The supercharging of Earth's water cycle by global warming is making some parts of our oceans saltier, while others parts are getting fresher, according to a new study.
The study, by CSIRO Marine and Atmospheric Science shows a clear link between salinity changes at the surface, caused by warming, and changes in the deeper waters over the last six decades.
CSIRO scientist Paul Durack says the saltiness, or salinity, of the oceans is controlled by evaporation and rainfall. The more heat and 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 a lot of rain is falling, the salty water gets more diluted and fresher.
Tracking the salinity changes over the oceans is, then, a great way to monitor the water cycle over the oceans. That's pretty important, says Durack, since the 97% of the water on Earth is in the oceans that cover 70% of the planet. So when the oceans start saying the global climate is changing, it's truly a global matter.
"The thing is, the general (population) doesn't live in the oceans," says Durack who has co-authored with Dr Susan Wijffels a paper in the latest issue of the Journal of Climate.
There are few actual rain gauges in the oceans. "So salinity is effectively a proxy for rainfall." In other words, the salinity changes are the rain gauges for the oceans.
Durack analysed more than 1.6 million oceanic readings from the now 2000-strong army of autonomous ARGO buoys, to create a framework into which they could fit the 'spotty' ocean salinity data collected since 1950.
After subtracting out such things as cyclical seasonal salinity changes seen by ARGO buoys, El Nia33;os and other extreme events, they identified a strong signal of more evaporation and rainfall over the oceans.
What they found is that the tropical waters of the Indian, Atlantic and Pacific oceans are getting saltier while the higher latitude waters are getting fresher.
But the matter goes deeper than just the ocean surface water, says Durack.
Under the surface
The ARGO buoys don't just float around, they can sink down to three kilometres under the surface and then rise again, gathering data the whole way to create three dimensional ocean profiles. These show that the salinity changes are actually moving into the depths.
"While such changes in salinity would be expected at the ocean surface (where about 80% of surface water exchange occurs), sub-surface measurements indicate much broader, warming-driven changes are extending into the deep ocean," says Durack.
"Before we had the global array of ARGO we really didn't have any idea," he says.
Previous studies 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," says Dr 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 paper on this topic."