Argo on I8S w Steve Riser
(from an interview in Dunedin, NZ on January 31, 2007)
In February and March, 2007, the R/V Roger Revelle made its way from the ice edge of Antarctica to the west coast of Australia for I8S, one of about sixteen global transects that are resurveyed every ten years as part of the Climate-Variability/CO2 Repeat Hydrography Program, also known as CLIVAR. Along the way, it was relieved of some cargo in the form of fourteen Argo floats, some of which had very special characteristics.
The I8S Outreach Team caught up with Dr. Steve Riser, a pioneer in Argo technology, as he fine-tuned the fourteen Argo floats for deployment in the Southern and Indian oceans.
Steve: Each one of them has a small PC in it, and there are lots of parameters to set for the missions of these. They've all been set in the lab in Seattle, before they were shipped, but we always send somebody, this time me, to check that the parameters, that the floats are working. They go through a self-check, they make sure that the parameters that were set are still set, before they're deployed.
This instrument is kind of an evolution of something that's been around for maybe 20 years now. These are much smarter and much easier to work with. I'm getting these all set up so the person who gets on the cruise doesn't have to do anything. Just put them in the water, that's it. In the old days, there were a lot of start-up steps, and a lot of things that could go wrong, but they're pretty much idiot-proof right now.
Steve also showed us some of the more recent innovations on the floats we were deploying on I8S. 8 of them had oxygen sensors, and 2 used the faster, two-way Iridium satellite system. Those 2 are also programmed to detect ice, which made it possible for them to be working in the Antarctic.
Steve: South of about 60S, very few floats. So this cruise, we're putting out a few, I have two other cruises this year, one from Punta Renas to the Antarctic and one from Hobart, Australia to the Antarctic, where we're going to put 60 more just in the Antarctic, and we're starting to be able to do these in the ice.
The floats detect and react to the characteristic temperature of water under sea ice. In the water column under ice, there's usually a mixed layer that is 80-100 m thick, with a temperature equal to the freezing point of the ice, which is about -1.8°C. As the float ascends through the mixed layer it calculates the median temperature from 50 to 30 m deep. If this temperature is below a threshold the float will reverse the buoyancy pump at about 5-7 m below the ice, and sink without surfacing.
In their present configuration, the floats can store up to 68 profiles, or about 2 years' worth. That way, they can stay safe below the winter ice, and transmit their data in a batch in the summer, when much of the ice has melted.
One of Dr. Riser's graduate students is comparing Argo data to the data collected during the World Ocean Circulation Experiment, or WOCE, a baseline study of the world's ocean properties during the '90's on which CLIVAR is based.
Steve: I have a graduate student now that's comparing all of the data we've gotten from Argo in the last few years to the data from WOCE 15 years ago, such as this line that was done 12 or 13 years ago. She's taking all of the WOCE lines from around the world and comparing the Argo data to it and you can see surprising global trends, just in 15 years, things like salinity especially. Nobody expected there to be such a large salinity, and at least in the Southern hemisphere, the salinity is getting very much pressure, the salinity is going down in the upper 2000m. It's some ocean signature of climate change.
I'm Steve Riser, I'm a Professor of Oceanography at the School of Oceanography at the University of Washington in Seattle. And for I8S Outreach, I'm Pien Huang.