Characterising Southern Ocean CO$$_2$$ flux variability

Lenton, AA 2006 , 'Characterising Southern Ocean CO$$_2$$ flux variability', PhD thesis, University of Tasmania.

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Abstract

The Southern Ocean, defined in this study as south of 40°S, plays an important role in mitigating climate change by sequestering atmospheric CO$$_2$$. which has continued to rise at unprecedented rates due to anthropogenic activities. The Southern Ocean is a highly variable net sink of atmospheric CO$$_2$$, but remains globally the most under-sampled ocean region for quantifying CO$$_2$$ uptake. Therefore Southern Ocean CO$$_2$$ flux estimates are subject to large uncertainties and hence the carbon budget remains poorly determined in this region. Consequently any prediction about how the Southern Ocean responds to climate change is also highly uncertain.
To compensate for limited Southern Ocean observations, a coarse-resolution (1° x 2°) prognostic, biogeochemical ocean general circulation model, driven with NCEP R-1 atmospheric forcing, was used to simulate variability in the carbon cycle. My model simulations of the ocean carbon cycle were used to tackle two important questions: 1) What level of sampling is required to minimise uncertainty in the annual net uptake of CO$$_2$$ in the Southern Ocean? ; and 2) Does the large amount of interannual variability that has been simulated as well as observed have its origin in the large scale variation of atmospheric pressure in the region known as the Southern Annular Mode (SAM)?
A sampling strategy was developed by applying 2D Fourier transforms and signal-to-noise ratios to the daily-simulated air-sea CO$$_2$$ fluxes and ΔpCO$$2$$ between 1990-1999. Oceanic pCO$$_2$$ observations were used to validate the statistical properties of the model and to estimate the mesoscale variability not captured by the model resolution. The results showed that a sampling strategy of measuring 3-monthly, at every 30° in longitude and 3° in latitude was sufficient to determine the net annual Southern Ocean CO$$_2$$ uptake. Applying this strategy to the total simulated air-sea fluxes, the net annual mean CO$$_2$$ uptake was estimated to be 0.6 ±0.1 PgC/yr (1990-1999).
The estimated uncertainty in the sampling strategy developed was dominated by the simulated interannual variability, and not by errors in the sampling or unresolved mesoscale variability. Therefore, sampling at higher resolutions in space and time would not reduce the uncertainty in the Southern Ocean annual mean uptake any further. The results showed that a doubling of the current Southern Ocean sampling (in longitude) would be required to constrain the net annual mean air-sea CO$$_2$$ fluxes to within the natural variability of the system.
The Southern Annular Mode (SAM), identified as the leading mode of atmospheric variability, has been suggested to be the driver of this large interannual variability. To explore what role the SAM played in driving Southern Ocean CO$$_2$$ fluxes between 1980-2000, the simulated air-sea CO$$_2$$ flux and its drivers were regressed against the SAM. The results showed that the change in CO$$_2$$ uptake was 0.18 PgC/yr per unit change in SAM and that 47% of the variance in interannual air-sea CO$$_2$$ fluxes in the Southern Ocean was explained by the SAM with a 4-month phase lag. This region acted as region of decreased CO$$_2$$ uptake during the positive SAM phase and increased CO$$_2$$ uptake during the negative SAM phase.
The response of the Southern Ocean to the SAM was governed by changes in ΔpCO$$_2$$ and not by changes in the gas exchange co-efficient. Component analyses showed that changes in ΔpCO$$_2$$ were due to SAM-induced changes in ocean physics controlling the supply of nutrients, primarily DIC, to the upper ocean. The SAM is predicted to become stronger and more positive in response to climate change; suggesting that this would in turn result in a net decrease in Southern Ocean CO$$_2$$ uptake.

Item Type: Thesis - PhD Lenton, AA Carbon dioxide mitigation, Carbon sequestration, Carbon dioxide sinks, Carbon cycle (Biogeochemistry) Copyright 2006 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Chapter 3 appears to be the (post-print) equivalent of an edited version of an article published as: Lenton, A., Matear, R. J., Tilbrook, B., 2006. Design of an observational strategy for quantifying the Southern Ocean uptake of CO$$_2$$, Global biogeochemical cycles, 20(4), GB4010, doi: 10.1029/2005GB002620. Copyright 2006 American Geophysical UnionChapter 4 appears to be the (post-print) equivalent of an edited version of an article published as: Lenton, A., Matear, R. J., 2007. Role of the Southern Annular Mode (SAM) in Southern Ocean CO$$_2$$ uptake, Global biogeochemical cycles, 21(2), GB2016, doi: 10.1029/2006GB002714. Copyright 2007 American Geophysical Union View statistics for this item