Lynne Talley (SIO) and Brendan Carter (Princeton)
On 20 March, 2014, we sailed from Hobart, Tasmania on the U.S. Antarctic Program’s Nathaniel B. Palmer, passing under the Tasman Bridge and on out to sea. We are underway to our first bio-Argo float deployment far south of New Zealand, which is on the way to GO-SHIP section P16S, commencing at 67°S, 150°W. The transit to the first float deployment is taking about 6 days. By the end of the cruise in Tahiti on May 5 we will have deployed 12 profiling biogeochemical floats, 30 surface drifters, made daily biogeochemical observations for NASA ocean color satellite cal/val, and completed more than 105 stations with physical and chemical measurements from surface to bottom. We are sampling or deploying instruments for approximately 18 different principal investigators, from NSF funding, NOAA and NASA. Our science party of 29 includes 9 grad students from all over the U.S. and the world.
The 12 floats that we will be deploying will be part of the global Argo float array, profiling every 10 days to 2000 m depth. They include the first set of fully-equipped Southern Ocean biogeochemical profiling floats, measuring oxygen, nitrate, fluorescence and backscatter, and―the newest addition―pH sensors, with the southernmost group having the capability to sense and avoid coming up to the sea surface under sea ice. We hope that these will be the “tip of the iceberg” for the growing Southern Ocean Observing System. The goal is to observe the Southern Ocean’s important uptake of excess CO2 from the atmosphere, and directly observe its acidification resulting from global change.
The stations that we will occupy along 150°W are repeats of two earlier transects. This set of stations was occupied in 1991 as part of that decades’ global observing program WOCE, and then again in 2005 as part of the international repeat hydrography program, now called GO-SHIP, which criss-crosses all of the oceans. We are analyzing about 19 different properties in the water collected from the rosette water sampler, and collecting vertical profiles of conductivity, temperature, oxygen (2 sensors), and pressure (on the CTD), velocity from a Lowered Acoustic Doppler Current Profiler (LADCP), transmittance, fluorescence, and temperature microstructure from a new program (“chi-pod”) for U.S. GO-SHIP. The goal is to observe changes over the decades in the ocean’s heat, salt, nutrient, oxygen, and carbon content. In this part of the world ocean, many of the changes observed thus far have been linked to global change.
We’re currently steaming southeast just along the Australian/New Zealand EEZ boundary. We passed north of Macquarie Island earlier today, getting a push from the northernmost branch of the Antarctic Circumpolar Current, which roughly parallels the Campbell Plateau. Underway measurements commenced close to Hobart: velocity profiling with the ship’s two ADCP systems; meteorological and bathymetric measurements; surface seawater sampling for temperature, salinity, pCO2; and continuous surface sampling of optical properties (backscatter, chlorophyll, CDOM fluorometry).
We estimate that we’ll be in position for the first station/float deployment ―on a P14S waypoint measured in 2012 by a Japanese research expedition, and in 1996 as part of WOCE―in the afternoon on the 26th of March local time.
In between logistical meetings, we’ve been having daily science meetings. We’ve had four people give science talks already, and have a rough plan for 8 more talks, 2 software tutorials, and a host of ‘lab visits’ between our various analysts in the days to come.
Some analysts have already begun their watch schedules to deal with the underway sampling. The rest of us are looking forward to losing ourselves in the blur of deploying a rosette package every few hours, though having time for daily science meetings has been a luxury indeed.
Lynne Talley (SIO) and Brendan Carter (Princeton)
During our second week on the Nathaniel B. Palmer, we completed an 11-‐day transit from Hobart, Tasmania to the southernmost station of our GO-‐SHIP hydrographic section, at 67°S, 150°W. We are arriving on station 5 to commence that section today, which will go northward with stations every 30 nm (55 km) to 15°S.
During the second half of our transit, beginning south of New Zealand, we deployed 4 biogeochemical profiling floats, spaced roughly 1 days’ steam apart. Each float has a CTD (temperature/salinity), and sensors for oxygen, nitrate and fluorescence/backscatter. At each float location we made a CTD/36-‐place rosette/LADCP/transmissometer/fluorometer station to depth, and a profile of Inherent optical properties (IOP) to 200 m (nicknamed the “NASA Cage”). Three of the four CTD stations were to the ocean bottom. The first was at the location of a station on WOCE/GO-‐SHIP P14S, south of Chatham Rise, so it will be possible to not only use our water sample data (nutrients, oxygen, pH, alkalinity, HPLC pigments) to calibrate the float profile, but also to compare our water properties with those collected in 1996 in WOCE and in 2012 in GO-‐SHIP. Station and float deployment 2 was on the northern flank of the Pacific-‐Antarctic Ridge and that profile was also taken to the ocean bottom, given the minimal additional time required and the nearly unmeasured hydrographic nature of this region.
Station and float deployment 3, located south of the Pacific-‐Antarctic Ridge and within the Ross Sea gyre south of the Antarctic Circumpolar Current, were shifted farther east along the transit than originally planned because of sustained high winds and seas. During a break in the weather, the station was occupied to the minimum necessary depth of 2000 m, a bio-‐optics cast was carried out, and the third float was deployed. As a result of the weather and constant waves on the main deck, which has often been secured, the DIC analysis group moved operations from their van into the main lab.
Station and float deployment 4 were then also shifted eastward to lie between station 3 and the anchor station for the P16S repeat at 67°S, 150°W. Weather and seas calmed considerably prior to Station 4, and we began seeing tabular icebergs and smaller floating ice. The mid-‐day conditions were sufficiently calm that the NASA bio-‐optics group deployed their apparent optical properties (AOP) profiler (nicknamed the “Javelin” by popular acclaim), the farthest south such profile at the time. The subsequent “Javelin” profile at Station 5 broke that record, as it was slightly farther south at our southernmost station overall.
During the long transit we have been collecting underway data: ADCP velocities to 1200 m, surface water properties with sampling every four hours, meteorology, and bathymetry.
PI Ken Johnson sent us early data returned by float 7557, which we have compared to data from the CTD deployment and the shipboard measurements of water from the rosette on station 2. The float data captured the features we observed in the oxygen and nitrate profiles well, though with a distinct offset in both quantities. This comparison underscores the potential for these floats to autonomously measure biogeochemical properties in the ocean, as well as the need for floats to be cross-‐calibrated upon deployment on cruises like this one.
We are now more than ready to start our 4 station per day regimen after many long days of no stations or at most one station a day, which always seemed to fall on the noon-‐mid watch. Labs are more than ready, watchstanders ready, crew ready. We’ve enjoyed the comforts of the 03 conference room, with airport connections to computers and comfie chairs, daily science presentations and discussion, excellent and varied food, the start of a 2-‐team gym competition, various card tournaments, daily NY Times crossword puzzles printed large for team use, lots of movies and books, and fun spotting icebergs and building a (tiny) snowman on the bow. The ship’s crew and USAP ASC team are excellent; we are in great hands for the intense part of our work, which started today.