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ItemClimate indices in historical climate reconstructions: a global state of the art(COPERNICUS GESELLSCHAFT MBH, 2021-06-17)Abstract. Narrative evidence contained within historical documents and inscriptions provides an important record of climate variability for periods prior to the onset of systematic meteorological data collection. A common approach used by historical climatologists to convert such qualitative information into continuous quantitative proxy data is through the generation of ordinal-scale climate indices. There is, however, considerable variability in the types of phenomena reconstructed using an index approach and the practice of index development in different parts of the world. This review, written by members of the PAGES (Past Global Changes) CRIAS working group – a collective of climate historians and historical climatologists researching Climate Reconstructions and Impacts from the Archives of Societies – provides the first global synthesis of the use of the index approach in climate reconstruction. We begin by summarising the range of studies that have used indices for climate reconstruction across six continents (Europe, Asia, Africa, the Americas, and Australia) as well as the world's oceans. We then outline the different methods by which indices are developed in each of these regions, including a discussion of the processes adopted to verify and calibrate index series, and the measures used to express confidence and uncertainty. We conclude with a series of recommendations to guide the development of future index-based climate reconstructions to maximise their effectiveness for use by climate modellers and in multiproxy climate reconstructions.
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ItemTowards a more reliable historical reanalysis: Improvements for version 3 of the Twentieth Century Reanalysis system(WILEY, 2019-10)Historical reanalyses that span more than a century are needed for a wide range of studies, from understanding large‐scale climate trends to diagnosing the impacts of individual historical extreme weather events. The Twentieth Century Reanalysis (20CR) Project is an effort to fill this need. It is supported by the National Oceanic and Atmospheric Administration (NOAA), the Cooperative Institute for Research in Environmental Sciences (CIRES), and the U.S. Department of Energy (DOE), and is facilitated by collaboration with the international Atmospheric Circulation Reconstructions over the Earth initiative. 20CR is the first ensemble of sub‐daily global atmospheric conditions spanning over 100 years. This provides a best estimate of the weather at any given place and time as well as an estimate of its confidence and uncertainty. While extremely useful, version 2c of this dataset (20CRv2c) has several significant issues, including inaccurate estimates of confidence and a global sea level pressure bias in the mid‐19th century. These and other issues can reduce its effectiveness for studies at many spatial and temporal scales. Therefore, the 20CR system underwent a series of developments to generate a significant new version of the reanalysis. The version 3 system (NOAA‐CIRES‐DOE 20CRv3) uses upgraded data assimilation methods including an adaptive inflation algorithm; has a newer, higher‐resolution forecast model that specifies dry air mass; and assimilates a larger set of pressure observations. These changes have improved the ensemble‐based estimates of confidence, removed spin‐up effects in the precipitation fields, and diminished the sea‐level pressure bias. Other improvements include more accurate representations of storm intensity, smaller errors, and large‐scale reductions in model bias. The 20CRv3 system is comprehensively reviewed, focusing on the aspects that have ameliorated issues in 20CRv2c. Despite the many improvements, some challenges remain, including a systematic bias in tropical precipitation and time‐varying biases in southern high‐latitude pressure fields.
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ItemA historical climate dataset for southwestern Australia, 1830-1875(WILEY, 2021-08)Abstract Southwestern Australia is a high priority area for climate research as there has been a significant, anthropogenically influenced winter rainfall decline detected in the region since 1970. Here we present the oldest daily historical climate dataset for Perth, southwestern Australia, to provide an extended record for analysing pre‐industrial climate variability and extremes from the region. This newly digitized record contains sub‐daily observations of temperature, barometric pressure, wind direction and weather remarks, including rain days, from 1830 to 1875. Following quality control and homogeneity adjustments, we demonstrate that the historical dataset reliably resolves modern characteristics of Perth's weather and climate variability, including the seasonal cycle and the ability to capture daily extremes like storm and heatwave events. We compare covariations in temperature, pressure, wind and rain days to cross‐verify our historical dataset against modern observations for Perth. We also use the historical dataset, modern Australian Bureau of Meteorology observations, the Twentieth Century Reanalysis, and historical documents to further validate our results. The multivariate method introduced in this study demonstrates that using multiple variables and data sources is a useful approach to reconstruct past variability, extremes and their societal impacts from historical observations. This is the first long instrumental record to be developed for southwestern Australia; an important region for observing pre‐industrial Indian and Southern Ocean variability and extremes. The new record from Perth provides 19th century data from a data sparse region of the Southern Hemisphere that can now be compared with daily observations from South Africa, New Zealand and southeastern Australia. This comparison will improve our understanding of contemporary changes in Southern Hemisphere atmospheric circulation.