The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks
Earth system model (ESM) simulations exhibit large biases compares to observation-based estimates of the present ocean CO<sub>2</sub> sink. The inter-model spread in projections increases nearly 2-fold by the end of the 21st century and therefore contributes significantly to the uncertai...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-04-01
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| Series: | Earth System Dynamics |
| Online Access: | http://www.earth-syst-dynam.net/7/295/2016/esd-7-295-2016.pdf |
| Summary: | Earth system model
(ESM) simulations exhibit large biases compares to observation-based
estimates of the present ocean CO<sub>2</sub> sink. The inter-model spread in
projections increases nearly 2-fold by the end of the 21st century and
therefore contributes significantly to the uncertainty of future climate
projections. In this study, the Southern Ocean (SO) is shown to be one of the
hot-spot regions for future uptake of anthropogenic CO<sub>2</sub>, characterized
by both the solubility pump and biologically mediated carbon drawdown in the
spring and summer. We show, by analyzing a suite of fully interactive ESMs
simulations from the Coupled Model Intercomparison Project phase 5 (CMIP5)
over the 21st century under the high-CO<sub>2</sub> Representative Concentration
Pathway (RCP) 8.5 scenario, that the SO is the only region where the
atmospheric CO<sub>2</sub> uptake rate continues to increase toward the end of the
21st century. Furthermore, our study discovers a strong inter-model link
between the contemporary CO<sub>2</sub> uptake in the Southern Ocean and the
projected global cumulated uptake over the 21st century. This strong
correlation suggests that models with low (high) carbon uptake rate in the
contemporary SO tend to simulate low (high) uptake rate in the
future. Nevertheless, our analysis also shows
that none of the models fully capture the observed biophysical mechanisms
governing the CO<sub>2</sub> fluxes in the SO. The inter-model spread for the
contemporary CO<sub>2</sub> uptake in the Southern Ocean is attributed to the
variations in the simulated seasonal cycle of surface <i>p</i>CO<sub>2</sub>. Two
groups of model behavior have been identified. The first one simulates
anomalously strong SO carbon uptake, generally due to both too strong a net
primary production and too low a surface <i>p</i>CO<sub>2</sub> in
December–January. The second group simulates an opposite CO<sub>2</sub> flux
seasonal phase, which is driven mainly by the bias in the sea surface
temperature variability. We show that these biases are persistent throughout
the 21st century, which highlights the urgent need for a sustained and
comprehensive biogeochemical monitoring system in the Southern Ocean to
better constrain key processes represented in current model systems. |
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| ISSN: | 2190-4979 2190-4987 |