By David Ho, LDEO
About 3 years ago, I took on the challenge of planning and championing a gas exchange experiment in the Southern Ocean. I had no illusion that it was going to be easy, but I had plenty of help from the group that I assembled, which consisted of leadership from the previous GasEx experiments, and others who were as passionate about understanding air-sea gas exchange as I was. Our first job was to sell the idea to the community at large, and to the various funding agencies. After that, the real planning began.
From the beginning, SO GasEx was going to be a collaborative experiment, requiring everyone involved to work together towards a common goal. Since many of us involved in the planning of this experiment had previously worked on GasEx-98 and/or GasEx-2001, we knew the challenges involved in staging an experiment that requires the ship to operate in very different modes; combining that with trying to operate in the harsh conditions of the Southern Ocean added to the challenge.
On the cruise, every detail, from lab assignments on the ship, to distribution of water from the CTD, to scheduling of different sampling events, had to be planned carefully. As I mentioned in a previous blog, there are various projects on the ship that required it to operate in different modes. Without the cooperation of all participants involved, this would have been a difficult task.
Many people on the cruise made my job easier, both in my role as co-Chief Scientist and as a PI for the 3He/SF6 component of the experiment. My co-Chief Scientist Chris Sabine was a pleasure to work with. He and I shared responsibilities for various tasks and decisions making (and we had to make some difficult decisions). While we were on the same page on most things, sometimes we would disagree but our divergent opinions lead us to compromises and to better solutions than either of us had thought of. I’m most grateful for the fact that Chris took care of navigating the NOAA-specific bureaucracy, which made it a lot easier for me to concentrate on the scientific aspects of the cruise.
Kevin Sullivan did a masterful job of creating the 3He/SF6 tracer patches, and measured all the SF6 samples from the CTD casts. Matt Reid and Paul Schmieder took turns to chase the tracer patches around for weeks, and never lost it (the patch, but I’m not sure about their sanity). Pete Strutton, Dave Hebert, Roberta Hamme, Burke Hales and Bob Castle helped with the study site selection. Geoff Lebon, Steve Archer, Mike Rebozo, Sarah Purkey helped with various aspects of the tracer injection and sampling. Paul Covert and Byron Blomquist helped us with computer issues. Mete Uz, Program Manager of the Global Carbon Cycle Program in NOAA’s Climate Program Office, rallied the troops on land when it wasn’t clear if the ship could stay at our study site in high winds, and made sure that we were able to get back on track. The Captain and the crew of the NOAA Ship Ronald H Brown, in their various roles, ensured that we were safe, well fed, and to a large extent, able to execute our various scientific projects.
I want to acknowledge Kathy Tedesco, former Program Manager of the Global Carbon Cycle Program in NOAA’s Climate Program Office, with whom I worked closely during the planning stages of the experiment. She was professional yet approachable, and without her help, planning for SO GasEx would have been immensely more difficult.
Even though we did not encounter sustained wind speeds in the 15-25 m/s range at our study site, we had periods of sustained winds up to ~15 m/s, which will be a valuable addition to existing measurements of gas transfer velocities from previous experiments and other parts of the global ocean. Also, we encountered a range of wind speeds (see picture below), which should allow us to effectively evaluate existing parameterizations between wind speed and gas exchange.
The experiment had more hours of eddy covariance CO2 and DMS measurements than any other previous experiments, and the most number of 3He/SF6 samples ever taken in one gas exchange experiment. The combination of CO2, DMS, O3 flux measurements with 3He/SF6 measurements of gas transfer velocities is unprecedented; along with ancillary measurements of waves, turbulence, and bubbles from a buoy that was able to remain with the tracer patch, they should allow us to elucidate mechanisms controlling air-sea gas exchange, and determine if these mechanisms are unique to the Southern Ocean. The detailed carbon system (DIC, pCO2, TAlk), DMS, productivity, and phytoplankton measurements could also help us understand what controls CO2 and DMS dynamics in our Lagrangian patch. All in all, I think SO GasEx was a success, and the data will bear this out in time.
Today, we will pull into Montevideo, Uruguay, and the SO GasEx cruise will be officially over; however, the fun is just beginning. Over the next months, the next chapter will unfold, with all the PIs working up their respective data, coming together to synthesize their results, and disseminating their findings to the community at large.
The entire SO GasEx scientific party on the fantail, taken on the last day before arriving in Montevideo. The weather was completely unrepresentative of what we experienced during our trip, and a welcomed relief to everyone.
Wind speed histograms from SO GasEx, showing winds averaged over 24 hours and spanning 3 CTD stations, which is the time period necessary for one 3He/SF6-derived gas transfer velocity calculation. We encountered a nice range of wind speeds, from 4 to 14 m/s.
We enjoyed a nice sunset on our last evening out at sea…
…and were visited by a school of hundreds of dolphins right after sunset; a nice way to end the cruise.
Future gas exchange scientists? Kathy Tedesco’s niece and nephew proudly sporting their SO GasEx T shirts.