Over the past 15 years, physical and biogeochemical studies have established that the Southern Ocean, the region surrounding Antarctica, plays a disproportionately large role in modulating Earth’s climate. Dense water masses that reside near the ocean bottom throughout mid- and low-latitude basins reach the surface in the Southern Ocean through a combination of wind- and eddy-induced transport. These waters are exposed to heat, freshwater fluxes, and atmospheric gases, which ventilate the deep-ocean reservoirs of heat and carbon.
This region is also critical for the biogeochemical conditioning of nutrients that will be available for primary production (synthesis of organic compounds from carbon dioxide) at lower latitudes. These processes are typically linked to the large-scale depth-latitude structure of the Southern Ocean’s density and other property fields, such as temperature, dissolved oxygen, and nutrients.
Research during the past 5 years has increasingly shown that a two-dimensional, cross-sectional view of the Southern Ocean is too simplistic. Energy content peaks at mesoscales, 10–20 kilometers in the Southern Ocean, but the amplitude and organization of these mesoscale features are not uniform. Distributions of temperature, salinity, nutrients, and dissolved gases typically exhibit even greater structure and patchiness.
The functioning and evolution of Southern Ocean ecosystems and the ventilation of the ocean’s deep carbon reservoir depend on the transport of these tracers, which involves an interplay between the underlying circulation and the temporally and spatially varying tracers. The coupled variability of these fields is only beginning to be explored.
To identify gaps in our understanding of Southern Ocean circulation, air-sea fluxes, and tracer fluxes, a workshop on Southern Ocean dynamics and biogeochemistry was held at the California Institute of Technology in Pasadena this past February.
The meeting opened with overview lectures by Michael Meredith, Andy Hogg, Ric Williams, and Danny Sigman on sustained observations, the meridional overturning circulation, air-sea partitioning of carbon, and nutrient fluxes, respectively. The meeting continued over a 3-day period with shorter presentations and significant time for group discussions. Planning future observational strategies that integrate the physical and biogeochemical oceanographic communities was a key goal.
Three major research themes emerged from the meeting discussions:
- Three-dimensional pathways influencing heat uptake, carbon sequestration, and preformed nutrients in the upper ocean. Zonal variations in mass and tracer fluxes extend throughout the water column and localize transport across the Antarctic Circumpolar Current (ACC). It remains unclear how this localization and the three-dimensional structure of ACC transport properties will respond to a changing climate. Meeting participants also emphasized the need to explore the coupling between physics and biogeochemistry at submesoscales (10 kilometers and smaller) and its impact on setting preformed nutrient values.
- Buoyancy forcing and gas exchange in the marginal sea ice zone. Observations needed to resolve surface processes in the region spanning the winter sea ice extent to coastal polynyas (open water surrounded by sea ice) remain sparse. Specifically, measurements are needed to determine the strength of surface buoyancy forcing and the efficiency of gas equilibration in a divergent, spatially nonuniform (e.g., lead- or gap-filled) sea ice field. These processes significantly influence the lowermost overturning cell dynamics, as well as water mass properties and carbon dioxide concentrations.
- Shelf and slope controls on mass and tracer fluxes. Discussion focused on the need for high-resolution process modeling and better parameterizations of eddy processes over the continental shelf and slope, including localization of transport due to topographic features and the injection of nutrients, such as iron, into the water column due to flow-topography interactions.
Participants discussed observational strategies needed to provide insight into these research priorities. The group highlighted the use of heterogeneous autonomous platforms to acquire persistent (continuous) measurements that resolve seasonal variability in the dynamical processes highlighted above. As sensor development for these platforms continues, an intelligent blend of autonomous and ship-based measurements is needed to advance the science.
Sustained observations, both oceanic and atmospheric, that lead to continuous time series will remain critical for detecting changes in the Southern Ocean. Achievement of these goals in such a challenging environment requires a coordinated, international effort.
—Andrew F. Thompson, Environmental Science and Engineering, California Institute of Technology, Pasadena; email: [email protected]; and Nicolas Cassar, Division of Earth and Ocean Sciences, Duke University, Durham, N.C.
Citation: Thompson, A. F., and N. Cassar (2015), Physical-biogeochemical coupling in the Southern Ocean, Eos, 96, doi:10.1029/2015EO036829. Published on 9 October 2015.