate from a cluster of projects that were initiated 5 years ago. These projects aimed to provide climate change and climate change adaptation information for a wide range of societal areas for the lower parts of the deltas of the Rhine and Meuse rivers, and particularly for the Netherlands. The papers give an overview of our experiences, methods, approaches, results and surprises in the process to developing scientifically underpinned climate products and services for various clients. Although the literature on interactions between society and climate science has grown over the past decade both with respect to policy-science framing in post-normal science (Storch et al 2011 J. Environ. Law Policy 1 1–15, van der Sluijs 2012 Nature and Culture http://dx.doi.org/nc.2012.070204/nc.2012.070204 7 http://dx.doi.org/nc.2012.070204/nc.2012.070204 ), user-science framing (Berkhout et al 2014 Regional Environ. Change http://dx.doi.org/10.1007/s10113-013-0519-2 14 http://dx.doi.org/10.1007/s10113-013-0519-2 ) and joint knowledge production (Hegger et al 2014 Regional Environ. Change http://dx.doi.org/10.1007/s10113-012-0382-6 14 http://dx.doi.org/10.1007/s10113-012-0382-6 ), there is still a lot to gain. With this focus issue we want to contribute to best practices in this quickly moving field between science and society