On the Southern Ocean CO₂ uptake and the role of the biological carbon pump in the 21st century

Judith Hauck (1) , Christoph Völker (1) , Dieter A. Wolf-Gladrow (1) , Charlotte Laufkötter (2) , Meike Vogt (2) , O. Aumont (3) , Laurent Bopp (4) , Erik T. Buitenhuis (5) , Scott C. Doney (6) , John P. Dunne (7) , Nicolas Gruber (2) , T. Hashioka (8) , Jasmin G. John (7) , Corinne Le Quéré (5) , Ivan D. Lima (6) , Hideyuki Nakano (9) , Roland Séférian (10) , Ian J. Totterdell (11)

1 AWI - Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung = Alfred Wegener Institute for Polar and Marine Research = Institut Alfred-Wegener pour la recherche polaire et marine
2 Department of Environmental Sciences
3 LPO - Laboratoire de physique des océans
4 LSCE - Laboratoire des Sciences du Climat et de l'Environnement [Gif-sur-Yvette]
5 Tyndall Centre for Climate Change Research
6 WHOI - Woods Hole Oceanographic Institution
7 GFDL - NOAA Geophysical Fluid Dynamics Laboratory
8 JAMSTEC - Japan Agency for Marine-Earth Science and Technology
9 MRI - Meteorological Research Institute [Tsukuba]
10 CNRM - Centre national de recherches météorologiques
11 MOHC - Met Office Hadley Centre

We use a suite of eight ocean biogeochemical/ecological general circulation models from the Marine Ecosystem Model Intercomparison Project and Coupled Model Intercomparison Project Phase 5 archives to explore the relative roles of changes in winds (positive trend of Southern Annular Mode, SAM) and in warming- and freshening-driven trends of upper ocean stratification in altering export production and CO₂ uptake in the Southern Ocean at the end of the 21st century. The investigated models simulate a broad range of responses to climate change, with no agreement on a dominance of either the SAM or the warming signal south of 44°S. In the southernmost zone, i.e., south of 58°S, they concur on an increase of biological export production, while between 44 and 58°S the models lack consensus on the sign of change in export. Yet in both regions, the models show an enhanced CO₂ uptake during spring and summer. This is due to a larger CO₂(aq) drawdown by the same amount of summer export production at a higher Revelle factor at the end of the 21st century. This strongly increases the importance of the biological carbon pump in the entire Southern Ocean. In the temperate zone, between 30 and 44°S, all models show a predominance of the warming signal and a nutrient-driven reduction of export production. As a consequence, the share of the regions south of 44°S to the total uptake of the Southern Ocean south of 30°S is projected to increase at the end of the 21st century from 47 to 66% with a commensurable decrease to the north. Despite this major reorganization of the meridional distribution of the major regions of uptake, the total uptake increases largely in line with the rising atmospheric CO₂. Simulations with the MITgcm-REcoM2 model show that this is mostly driven by the strong increase of atmospheric CO₂, with the climate-driven changes of natural CO₂ exchange offsetting that trend only to a limited degree (∼10%) and with negligible impact of climate effects on anthropogenic CO₂ uptake when integrated over a full annual cycle south of 30°S.

Domaines: Océanographie

Judith Hauck, Christoph Völker, Dieter A. Wolf-Gladrow, Charlotte Laufkötter, Meike Vogt, et al.. On the Southern Ocean CO₂ uptake and the role of the biological carbon pump in the 21st century. Global Biogeochemical Cycles, 2015, 29 (9), pp.1451-1470. ⟨10.1002/2015GB005140⟩. ⟨hal-01575244⟩