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Past climate variability from an upper level perspective - Extension period

English title Past climate variability from an upper level perspective - Extension period
Applicant Brönnimann Stefan
Number 120871
Funding scheme SNSF Professorships
Research institution Institut für Atmosphäre und Klima ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Climatology. Atmospherical Chemistry, Aeronomy
Start/End 01.08.2008 - 31.07.2010
Approved amount 653'865.00
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All Disciplines (2)

Climatology. Atmospherical Chemistry, Aeronomy

Keywords (9)

climate variability; climate change detection and attribution; climate dynamics; climate reconstruction; atmospheric ozone; historical data; climate trend; drought; climate history

Lay Summary (English)

Lay summary
Understanding the mechanisms behind interannnual-to-decadal large-scale climate variations is a basis for assessing future climate change as well as for improving predictions of climate extremes such as droughts. The lack of understanding important climate variations during the past 100 years, including large-scale, multiannual droughts and strong trends in the first half of the 20th century, demonstrates that this goal is not yet reached. One reason for this is the lack of meteorological data above the Earth’s surface, which are important for understanding the processes and which currently reach back only to 1948 and do not cover three key periods of 20th century climate variability. In the proposed project, two important open questions will be addressed by using historical upper-level data, statistical reconstructions of atmospheric circulation, and simulations with a chemistry-climate model. First, a detection and attribution study will be performed with respect to climate change in the early 20th century (as well during the last decades). This will help to quantify the contributions of natural climate forcings (such as solar or volcanic forcing) and anthropogenic forcings. In contrast to other studies, we will use atmospheric circulation rather than surface air temperature to detect changes. Second, the relation between large-scale, multiannual droughts in different parts of the world and their relation to oceanic and other forcing as well as internal variability will be analysed. Finally, the data work started in the ongoing project phase will be harmonised with other efforts, facilitating future reanalysis projects. The proposed project is expected to provide an essential contribution to the fundamental understanding of interannual-to-decadal large-scale climate variability. The data provided through this project will allow much more detailed assessments of climate model simulations, thereby helping to promote the predictive capabilities.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants


Associated projects

Number Title Start Funding scheme
102731 Past climate variability from an upper level perspective 01.08.2004 SNSF Professorships
130407 Extension, Validation and Analysis of Historical Upper-Air Data Sets (EVALUATE) 01.10.2010 Project funding


Understanding the mechanisms behind large-scale, interannnual-to-decadal climate variations is a basis for understanding future climate change as well as for improving predictions of climate extremes. Studying past climate variability remains a key task in this endeavour. A particularly important time period in this context are the past 100 years. First, with respect to trends, this period marks the changeover of a climate system dominated by natural forcings to one dominated by anthropogenic forcings. Second, with respect to interannual variability, the past 100 years are the closest precedent to the present and near future. Understanding the underlying processes during the recent past may eventually contribute to improved forecasts. Third, with respect to extremes, the period comprises several volcanic eruptions, severe droughts and floods, heat waves and other climate extremes. Some of these extremes are expected to become more frequent in the future and therefore past extremes may serve as an analogue.The climate of the past 100 years demonstrates that our understanding of large-scale, interannnual-to-decadal climate variability is still incomplete. For instance, the surprisingly rapid warming of the globe during the first half of the 20th century lacks a proper explanation. Also, the mechanisms behind multi-annual, large-scale droughts, in particular the role of possible oceanic forcing, are not fully understood. These two areas represent major challenges in current climate research. The proposed project will address these two topics from a new perspective by including upper-level circulation.The project “Past climate variability from an upper-level perspective” has provided new results on climate variability and large-scale atmospheric circulation during the past century. It has demonstrated that the third dimension adds considerably to the interpretation of 20th century climate variability. Moreover, the project has provided a wealth of historical upper-air data reaching back to the early 1900s. In a complementary project, ensemble simulations of the 20th century were performed with a state-of-the-art chemistry-climate model (CCM), termed SOCOL (middle atmosphere version of ECHAM4, fully coupled with the chemical transport model MEZON). All forcings were prescribed in a transient way (sea-surface temperatures and sea ice, land cover, aerosols, solar irradiance, greenhouse gasses and ozone depleting substances, emissions of short-lived species, Quasi-Biennial Oscillation). To the best of our knowledge, these are the first CCM simulations of the 20th century that have ever been performed. Because of its detailed representation of stratospheric chemistry and dynamics, the model well reproduces the effects of solar and volcanic climate forcing.The proposed extension period of the project now aims at combining the historical upper-air data products with the model simulations in order to detect and attribute climate change in the early 20th century as well during the last decades. The focus of part one of the project will be on detecting climate change in the circulation of the troposphere and lower stratosphere, where the effect of several of the forcings can be detected more easily than in surface air temperature. The CCM simulations will be repeated with individual forcings switched on and off, and optimal fingerprint (or similar) techniques will be used for attribution. A second part of the project concerns the mechanisms behind large-scale, multiannual droughts in different parts of the world. Addressing a possible oceanic forcing (and comparison with climate models) requires analysing upper-level circulation. The ongoing project has demonstrated the value of upper-air data for the case of the decade-long “Dust Bowl” droughts in the Midwest of the USA. However, multiannual droughts are also important in other parts of the world, and the mechanisms may or may not operate in a similar way. In the proposed extension project, multiannual droughts will be analysed also in the Sahel region and in Europe, both in the first and second half of the 20th century. Moreover, the interannnual-to-decadal variability of the Indian and East Asian monsoon during the 20th century will be studied in collaboration with other groups and in comparison with climate model simulations.The third part of the project is concerned with harmonising the data work with other efforts in order to facilitate future reanalysis projects. Within the extension period, it is planned to bring the historical upper-air data into a format that can be fed into the ERA-Interim re-analysis system at ECMWF. This part of the project is coordinated with ECMWF and with a project at University of Vienna and will result in a “historical reanalysis” pilot project. The goal is to assimilate two years in the 1940s and gain experience for a future ERA-75 reanalysis. The two selected years were drought years in Europe and will be analysed in the second part of the project (see above).The project will be hosted by the Institute for Atmospheric and Climate Science, ETH Zürich. Close collaboration is planned with NCCR Climate and internationally with CRU (Norwich, UK), ECMWF (Reading, UK), NOAA (Boulder, USA), the University of Vienna, and the Free University of Berlin. As a part of the project, an international workshop will be organised at the end of the extension period.