Bates Nicholas R, Pequignet A Christine, Johnson Rodney J, Gruber Nicolas
Bermuda Biological Station For Research, Inc., Ferry Reach, Bermuda.
Nature. 2002 Dec 5;420(6915):489-93. doi: 10.1038/nature01253.
Large-scale features of ocean circulation, such as deep water formation in the northern North Atlantic Ocean, are known to regulate the long-term physical uptake of CO2 from the atmosphere by moving CO2-laden surface waters into the deep ocean. But the importance of CO2 uptake into water masses that ventilate shallower ocean depths, such as subtropical mode waters of the subtropical gyres, are poorly quantified. Here we report that, between 1988 and 2001, dissolved CO2 concentrations in subtropical mode waters of the North Atlantic have increased at a rate twice that expected from these waters keeping in equilibrium with increasing atmospheric CO2. This accounts for an extra 0.4-2.8 Pg C (1 Pg = 10(15) g) over this period (that is, about 0.03-0.24 Pg C yr(-1)), equivalent to 3-10% of the current net annual ocean uptake of CO2 (ref. 3). We suggest that the lack of strong winter mixing events, to greater than 300 m in depth, in recent decades is responsible for this accumulation, which would otherwise disturb the mode water layer and liberate accumulated CO2 back to the atmosphere. However, future climate variability (which influences subtropical mode water formation) and changes in the North Atlantic Oscillation (leading to a return of deep winter mixing events) may reduce CO2 accumulation in subtropical mode waters. We therefore conclude that, although CO2 uptake by subtropical mode waters in the North Atlantic--and possibly elsewhere--does not always represent a long-term CO2 sink, the phenomenon is likely to contribute substantially to interannual variability in oceanic CO2 uptake.
海洋环流的大规模特征，如北大西洋北部的深水形成，已知通过将富含二氧化碳的表层水转移到深海来调节大气中二氧化碳的长期物理吸收。但是，二氧化碳被吸收到为较浅海洋深度提供通气的水体中的重要性，如亚热带环流的亚热带模态水，却很少得到量化。我们在此报告，在1988年至2001年期间，北大西洋亚热带模态水中溶解的二氧化碳浓度增加的速率是这些水体与大气中二氧化碳增加保持平衡时预期速率的两倍。在此期间，这额外增加了0.4 - 2.8Pg C（1Pg = 10¹⁵g）（即约0.03 - 0.24Pg C yr⁻¹），相当于当前海洋每年净吸收二氧化碳量的3% - 10%（参考文献3）。我们认为，近几十年来缺乏深度超过300米的强烈冬季混合事件是造成这种积累的原因，否则这种积累会扰乱模态水层并将积累的二氧化碳释放回大气中。然而，未来的气候变率（影响亚热带模态水的形成）以及北大西洋涛动的变化（导致深冬混合事件再次出现）可能会减少亚热带模态水中二氧化碳的积累。因此，我们得出结论，尽管北大西洋（可能还有其他地方）亚热带模态水对二氧化碳的吸收并不总是代表一个长期的二氧化碳汇，但这种现象很可能对海洋二氧化碳吸收的年际变化有很大贡献。
