The Argo network of ocean-observing sensors is expected to pay its biggest dividends to climate researchers in the future after several decades’ worth of Argo data help them observe some of the largest-scale processes of the oceans.
But only two years after the network reached its planned operational capacity of 3,000 units, Argo has already uncovered a striking symptom of climate change. The world’s hydrological cycle is accelerating as warming speeds up evaporation of surface waters. This is making some ocean waters more salty and others fresher.
Dean Roemmich, an oceanographer at Scripps Institution of Oceanography at UC San Diego and an Argo steering committee member, co-authored a 2009 paper reporting an initial finding of changes in ocean surface salinity. Now colleagues at other major research centers are adding quantitative detail. Two upcoming papers assess how fast the hydrological cycle is accelerating.
“This is the most compelling information there is that the global process of evaporation and precipitation has sped up,” said Roemmich. “There’s little ambiguity in the data. Prior to Argo these changes were suspected, but there just wasn’t enough salinity data to map the global pattern.”
As of Feb. 10, 3,198 Argo sensors were operational in the world’s oceans. The instruments employ technology developed largely at Scripps that allows them to measure water temperature and salinity while sinking to depths of 1,800 meters (6,000 feet). The floats drift on ocean currents for 10 days before resurfacing and transmitting data via satellite.
The first Argo floats were deployed in 2000. Roemmich and fellow Scripps researcher John Gilson used Argo salinity data gathered from 2004 to 2008 to detect the acceleration of evaporation and precipitation cycles, a phenomenon that had been expected as ocean waters warmed but had been extremely difficult to observe before Argo.
As water warms, it evaporates faster and escapes into the atmosphere. The atmosphere cannot hold water for long and releases it as rain and snow. Salinity maps generated by Argo show the ocean regions in which the water is evaporating most rapidly and the others where it is being redeposited on the surface as salt-free rainwater.
Paul Durack of Australia’s Commonwealth Science and Industrial Research Organizations (CSIRO) describes the oceans as the “engine room” of the global hydrological cycle, with about 80 percent of the evaporation and precipitation of fresh water taking place over the ocean surface.
“Clearly, if we’re able to get a better estimate of changes to the 80 percent of global freshwater fluxes over the 20th century, we can much better validate global climate models and start to narrow the wide uncertainty ranges associated with hydrological cycle changes both in the past and the future,” Durack said.
An upcoming paper by Durack and Susan Wijffels finds that model estimates of ocean salinity patterns reported in the Intergovernmental Panel on Climate Change’s 2007 report underestimate the strength of the patterns when compared to the salinity profiles made by the Argo network and preceding efforts. Their previous paper describing the quantitative analysis of the global ocean salinity changes has been submitted to the Journal of Climate, and a second analysis of Argo salinity has been published in Japan’s Journal of Oceanography by Shigecki Hosoda and colleagues of the Japan Agency for Marine-Earth Science and Technology.