Variability of the transport of anthropogenic CO<sub>2</sub> at the Greenland–Portugal OVIDE section: controlling mechanisms
The interannual to decadal variability in the transport of anthropogenic CO<sub>2</sub> (Cant) across the subpolar North Atlantic (SPNA) is investigated, using summer data of the FOUREX and OVIDE high-resolution transoceanic sections, from Greenland to Portugal, occupied six times from 1...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2014-04-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/11/2375/2014/bg-11-2375-2014.pdf |
| Summary: | The interannual to decadal
variability in the transport of anthropogenic CO<sub>2</sub> (Cant) across the
subpolar North
Atlantic (SPNA) is investigated, using summer data of the FOUREX and OVIDE
high-resolution transoceanic sections, from Greenland to Portugal, occupied
six times from 1997 to 2010. The transport of Cant across this section,
<i>T</i><sub>cant</sub> hereafter, is northward, with a mean value of
254 ± 29 kmol s<sup>−1</sup> over the 1997–2010 period. We find that
<i>T</i><sub>cant</sub> undergoes interannual variability, masking any trend
different from 0 for this period. In order to understand the mechanisms
controlling the variability of <i>T</i><sub>cant</sub> across the SPNA, we propose
a new method that quantifies the transport of Cant caused by the diapycnal
and isopycnal circulation. The diapycnal component yields a large northward
transport of Cant (400 ± 29 kmol s<sup>−1</sup>) that is partially
compensated by a southward transport of Cant caused by the isopycnal
component (−171 ± 11 kmol s<sup>−1</sup>), mainly localized in the
Irminger Sea. Most importantly, the diapycnal component is found to be the
main driver of the variability of <i>T</i><sub>cant</sub> across the SPNA. Both the
Meridional Overturning Circulation (computed in density coordinates,
MOC<sub>σ</sub>) and the Cant increase in the water column have an important
effect on the variability of the diapycnal component and of <i>T</i><sub>cant</sub>
itself. Based on this analysis, we propose a simplified estimator for the
variability of <i>T</i><sub>cant</sub> based on the intensity of the MOC<sub>σ</sub> and on the difference of Cant between the upper and lower limb of the
MOC<sub>σ</sub> (ΔCant). This estimator shows a good consistency with
the diapycnal component of <i>T</i><sub>cant</sub>, and help to disentangle the
effect of the variability of both the circulation and the Cant increase on
the <i>T</i><sub>cant</sub> variability. We find that ΔCant keeps
increasing over the past decade, and it is very likely that the continuous
Cant increase in the water masses will cause an increase in <i>T</i><sub>cant</sub>
across the SPNA at long timescale. Nevertheless, at the timescale analyzed
here (1997–2010), the MOC<sub>σ</sub> controls the <i>T</i><sub>cant</sub>
variability, blurring any <i>T</i><sub>cant</sub> trend. Extrapolating the observed
ΔCant increase rate and considering the predicted slow-down of
25% of the MOC<sub>σ</sub>, <i>T</i><sub>cant</sub> across the SPNA is expected
to increase by 430 kmol s<sup>−1</sup> during the 21st century. Consequently, an
increase in the storage rate of Cant in the SPNA could be envisaged. |
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| ISSN: | 1726-4170 1726-4189 |