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Ocean Salinity Reveals Information on Amplification of Global Water Cycle

Collaborative study finds evidence of amplification of the global water cycle over the past 50 years through studying of ocean salinity.

As a result of climate change, the global water cycle is also found to be changing in a number of crucial ways. Through theory and proposed models, as the earth’s temperature rises, the global water cycle is thought to amplify. This means that due to the higher atmospheric temperature more water is being evaporated from the ocean, and subsequently results in higher volume of rainfall.

Observational confirmation of this theory has been challenging and yet to be proven due to the difficulty in measuring global-scale evaporation and precipitation because of elaborate spatial and temporal variability.

In a study published in the Journal of Climate has shown to overcome many of these challenges, and devised a new way to estimate changes in the water cycle based on salinity data produced since 1960. The team was able to provide substantial evidence that the global water cycle has been amplifying over the past 50 years. This evidence therefore confirms previous theories and models of the changes in water cycle.

The study was led by Cheng Lijing from the Institute of Atmospheric at the Chinese Academy of Sciences, who collaborated with a group of international scientists from National Center for Atmospheric Research (NCAR), U.S.; ETH Zurich; University of St. Thomas, U.S.; Pennsylvania State University, U.S.

“Ocean salinity change can be used to estimate water cycle change, because it reveals the modification of global surface freshwater exchanges: evaporation takes freshwater from the ocean into the atmosphere and increases the ocean salinity; precipitation puts more freshwater into the ocean and reduces the salinity. Consequently, salinity changes integrate effects over broad areas and provide an excellent indicator for water cycle change.” Shared Cheng Lijing.

The salinity change is spatially complicated, this study used a simple index to synthesize these changes, known as the Salinity Contrast (SC) index. This index is defined as the difference between the salinity averaged over high-salinity and low-salinity regions.

“This metric provides a simple but powerful means of synthesizing the observed salinity pattern changes,” said Nicolas Gruber, a co-author of this study from ETH.

“We demonstrated that 0 to 2000 metre salinity pattern has amplified by 1.6 percent and surface salinity has amplified by 7.5 percent We also show that this increase is due to human influence, and this anthropogenic signal has exceeded the natural background variability.”

John Abraham explained that, “To perform interpolation across data sparse intervals and regions, the method uses information on the spatio-temporal co-variability of salinity taken from the historical coupled climate model simulations. The method is then rigorously verified.”

“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.”

Co-author of the study, Kevin Trenberth from NCAR said, “The new data demonstrate that the existing salinity pattern has amplified. In other words, ‘the fresh gets fresher, and the salty gets saltier’ in much of the ocean. Also, we show, for the first time, that the ocean 0-2000 metre averaged salinity trend indicates freshening in almost the entire Pacific Ocean, broad salinification in the low- and middle-latitudes Atlantic, pronounced freshening in North Atlantic, and contrasts between the north and south Indian ocean.”

An improved estimate of the global water cycle change has been compiled based on the new salinity data, salinity-contrast metrics and model simulations. It shows that the water cycle has been already amplified by two to four percent per degree Celsius since 1960.

Results of the study are of significance to the implications on the future of climate change. The more accurate estimate of salinity to confirm amplification of the global water cycle through the Salinity-Contrast index will provide a key measure of how climate change can affect the water cycle which could lead to increased risk of heavy rains and extreme flooding across the world. [APBN]