- Global warming has ramped up the planetary water cycle since records began in 1950.
- The salinity of different parts of the oceans serves as a rain gauge for researchers.
- An army of 3,200 autonomous ARGO buoys gathered data used in this study.
The supercharging of Earth's water cycle by global warming is already making some parts of Earth's oceans much saltier while others parts are getting fresher.
A new study by Australian scientists shows a clear link between salinity changes at the surface and changes in the deeper waters over the last six decades caused by the warming seen over the same period.
The reasoning goes like this: The saltiness, or salinity, of the oceans is controlled by evaporation and rainfall at Earth's surface, explains Paul Durack of CSIRO, the Australian government's research agency.
The more evaporation there is at a given patch of ocean, the more concentrated the salts get in the seawater, and the higher the salinity. In places where lots of rain is falling, the water gets more diluted, becoming fresher.
Tracking salinity changes over the oceans is, then, a great way to monitor the water cycle over the oceans.
"A key point is that the general population doesn't live in the oceans, so why should they care about these changes," said Durack who has co-authored with Susan Wijffels a paper in the latest issue of the Journal of Climate.
There are few actual rain gauges in the oceans. Some small island states provide estimates of rainfall over the oceans but largely the rainfall is unmonitored. "So salinity is our proxy for rainfall."
In other words, the salinity changes are, in effect, the rain gauges for the oceans.
Reading those proxy gauges takes a great deal of work. Durack analyzed more than 460,000 oceanic readings from the now 3,200-strong army of autonomous ARGO buoys that have been bobbing through the oceans since 2003 to create a framework into which they could fit the more spotty ocean salinity data collected since 1950 -- a process which analyzed over 1.6 million observations of ocean salinity.
After subtracting out such things as cyclical seasonal salinity changes seen by ARGO buoys, El Ninos and other extreme events, they found a strong signal of more evaporation and rainfall over the oceans -- an enhancement to the average surface salinity.
What they found is that the subtropical, evaporation-dominated waters of the Indian, Atlantic and Pacific oceans are getting saltier while the tropical and higher latitude waters are getting fresher -- these later areas being where there is more rainfall than evaporation over the year.
But the matter goes deeper than just the ocean surface water, says Durack. The ARGO buoys don't just float around on the surface, they can sink down to two kilometers below the surface and rise again, gathering data the whole way to create three dimensional ocean profiles. These show that the salinity changes are actually moving, following the paths that ocean water circulates from the surface into the depths.
"While such changes in salinity would be expected at the ocean surface (where about 80 per cent of surface water exchange occurs), sub-surface measurements indicate much broader, warming-driven changes are extending into the deep ocean," said Durack.
"Before we had the global array of ARGO we really didn't have any idea," said Durack. Previous studied were, by necessity, snap shots that could see changes, but not really address the amounts of change in the oceans.
"This is probably one of the most significant papers we've seen yet in this area," said Dean Roemmich, part of the ARGO leadership team and a professor at the University of California at San Diego's Scripps Institution of Oceanography. "This is only the most recent paper on this topic and I think it is the best paperis the best paper on this topic."