Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

Rodgers, Keith B.; Aumont, Olivier; Fletcher, S. E. Mikaloff; Plancherel, Yves; Bopp, Laurent; Montégut, C. de Boyer; Iudicone, Daniele; Keeling, Ralph F.; Madec, Gurvan; Wanninkhof, Rik

doi:10.5194/bg-11-4077-2014

Cited by 43 publications

(53 citation statements)

References 58 publications

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“…This is encouraging given that many ocean models tend to underestimate winter MLDs (Sallée et al, 2013;Downes et al, 2015). Simulating winter mixed layers correctly is critical for setting interior ocean properties supplying nutrients to the upper ocean to fuel the biologically active growing season (Rodgers et al, 2014). However, in contrast to the winter, ACCESS-ESM1 appears to systematically underestimate MLDs in the high-latitude ocean in summer, 60 % (or 30-40 m) in the Southern Ocean, Pacific and Atlantic oceans.…”

Section: Sea Surface Temperature and Mixed Layer Depthmentioning

confidence: 83%

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

et al. 2017

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Abstract. Over the last decade many climate models have evolved into Earth system models (ESMs), which are able to simulate both physical and biogeochemical processes through the inclusion of additional components such as the carbon cycle. The Australian Community Climate and Earth System Simulator (ACCESS) has been recently extended to include land and ocean carbon cycle components in its ACCESS-ESM1 version. A detailed description of ACCESS-ESM1 components including results from pre-industrial simulations is provided in Part 1. Here, we focus on the evaluation of ACCESS-ESM1 over the historical period in terms of its capability to reproduce climate and carbon-related variables. Comparisons are performed with observations, if available, but also with other ESMs to highlight common weaknesses. We find that climate variables controlling the exchange of carbon are well reproduced. However, the aerosol forcing in ACCESS-ESM1 is somewhat larger than in other models, which leads to an overly strong cooling response in the land from about 1960 onwards. The land carbon cycle is evaluated for two scenarios: running with a prescribed leaf area index (LAI) and running with a prognostic LAI. We overestimate the seasonal mean (1.7 vs. 1.4) and peak amplitude (2.0 vs. 1.8) of the prognostic LAI at the global scale, which is common amongst CMIP5 ESMs. However, the prognostic LAI is our preferred choice, because it allows for the vegetation feedback through the coupling between LAI and the leaf carbon pool. Our globally integrated land-atmosphere flux over the historical period is 98 PgC for prescribed LAI and 137 PgC for prognostic LAI, which is in line with estimates of land use emissions (ACCESS-ESM1 does not include land use change).The integrated ocean-atmosphere flux is 83 PgC, which is in agreement with a recent estimate of 82 PgC from the Global Carbon Project for the period 1959-2005. The seasonal cycle of simulated atmospheric CO 2 is close to the observed seasonal cycle (up to 1 ppm difference for the station at Mace Head and up to 2 ppm for the station at Mauna Loa), but shows a larger amplitude (up to 6 ppm) in the high northern latitudes. Overall, ACCESS-ESM1 performs well over the historical period, making it a useful tool to explore the change in land and oceanic carbon uptake in the future.

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Section: Sea Surface Temperature and Mixed Layer Depthmentioning

confidence: 83%

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

et al. 2017

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“…In their model study applying an ad hoc parameterization of wind stirring, Rodgers et al (2014) demonstrate that changes in wind stirring have a large impact on the mean carbon uptake and seasonal cycle phasing in the SO (south of 45 • S). They show that resultant changes in the seasonal onset of stratification influence both entrainment and the biological pump.…”

Section: Discussionmentioning

confidence: 99%

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

Kessler

Tjiputra

2016

Earth Syst. Dynam.

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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.

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“…This is due to the opposing influences of the dominant winter and summer drivers, partially damped by the seasonal cycle of temperature (Takahashi et al, 2002;Thomalla et al, 2011;Lenton et al, 2013). In winter, the dominant processes of mixing and entrainment results in increased surface pCO 2 and thus outgassing (Takahashi et al, 2009;Lenton et al, 2013;Rodgers et al, 2014). In summer, stratification allows for increased biological production and the consequent uptake of CO 2 , thus reducing the entrained winter DIC and associated pCO 2 (Bakker et al, 2008;Thomalla et al, 2011).…”

Section: Ensemble Seasonal Cyclementioning

confidence: 99%

Interannual drivers of the seasonal cycle of CO<sub>2</sub> in the Southern Ocean

Gregor

Kok

Monteiro³

2018

Biogeosciences

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Abstract. Resolving and understanding the drivers of variability of CO 2 in the Southern Ocean and its potential climate feedback is one of the major scientific challenges of the ocean-climate community. Here we use a regional approach on empirical estimates of pCO 2 to understand the role that seasonal variability has in long-term CO 2 changes in the Southern Ocean. Machine learning has become the preferred empirical modelling tool to interpolate time-and locationrestricted ship measurements of pCO 2 . In this study we use an ensemble of three machine-learning products: support vector regression (SVR) and random forest regression (RFR) from , and the self-organising-map feedforward neural network (SOM-FFN) method from Landschützer et al. (2016). The interpolated estimates of pCO 2 are separated into nine regions in the Southern Ocean defined by basin (Indian, Pacific, and Atlantic) and biomes (as defined by Fay and McKinley, 2014a). The regional approach shows that, while there is good agreement in the overall trend of the products, there are periods and regions where the confidence in estimated pCO 2 is low due to disagreement between the products. The regional breakdown of the data highlighted the seasonal decoupling of the modes for summer and winter interannual variability. Winter interannual variability had a longer mode of variability compared to summer, which varied on a 4-6-year timescale. We separate the analysis of the pCO 2 and its drivers into summer and winter. We find that understanding the variability of pCO 2 and its drivers on shorter timescales is critical to resolving the long-term variability of pCO 2 . Results show that pCO 2 is rarely driven by thermodynamics during winter, but rather by mixing and stratification due to the stronger correlation of pCO 2 variability with mixed layer depth. Summer pCO 2 variability is consistent with chlorophyll a variability, where higher concentrations of chlorophyll a correspond with lower pCO 2 concentrations. In regions of low chlorophyll a concentrations, wind stress and sea surface temperature emerged as stronger drivers of pCO 2 . In summary we propose that sub-decadal variability is explained by summer drivers, while winter variability contributes to the long-term changes associated with the SAM. This approach is a useful framework to assess the drivers of pCO 2 but would greatly benefit from improved estimates of pCO 2 and a longer time series.

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Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

Cited by 43 publications

References 58 publications

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

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

Interannual drivers of the seasonal cycle of CO<sub>2</sub> in the Southern Ocean

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Strong sensitivity of Southern Ocean carbon uptake and nutrient cycling to wind stirring

Cited by 43 publications

References 58 publications

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

The carbon cycle in the Australian Community Climate and Earth System Simulator (ACCESS-ESM1) – Part 2: Historical simulations

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

Interannual drivers of the seasonal cycle of CO&lt;sub&gt;2&lt;/sub&gt; in the Southern Ocean

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Product

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Interannual drivers of the seasonal cycle of CO<sub>2</sub> in the Southern Ocean