全球海洋CFC-11吸收对传输速度的敏感性

本文在中国科学院大气物理研究所发展的全球海洋模式(LICOM)中使用五个不同的海气交换的气体 传输速度公式对CFC-11(一氟三氯甲烷)在海洋中的分布和吸收做了模拟。讨论了不同气体传输速度的差异, 发现差异最大的两个公式得到的全球年平均传输速度相差81%。对CFC-11的海表浓度分布、海气通量、水柱总量、海水累积吸收量以及在大洋断面的垂直浓度分布进行了分析。分析结果显示, 使用Liss and Merlivat (1986) 的传输速度公式的试验在海气通量和海洋吸收总量的模拟上均小于其他试验, Nightingale et al. (2000)、Ho et al. (2006) 和Sweeney et al. (2007) 等的公式虽然全球年平均值相近, 但在高风速地区Nightingale et al. (2000) 公式的传输速度要小于后两者, 导致了使用该公式的试验模拟结果在主吸收区和存储区的强度比后二者偏小。Wanninkhof (1992) 的公式在形式上与Ho et al. (2006) 以及Sweeney et al. (2007) 的公式一致, 但在系数上存在差别, 这使得模拟的水柱总量在南大洋的分布明显好于其他试验, 尽管其最大值仍比观测资料略小。在海洋累积吸收量的计算上, 使用Wanninkhof (1992) 传输速度公式得到的模拟结果比观测资料小8%左右。计算了Liss and Merlivat (1986) 和Wanninkhof (1992) 的传输速度公式模拟的单年吸收量相对差, 其总体上一直保持持续下降的趋势, 到2007年仅为2%。从该相对差变化趋势看, 在最初的前10年, 海气CFC-11交换通量对海气交换传输速度的敏感性更强, 而在更长时间的模拟上, 海洋对CFC-11的吸收则更依赖于物理模式的通风速率。通过对CFC-11垂直断面分布的分析可知, 不同的传输速度在主要吸收区的不同导致了一定的垂直分布差异。基于本文的结果可以认为Wanninkhof (1992) 的海气气体传输速度公式更适合本模式对CFC-11的模拟。

Sensitivity of CFC-11 Uptake in a Global Ocean Model to Air-Sea Gas Transfer Velocity

Using five different formulas for the air-sea gas transfer velocity, the oceanic uptake, distribution, and storage of CFC-11 are simulated in the global ocean general circulation model (OGCM) LICOM (LASG/IAP Climate System Ocean Model), which was developed by the Institute of Atmospheric Physics of the Chinese Academy of Sciences. The difference in the transfer velocities calculated using each formula is discussed. The largest difference in global annual mean transfer velocity among these formulas is 81%. The sea surface CFC-11 concentrations, air-sea CFC-11 fluxes, water column inventories, cumulative uptake, and vertical distributions are analyzed. The results show that the air-sea CFC-11 flux and the total uptake obtained in an experiment using the formula of Liss and Merlivat (1986) are less than those from other experiments using other formulas. Although the global annual mean transfer velocities obtained using the formulas of Nightingale et al. (2000), Ho et al. (2006), and Sweeney et al. (2007) are quite close to each other, the transfer velocity from the formula of Nightingale et al. (2000) in regions with high wind speed is smaller than those from the latter two formulas because of differences in the formulas, leading to a relatively small strength in the main uptake and storage area. The formula of Wanninkhof is formally the same as those of Ho et al. and Sweeney et al. except for the proportional coefficient, and thus it results in a significantly better simulation of the distribution of the CFC-11 water column inventory in the Southern Ocean. However, its maximum value is still slightly smaller than the data-based estimate. The simulated global accumulative uptake with the transfer velocity from Wanninkhof is 8% smaller than the data-based estimate. The relative difference in the oceanic CFC-11 uptake between two experiments that used the formulas of Wanninkhof and Liss and Merlivat, respectively, on data from individual years decreases overall with time until it is about 2% in 2007. This demonstrates that for a short time period (10 years) from the beginning of the simulation, the sensitivity of CFC-11 uptake to the air-sea gas transfer velocity is greater, whereas during a long-term simulation, the oceanic uptake of tracers depends more on the physical model. In addition, the simulated CFC-11 results are compared with cruise data for two sections. The results show that the different transfer velocities used in these experiments produce certain differences in the vertical distributions of CFC-11 in the main uptake regions. For our OGCM, the simulated CFC-11 results from Wanninkhof's formula for air-sea gas transfer velocity are closer to the data-based estimates than those from other formulas.