Designing a carbon and heat observing system for the Southern Ocean

January 18, 2018
Simulated carbon flux from the Southern Ocean to the atmosphere [mol m−2 yr−1] from a 2008 - 2012 bigeochemical state estimate (available at Red denotes ocean losing carbon to the atmosphere, and blue denotes ocean carbon uptake. The white contour denotes ice extent. Animation created by Matt Mazloff.

A central goal for understanding the climate state and its variability is to quantify the oceanic inventories of carbon and heat, as well as the carbon and heat exchanges between the atmosphere and ocean. New technologies have enhanced our ability to measure interior ocean properties. The last decade saw a widespread deployment of profiling floats equipped with temperature and salinity sensors, and the next decade offers the possibility of adding biogeochemical sensors to the floats. Because of the significant added cost for such sensors, an estimate of the minimum number of such enhanced platforms is crucial. In a new paper in the Journal of Geophysical Research-Oceans, Mazloff and coauthors estimate spatial correlations of properties at each location in the Southern Ocean, revealing how many observing platforms are necessary to adequately observe carbon and heat. The correlation information from their study also has the potential to improve mapped products, once measurements are obtained.

The time series information needed to quantify spatial correlation of ocean interior properties is limited. In this paper, Mazloff and coauthors use a data-assimilating model solution to calculate correlation lengths of carbon and heat inventories and air-sea exchanges in the Southern Ocean. Biases in the model scales are diagnosed via comparison to satellite-derived products. The rich structure simulated by the model is demonstrated by the animation of air-sea carbon exchanges. This carbon flux variability is partly due to the seasonal cycle; outgassing occurs when the ocean warms and ingassing when the ocean cools. This is most apparent in the subtropical Pacific Ocean. Biological activity results in carbon uptake, and so the most productive regions are primarily characterized by ingassing (e.g., the blue coloring around Tasmania). Ocean circulation and mixing are responsible for much of the rich structure of the animation. Strong outgassing, denoted by the bright reds, is shown where upwelling brings carbon-rich waters to the surface. Sea ice cover damps the air-sea exchanges, playing a primary role in governing the overall net exchange.

The paper answers the question: when one observes a property change in the ocean, how far away in space does one also expect to see a similar change? The authors find the structure is correlated on lengths of approximately 2,000 km in the zonal direction and 1,000 km in the meridional direction. Carbon and heat inventories show similar scales, while heat flux has far longer scales, reflecting faster equilibration with the atmospheric state. Given the scales derived for property inventories, the authors estimate that it requires approximately one platform every 20° longitude by 6° latitude, or about 100 optimally spaced measurement platforms, to constrain a heat and carbon inventory between 35°S and 70°S on timescales longer than 90 days. The Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) is deploying nearly 200 floats in the Southern Ocean equipped with biogeochemical sensors. While it is impossible to optimally deploy and maintain these floats at separation intervals of one correlation length, these findings confirm that this array should allow skilled assessment of ocean carbon cycle variability.

Written by 
Matt Mazloff, Scripps Institution of Oceanography

Matthew R. Mazloff, Bruce D. Cornuelle, Sarah T. Gille, and Ariane Verdy

Scripps Institution of Oceanography