Sometime this fall, probably in a remote stretch of Southern Ocean between New Zealand and Chile, a measurement device trussed in a cardboard box will be dispatched from the back of a ship.
It will be a business-as-usual ceremony resembling a burial at sea without the salutes or prayers — this despite the fact that this marks an unprecedented scientific achievement.
For the worldwide measurement network known as Argo, which was first conceived at Scripps Institution of Oceanography at UC San Diego a decade ago, this is float number 3,000. Along with the other 2,999, this float takes the “vital signs” of the oceans: temperature, salinity and velocity. This float represents the end of Argo’s deployment phase and the beginning of the program’s “sustained maintenance phase.”
Despite the dry moniker, this newer phase is what excites the people behind Argo. A layman may appreciate the quest for 3,000 the way he or she might follow Barry Bonds’ quest for 756 but for the scientists, the value of Argo will come not from a number but from decades of data-gathering. Only with “sustained maintenance” will Argo reach its goal to transform the way society understands the oceans.
Argo is the name of an array of floats spread out over the world’s oceans. It was conceived as a more sophisticated successor to the World Ocean Circulation Experiment (WOCE), which, for the better part of the 1990s, broke ground in its use of autonomous floats to track ocean currents.
Argo was designed to measure more water properties than the WOCE floats and, more importantly, to have an indefinite run. Scientists named it after the vessel from Greek mythology; its counterpart satellite was already named Jason.
When depicted on animated maps, the Argo armada takes on the appearance of having spread like an epidemic since Australian researchers deployed the first half-dozen floats in 1999. The floats are dropped from ships and the backs of aircraft and begin recording data autonomously as soon as they hit the water. They continue to do so as long as their batteries last, usually about four to five years.
About 30 percent of the floats, called Sounding Oceanographic Lagrangian Observer (SOLO) instruments, are based on a design co-invented by Scripps research oceanographer Russ Davis, whose breakthroughs in instrument development helped make WOCE possible 20 years ago. Most SOLO floats are still made at Scripps. The rest of the array is filled by floats designed either by a private American firm or a French research agency.
Each tube-shaped float descends to depths up to 1,800 meters (6,000 feet), drifts on ocean currents for 10 days, then returns to the surface when oil is pumped to an external bladder on the bottom of the instrument.
At the surface, the float beams results to a passing satellite. The profile of ocean conditions they generate is then processed and posted within 24 hours of transmission. The raw temperature and salinity data as well as the position of the floats relative to where they were during their last contact with the satellite allow for a wide range of interpretive study.
New floats added to the array have improved the resolution of its dataset the way additional pixels sharpen the image on a television screen. The challenge for the Argo steering team, an international committee co-chaired by Scripps physical oceanographer Dean Roemmich, has been to add new floats faster than old ones wear out.
In some cases, they experience other unanticipated service disruptions. Earlier this year, for example, satellites tracked one wayward unit making measurements on land. It turned out that the unit was dutifully measuring temperature and salinity levels on the streets of downtown Brisbane, Australia. The unit was touring the city in the bed of a pickup truck owned by the fisherman who had trawled it from the seafloor just off the coast. The angler planned to convert it into a fashionable mailbox.
In 2004, when Argo reached its halfway point, there were enough floats operational to allow the network to offer what could be considered worldwide coverage, a hazy view of things to come.
Oceanographers usually need decades worth of data to follow cycles that last that long. However, Argo data, viewable by anyone with an Internet connection, has already been cited in more than 100 research papers. In several key areas, it is already yielding new perspectives:
Ocean heat content – What people call “global warming” typically refers to air temperatures, but knowledge of ocean temperatures is just as important to climate researchers because the oceans store far more heat energy than the atmosphere. For this reason, the top of the ocean acts as a “thermostat” in controlling the temperature of the air.
Though the ocean’s temperature has been taken by naval, commercial, and oceangoing ships of all kinds for centuries, the effort has been piecemeal compared to what Argo offers. A recent research paper by Roemmich reported the discovery that the strongest warming in the oceans is taking place between 40 and 50 degrees south latitude in a region of the Southern Ocean. Such findings will enable scientists to identify cause-and-effect relationships between ocean heat and climate phenomena experienced on land.
Sea level rise – The height of the ocean surface is rising for two reasons. First, ocean warming causes the water to expand, just as water in a tea kettle expands and overflows on a hot stove. Second, mountain glaciers and polar ice are melting and adding new water to the oceans. Argo observes the first of these effects directly by measuring ocean temperature. A study by former Scripps student Josh Willis using Argo data found that the expansion of ocean water caused by rising temperatures has been responsible for half of the sea level rise documented in the last 14 years. Argo observes the second effect indirectly by measuring salinity. As land-bound ice melts and runs into the sea, it makes the seawater fresher by a small amount. Argo data have shown that present day salinities are fresher than a few decades ago.
Global hydrological cycles — Rain and snow originate in the oceans, the reservoir of precipitation. Rain and snowfall patterns are easy to observe over land but until Argo have been difficult to trace in the oceans. The geographic patterns of salinity readings of Argo floats reveal how fresh water passes back and forth between oceans and the atmosphere and let scientists test the hypothesis that global warming will cause the hydrological cycle to intensify.
Extreme weather events – The durable floats have beamed data through hurricanes and typhoons to show what happens below the surface during such events, providing data that would be too hazardous to try to collect with ship-based instruments. Not only has this led to a better understanding of cyclone dynamics but the frequently updated data helps meteorological agencies around the world create better extreme weather forecasts. Knowledge of current conditions in remote ocean stretches has also been useful to cargo shippers, fishing fleets, and oil exploration firms.
Three thousand floats was the number that Argo scientists decided would provide adequate density and be practical to sustain. The floats, which cost $15,000 to build and another $15,000 to operate over a four-year lifespan, cover the oceans at a rate of one per every three degrees latitude and longitude.
The science team hopes that funding agencies in the United States and elsewhere will continue to consider support of Argo as money well spent. Every year will require the replacement of one-fourth of the array.
For its part, the National Oceanic and Atmospheric Administration (NOAA) has committed to funding Argo through 2011 but there are more than 50 research agencies around the world involved in administering Argo. There is, said Roemmich, a tendency among Argo scientists to downplay the 3,000-float milestone lest support wane just as what Davis calls the “first data point” of Argo is collected.
“If we were to say that the Argo array is complete, that could be interpreted to mean no more floats are required,” said Roemmich. “The message that we want to convey is that Argo has achieved a lot in getting a global array out there. The hard work is just beginning.”