Researchers Camille Akhoudas and Christian Stranne at the Department of geological sciences, Stockholm University together with colleagues in France, Germany and the US recently published an article (https://www.nature.com/articles/s41467-023-38425-5) in the open access journal Nature Communications. They used a 30-year time series to analyze how surface salinity in the Southern Ocean has changed. By studying changes in ocean surfaces properties the researchers were able to trace back to the processes behind salinity changes, proving that the observed decrease in salinity in the Indian sector of the Southern Ocean is mainly due to an intensification of the hydrological cycle, with a lesser contribution from polar ice sheet melting.

Water is essential for life and the climate system, as it connects the atmosphere, land, and ocean, influencing weather and climate phenomena. The global ocean holds 97 percent of Earth's surface water, and the distribution of freshwater fluxes affects global ocean circulation, with the Southern Ocean playing a vital role as a major heat and carbon dioxide sink. Although it is believed that the water cycle intensifies as the Earth warms, confirming this through observations has been challenging.

Investigation on potential contribution of atmospheric freshwater flux

Previous explanations for changes in salinity, particularly freshening of surface waters in the Southern Ocean, have centered around increased glacial meltwater input and changes in Antarctic sea-ice production and transport. However, these explanations have limitations, as the former does not account for freshening in open ocean waters away from the coast, and the latter is highly variable across different regions.

Therefore, in this study, we sought to investigate the potential contribution of atmospheric freshwater flux to the observed changes in salinity. While ocean salinity change is an excellent indicator of freshwater flux changes over broad areas, it alone cannot be used to accurately quantify these changes. Additionally, determining trends in net precipitation-driven freshwater is challenging due to inconsistencies among atmospheric reanalysis products and a lack of reliable data in the Southern Ocean.

Independent approach to estimate net precipitation changes

To address these challenges, the researchers applied an independent approach to estimate net precipitation changes from surface ocean properties (salinity and oxygen isotope) and surface flux estimates of sea ice and glacial meltwater for the period from 1993 to 2021. This approach provides a simple but powerful means of explaining the observed changes in surface salinity by combining changes in net precipitation, sea ice, and glacial meltwater inputs.

Intensity of the atmospheric hydrological cycle has increased

The study reveals that the intensity of the atmospheric hydrological cycle has increased by 6 percent between 1993 and 2021 in the Southern Ocean, explaining the observed changes in surface ocean salinity in the Indian sector. The results align with previous studies, providing further evidence that climate change amplifies the hydrological cycle. This finding has critical implications for the future climate. With a 2°C rise in global temperatures (the upper limit of the Paris Agreement target), the water cycle is projected to amplify by 4-8 percent, leading to a more pronounced ocean salinity pattern, where 'the fresh gets fresher, and the salty gets saltier.' Furthermore, the anticipated decline in sea ice and loss of the Antarctic ice sheet are likely to play a significant role in changing the Southern Ocean's overturning circulation.

The study represents a significant step forward in this field, with the more precise estimates of net precipitation changes providing a better basis for comparison with climate model simulations. Additionally, the concurrent use of long-term salinity and seawater oxygen isotopes provides a crucial measure of the impact of climate change on the hydrological cycle and helps distinguish the signal.

Read article in Nature Communications by Camille Akhoudas and Christian Stranne et al (published 13 May 2023)

“Isotopic evidence for an intensified hydrological cycle in the Indian sector of the Southern Ocean”