Past climate variability from an upper-level perspective

The project aims at improving the current understanding of 20th century climate variability, which is a prerequisite for assessing and predicting current and future climate change. While climate variability during the second half of the 20th century has been studied extensively, relatively little is still known about the first half of the century, with its huge climatic variations such as the decade-long “Dust Bowl” droughts in the Midwest of the USA during the 1930s, the warming of the Arctic from 1920 to 1945, or the global climate anomalies in the 1940s. One reason for this is the lack of data above the Earth’s surface prior to 1948, inhibiting the understanding of large-scale dynamical processes. Upper-air data would allow addressing important dynamical features such as the position of the jet streams, the planetary wave structure, or the strength of the stratospheric polar vortex. Historical upper-air data, obtained with balloons and aircraft, can still be found today on paper in various meteorological archives. In the proposed project a large amount of these data, covering the 1925 to 1948 period, will be digitised and re-evaluated. The upper-air data will be used to construct upper-level meteorological fields back to the 1920s using statistical methods. In addition, historical total ozone data will be re-evaluated. With these novel data sets, combined with available climate model simulations, the key periods of early 20th century climate variability will be studied and the dynamical processes will be addressed in the upper-level fields. These analyses will provide new insights into the processes of large-scale climate coupling.