Seawater salinity is not all the same, and ocean basins are becoming more disparate in their saltiness.
Data from ship- and satellite-based sensors show that the salinity contrast between the North Atlantic and North Pacific oceans has increased by about 6% over the past 50 years. Specifically, subtropical regions of the Pacific are becoming fresher, whereas the same regions in the Atlantic are becoming saltier. A new study in Nature Climate Change points out that this shift may have been driven by ocean warming and changes in wind circulation.
“The finding is exciting,” said study coauthor Yuanlong Li, an oceanographer at the Institute of Oceanology, Chinese Academy of Sciences, in Qingdao. Whereas most of the existing studies on ocean saltiness assume that freshwater fluxes alone control the salinity, “we show that ocean dynamics coming from surface wind and ocean warming also matter,” he added.
Li and his colleagues set out to investigate how differences in salinity between the Pacific and Atlantic may be contributing to contrasting climate change effects such as sea level rise and oxygen loss between the two oceans.
They compared data on ocean salinity with climate simulations to look at how much the water column moved. This approach allowed the researchers to isolate how much of the salinity change was because of changes in ocean circulation due to sea surface winds or surface warming compared to changes in freshwater addition such as increased rainfall.
The datasets confirmed that in the top 800 meters of the tropical and subtropical regions of the oceans, the difference in salinity between the Atlantic and Pacific has increased by about 3.6%. Near the surface, the difference increased by 6%.
To look into the causes of salinity change at different depths, the researchers evaluated two main phenomena: heaving and spicing.
Winds and Warming
Heaving is the raising or lowering of a column of water without altering its temperature structure—like moving an entire layered cake up or down. In the ocean, heaving is caused by changes in ocean currents and temperatures and wind-driven upwelling and downwelling.
Overall, spicing and heaving lead to a noticeable increase in saltiness in the North Atlantic compared with the North Pacific.
In the North Atlantic, researchers found that the salinity increase in the upper 800 meters was mainly due to heaving driven by wind. Changes in surface winds pushed water together in the middle latitudes, causing saltier water to accumulate in these regions. Westerly winds in the North Atlantic have gotten stronger and are blocking salty water from moving north, leading to increased salinity at midlatitudes.
Spicing occurs when water masses mix or exchange heat and salt—like mixing the ingredients between each layer of a cake to change their flavors. The researchers found that the phenomenon was mainly driven by ocean warming, which led to changes in the density of the water. The boundary between warm surface water and deep colder water varies with latitude: It’s shallower or at the surface closer to the poles, and deeper toward the equator. As the upper layer of the North Atlantic warms, the boundary migrates northward. Mixing occurs along that boundary. The group’s modeling work revealed that the North Pacific saw a slight freshening near the surface due to spicing. The top 400 meters of the North Atlantic saw a small additional increase in salinity due to spicing.
Overall, spicing and heaving lead to a noticeable increase in saltiness in the North Atlantic compared with the North Pacific.
A Fresh Look
“I think their analysis is new and looks at salinity changes in the Atlantic from a more ocean-centric perspective, rather than just focusing on the atmosphere,” said Aixue Hu, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo. Most previous research has centered on how atmospheric factors such as evaporation and precipitation affect salinity. This study, however, dives deeper into ocean circulation patterns and water properties to understand these changes better.
Though the study is a “great work of quantitative analysis,” various other climate factors could also be playing a large role, said Arnold L. Gordon, a climate scientist and professor emeritus at Columbia University. Fluctuations between El Niño and La Niña, the North Atlantic Oscillation, and the Pacific Decadal Oscillation might have had a significant effect on wind-sea interactions, and they may also be driving the salinity changes, Gordon said.
In addition, a boundary current near the southern tip of Africa leaks salty and warm water from the Indian Ocean into the Atlantic Ocean. “This leakage has likely increased over recent decades and centuries,” Gordon added. “Future research should expand looking into these relationships.”
—Saugat Bolakhe (@saugat_optimist), Science Writer
