Ocean salinity, a powerful tool for understanding changes in Earth’s water cycle
To see how global warming is changing Earth’s water cycle, all we have to do is turn on a TV or look around us. Some parts of the world are getting deluged by heavy rains and storms, while other regions are becoming drier. This is an indication that our changing climate has amplified the global water cycle’s patterns of evaporation, water vapour transport through the atmosphere and precipitation. An important tool for understanding these changes is the measurement of ocean salinity. This is because variations in salinity are a reliable gauge of the freshwater exchange between the ocean and the atmosphere. “Evaporation takes freshwater from the ocean into the atmosphere and increases the ocean salinity; precipitation puts freshwater into the ocean and reduces its salinity. Consequently, salinity changes integrate effects over broad areas and provide an excellent indicator for water cycle change,” Lijing Cheng, lead author of a recent study, explained in a news item posted on the ‘Mirage News’ website. The study, which was published in the ‘Journal of Climate’ and received support from the EU-funded SO-CHIC and 4C projects, has addressed previous methods’ inconsistencies that have made studying the changes in long-term salinity challenging.
Measuring salinity not only on the surface
To tackle these inconsistencies, the researchers adopted a different technique to estimate changes in ocean salinity since 1960. Unlike many previous efforts, they don’t only focus on changes in surface salinity but also measure salinity up to a depth of 2 000 m. “The new product is clearly more reliable for examining long-term salinity changes, as we show that this new salinity reconstruction has much better continuity through changes in the observing-system (from altimeters on satellites and profiling floats (Argo) in the ocean),” observed study co-author Kevin Trenberth. Their findings have confirmed that the contrasts between surface and subsurface salinity have in fact increased and that the water cycle has amplified. The project team captured the water cycle’s amplification using a tool called the Salinity Contrast (SC) index. The SC index was calculated every month and measured the difference between the salinity averaged over high- and low-salinity regions. “This metric provides a simple but powerful means of synthesizing the observed salinity pattern changes,” stated Nicolas Gruber, another study co-author. “We show that the 0-2000m salinity pattern has amplified by 1.6% and and [sic] that at the surface by 7.5%. We also show that this increase is due to human influence, and that this anthropogenic signal has exceeded the natural background variability,” Gruber went on to say. Furthering the objectives of SO-CHIC (Southern Ocean Carbon and Heat Impact on Climate) and 4C (Climate-Carbon Interactions in the Current Century), the study highlights the major implications of salinity changes for the ocean system and the planet’s future climate. “This study is a significant advance in the field,” noted co-author Michael Mann. “First, the new, more accurate estimates of salinity changes provide a better basis for comparison with climate model simulations. Secondly, the Salinity-Contrast index provides a key measure of climate change impact on the global water hydrological cycle …. We find that it takes a little more than a decade to isolate the climate change signal from background noise in this particular metric, suggesting it should be used more widely by the climate research community.” For more information, please see: SO-CHIC project website 4C project website
Keywords
SO-CHIC, 4C, climate change, ocean salinity, water cycle