The Southern Ocean as a constraint to reduce uncertainty in future ocean carbon sinks

Abstract

Abstract. Earth system model (ESM) simulations exhibit large biases compares to observation-based estimates of the present ocean CO 2 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 2 , 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 2 Representative Concentration Pathway (RCP) 8.5 scenario, that the SO is the only region where the atmospheric CO 2 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 2 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 2 fluxes in the SO. The inter-model spread for the contemporary CO 2 uptake in the Southern Ocean is attributed to the variations in the simulated seasonal cycle of surface pCO 2 . 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 pCO 2 in December-January. The second group simulates an opposite CO 2 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.

Narrowing the spread in CMIP5 model projections of air-sea CO2 fluxes

“…Hence, the air-sea CO 2 flux simulation spread needs to be narrowed to reduce the uncertainty in future climate projections. Kessler and Tjiputra31 have proposed that the air-sea CO 2 fluxes in the Southern Ocean could be used to constrain the future global ocean carbon uptake uncertainty. In this paper, two other methods are used to narrow the projected spread of air-sea CO 2 fluxes.…”

The seasonal cycle of <i>p</i>CO<sub>2</sub> and CO<sub>2</sub> fluxes in the Southern Ocean: diagnosing anomalies in CMIP5 Earth system models

“…Though this relationship between dSST and dpCO 2 is based on a linear assumption (Takahashi et al, 1993), this formulation has been shown to hold and has been widely used in the literature (e.g., Bakker et al, 2014;Feely et al, 2004;Marinov and Gnanadesikan, 2011;Takahashi et al, 2002;Wanninkhof et al, 2009;Landschützer et al, 2018). We show in the Supplement that the extension of this expression into polar temperature ranges (SST < 2 • C) only introduces a minor additional uncertainty of 4-5 % (SM Fig.…”

Climate feedbacks in the Earth system and prospects for their evaluation

“…The emergent constraint approach has been applied to link air-sea flux changes in the tropics (Wenzel et al, 2014) and the Southern Ocean (Kessler and Tjiputra, 2016) as simulated by different ESMs to observational signals (tropical temperatures) or potentially observable flux changes (Southern Ocean CO2 uptake strength). Kwiatkowski (2017) found an emergent constraint on narrowing down the uncertainties in declining primary production at low latitudes.…”

“…2017)) focuses on diagnosing forcings and feedbacks from near-term climate forcers.In addition to the CMIP6 experimental design, progress in constraining individual feedbacks and in equilibrium climate sensitivity, TCR, and the transient climate response to cumulative carbon emissions (TCRE) can be expected from emergent constraint studies (e.g Cox et al (2013)Hall and Qu (2006);Kessler and Tjiputra (2016);Wenzel et al (2014); Wenzel et al…”