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The Southern Hemisphere Sites of the U.S. NSF Ocean Observatories Initiative

"Southern Ocean" and "Argentine Basin" Observatories 

  • This e-mail address is being protected from spambots. You need JavaScript enabled to view it. , Senior Scientist, Woods Hole Oceanographic Institution
  • August 2016

Two new ocean observatories in the Southern Hemisphere have been installed and are operational, providing their data openly to all users. These new locatins (Figure 1) are two of the four global sites of the United States National Science Foundation’s (NSF) Ocean Observatories Initiative (OOI). One is located at 55°S, 90°W and named Southern Ocean, and the other at 42°S, 42°W and named Argentine Basin.

  

Figure 1:  Locations of the Southern Ocean and Argentine Basin observatories of the OOI.  Adapted from a map from the OOI Cabled Array program and the Center for Environmental Visualization, University of Washington.

The OOI Global Arrays consist of a total of four sites, two in each hemisphere.  Northern hemisphere sites include PAPA in the Gulf of Alaska at 50°N, 144.5°W, and Irminger Sea southeast of the southern tip of Greenland at 60°N, 39°W. The establishment and occupation of these four high latitude sites is a a major component of the NSF’s OOI, whose goal is to advance observing capabilities and support cutting edge ocean science in these sites for 25 years.  These four sites were chosen as they were judged by the ocean science community to be of high scientific interest, but they have been sparsely sampled up until now, due to their environmentally challenging locations. 

The scientific community identified the Southern Ocean site as a key study location due to its high wind, high waves, and large buoyancy flux. These dynamics, coupled with its long fetch and location of water mass transformation, make this location a prime area to examine changes in climate, westerly wind strength, turbulent mixing, and biophysical interactions. Similar dynamics also drew researchers to select the Argentine Basin site. This location also provides the ability to examine possible micronutrient fertilization by dust from South America, as well as kinetic energy dynamics similar to the Gulf Stream with low mode velocity structures, strong enough to make mud waves on the sea floor.

Requirements for Global Observatories

Community guidance was sought whilst designing the global observatories. This lead to requirements to sample from the sea surface - including the meteorology and air-sea fluxes - down to the sea floor; to field multidisciplinary instrumentation; and to address the variability associated with horizontal scales up to the mesoscale.  To address these requirements, a roughly triangular array of moorings was deployed (Figure 2), with the sides set roughly by the local Rossby radius of deformation, about 10 times the water depth (water depth at the Southern Ocean site is 4,800 m, and water depth at Argentine Basin is 5,200 m). The moored array is complemented by ocean gliders. Gliders are deployed for a year at a time and recovered on the annual servicing cruises. Each site is serviced once per year, with both cruises departing from Punta Arenas, Chile, where the U.S. Antarctic Program support facility is located.

 

Figure 2:  Schematic of the OOI Southern Ocean observatory. From www.oceanobservatories.org. 

The design of the global observatories emphasizes expanding capabilities. At one corner of the moored array (shown to the right in Figure 2) a pairing of two moorings provide the capabilities of sampling from the sea surface to the sea floor.  A surface mooring has a well-instrumented surface buoy, a vertically dense array of multidisciplinary instrumentation in the upper 100m, and a sparser array of physical (velocity, temperature, and salinity) sensors down to 1,500m.  This is the depth where the wire rope ends and the surface mooring line transitions to a synthetic rope, making it much less useful for mounting instrumentation.

A nearby (~10 km) taut subsurface mooring in this same corner of the triangle has an instrumented surface sphere at about 30 m depth and two wire-following profilers distributed along the wire below, each covering roughly half the water depth. At the other two corners of the triangular array there are taut subsurface moorings, called flanking moorings. These have sensors at fixed depths between the sphere (at roughly 30 m depth) down to 1,500m. In this way, the three corners of the array provide similar coverage over the upper 1,500 m. To improve further on the observatories’ ability to sample horizontal variability, three gliders are deployed. These are to be used over the year to patrol and/or sample the variability in and around the moored array. Additional capability to sample through the water column and up through the sea surface is provided by two gliders that are dedicated to vertical profiling in the vicinity of the surface and profiler moorings.

Instrumentation

The surface buoy carries meteorological sensors. In addition to providing typical observations, these support computation of the air-sea fluxes of heat, freshwater, and momentum by the bulk formulae, and include a surface wave package, a motion package, and an air-sea PCO2 instrument. The Southern Ocean surface buoy also carries a Direct Covariance flux package.  The surface mooring carries the following in water instrumentation types:  absorption spectrophotometer, 2 and 3-wavelength fluorometers, temperature/salinity (T/S), velocity, pCO2, dissolved oxygen, nitrate, spectral irradiance, and pH.  The nearby profiler mooring carries:  bio-acoustic sonar, T/S, 2-wavelength fluorometer, dissolved oxygen, and velocity sensors. The two flanking moorings carry dissolved oxygen, pH, 3-wavelength fluorometers, T/S, and velocity sensors. The gliders that patrol the array are instrumented with:  dissolved oxygen, 2-wavelength fluorometers, and T/S sensors. The vertically profiling gliders are instrumented with dissolved oxygen, 3-wavelength fluorometers, nitrate, photosynthetically available radiation, T/S, and nitrate. Further details on the instrumentation used at both sites can be found on the OOI website (www.oceanobservatories.org). 

Data from these sites are sent back to shore in near-real time wherever possible. The surface mooring has the power and telemetry capabilities to send most of its basic data (down to one-minute sampling for some sensors) back via satellite. The gliders act as data carriers, obtaining data acoustically from the three subsurface moorings, and transmitting it via satellite when they are at the surface. Communications capabilities are two-way, with the intent of enabling not only engineering and maintenance functionality but also allowing the observing community to adapt sampling as needed.

Data are flowing from these observatories, and users are encouraged to obtain and use the data. Those interested in the Southern Ocean and Argentine Basin observatories and in obtaining their data should visit the OOI website (www.oceanobservatories.org). Navigate to the OOI Data menu on the left of the page to access the OOI Data Portal, a raw data archive, a THREDDS server, information about the cruises, and more useful information.

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