Preprints
https://doi.org/10.5194/cp-2023-90
https://doi.org/10.5194/cp-2023-90
01 Dec 2023
 | 01 Dec 2023
Status: a revised version of this preprint was accepted for the journal CP and is expected to appear here in due course.

Early Twentieth Century Southern Hemisphere Cooling

Stefan Brönnimann, Yuri Brugnara, and Clive Wilkinson

Abstract. Global surface air temperature increased by ca. 0.5 °C from the 1900s to the mid-1940s, also known as Early Twentieth Century Warming (ETCW). However, the ETCW started from a particularly cold phase, peaking in 1908–1911. The cold phase was global but more pronounced in the Southern Hemisphere than in the Northern Hemisphere and most pronounced in the Southern Ocean, raising the question whether uncertainties in the data might play a role. Here we analyse this period based on reanalysis data and reconstructions, complemented with newly digitized ship data from 1903–1916 as well as land observations. The cooling is seen consistently in different data sets, though with some differences. Results suggest that the cooling was related to a La Niña-like pattern in the Pacific, a cold tropical and subtropical South Atlantic, a cold extratropical South Pacific, and cool Southern midlatitude land areas. The Southern Annular Mode was positive, with a strengthened Amundsen-Bellingshausen seas low, although the spread of the data products is considerable. All results point to a real climatic phenomenon as the cause of this anomaly and not a data artefact. Atmospheric model simulations are able to reproduce temperature and pressure patterns, consistent with a real and perhaps ocean-forced signal. Together with two volcanic eruptions just before and after the 1908–1911 period, the early 1900s provided a cold start into the ETCW.

Stefan Brönnimann, Yuri Brugnara, and Clive Wilkinson

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2023-90', Ryan Fogt, 03 Jan 2024
    • AC1: 'Reply on RC1', Stefan Bronnimann, 29 Jan 2024
  • RC2: 'Comment on cp-2023-90', Laura Slivinski, 03 Jan 2024
    • AC2: 'Reply on RC2', Stefan Bronnimann, 29 Jan 2024

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on cp-2023-90', Ryan Fogt, 03 Jan 2024
    • AC1: 'Reply on RC1', Stefan Bronnimann, 29 Jan 2024
  • RC2: 'Comment on cp-2023-90', Laura Slivinski, 03 Jan 2024
    • AC2: 'Reply on RC2', Stefan Bronnimann, 29 Jan 2024
Stefan Brönnimann, Yuri Brugnara, and Clive Wilkinson
Stefan Brönnimann, Yuri Brugnara, and Clive Wilkinson

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Short summary
The early twentieth century warming – the first phase of global warming in the 20th century – started from a peculiar cold state around 1910. We digitised additional ship log books for these years to study this specific climate state and found that it is real and likely an overlap of several climatic anomalies, including oceanic variability (La Niña) and volcanic eruptions.