On the Southern Ocean CO2 uptake and the role of the biological carbon pump in the 21st century, links to supplementary material

We use a suite of eight ocean biogeochemical/ecological general circulation models from the MAREMIP and CMIP5 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 ona 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 CO 2 (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.

本研究采用MAREMIP与CMIP5档案库中的8套海洋生物地球化学/生态通用环流模型，旨在探究21世纪末南半球海洋中，风场变化（南极环状模（Southern Annular Mode, SAM）呈正趋势）以及增温与淡化驱动的上层海洋层结变化，对生物输出生产与CO₂吸收的相对作用。所评估的模型对气候变化的响应跨度较大，在南纬44°以南区域，尚未就SAM信号或增温信号何者占主导达成共识。在最南区域（即南纬58°以南），所有模型一致认为生物输出生产将增加；而在南纬44°至58°之间，模型对输出生产的变化趋势正负尚无统一结论。但在这两个区域，模型均显示春夏季的CO₂吸收能力有所增强。这一现象源于21世纪末雷维尔因子（Revelle factor）更高的背景下，同等量级的夏季输出生产会引发更大幅度的溶解态CO₂抽取。这将大幅提升生物碳泵（biological carbon pump）在整个南半球海洋中的作用权重。在南纬30°至44°的温带区域，所有模型均显示增温信号占主导，且营养盐供给变化会导致生物输出生产降低。据此预计，到21世纪末，南纬44°以南区域在南纬30°以南南半球海洋总CO₂吸收量中的占比将从47%升至66%，而北部区域的占比则会出现相应幅度的下降。尽管主要CO₂吸收区域的经向分布发生了重大重构，但总吸收量仍基本随大气CO₂浓度上升而同步增加。基于MITgcm-REcoM2模型的模拟结果显示，该现象主要由大气CO₂浓度的大幅升高驱动；气候驱动的自然CO₂交换变化仅能小幅抵消该趋势（约10%），而在南纬30°以南区域全年积分后，气候效应对人为CO₂吸收的影响可忽略不计。