季节性层化对北黄海海气CO2交换通量的影响

Anthropogenic processes have significantly affected the global carbon cycle. The increasing of atmospheric CO2 caused by the burning of fossil fuel has resulted in lots of undesirable impacts such as global warming, ocean acidification and so on. The global ocean plays an important role in absorbing the anthropogenic CO2 for the atmosphere, especially for the continental shelf seas which are disproportionately important to ocean carbon cycles and budgets. Compared with the East China Sea and South China Sea, the uncertainties of sea-air CO2 fluxes in the northern Yellow Sea are still considerable and their controlling mechanisms are still quite unclear. Even, the patterns for the atmospheric CO2 sink/source for the northern Yellow Sea are still contradiction in different studies. Based on the previous studies and the characters for the temperature continental shelf seas, we proposed that the seasonal stratification should be an important factor to control the sea-air CO2 fluxes in the northern Yellow Sea. In this study, we firstly re-evaluate the sea-air CO2 fluxes in the northern Yellow Sea based on high temporal and spatial resolution dataset from our group which are constructed from more than 20 cruises. We will evaluate the monthly, seasonal and intra-seasonal variation of sea-air CO2 fluxes in the northern Yellow Sea. The climatology seasonal distribution of sea-air CO2 fluxes will also be proposed. Moreover, we will examine the controlling mechanisms for the surface seawater pCO2 in the northern Yellow Sea. We will decouple the effects between seasonal stratification and some other processes affecting the surface seawater pCO2, such as, temperature and biological processes. Then the relationship between seasonal stratification and variation of surface seawater pCO2 will be quantified to estimate the variation and the uncertainties for the sea-air CO2 fluxes in the northern Yellow Sea. The results of study are helpful for us to understand sea-air CO2 fluxes and the carbon cycle in the China coastal sea.

工业革命以来，人类活动导致大气CO2浓度持续升高，其引起的全球变暖，海洋酸化等负面效应成为当今炙手可热的社会问题和科学问题。陆架边缘海在吸收大气CO2方面发挥着重要的作用。相比较东海和南海，北黄海海气CO2交换通量的研究还比较薄弱。其海气CO2交换通量的量级还存在较大不确定度，控制机制也尚不清晰。我们通过对已有的研究进行分析总结，提出季节性层化过程是影响北黄海海气CO2交换通量的重要过程。本研究，一方面利用项目组已有的高时空分辨率的数据集，评估北黄海海气CO2交换通量月际、季节分布，降低北黄海海气CO2交换通量评估的不确定度；另一方面，以季节性层化为主线，结合温度、生物等效应的影响，研究北黄海表层海水pCO2的季节变化和控制机制，从机理上认识该海域海气CO2交换通量的时空变化规律。研究成果可以完善我国北方海域温室气体的基础数据，有助于提高我们对中国近海碳循环过程的认识。

本研究基于24个航次的高时空分辨率数据集以及一维和三维数值模型的结果，得到如下主要研究结论：北黄海月平均海水pCO2范围为300-430 μatm，从1月份开始下降，3月份达到最低值约300 μatm，然后开始逐渐升高，7月份达到较大值约400 μatm，然后小幅度下降，再继续升高，11月份到达最大值约430 μatm，并逐渐下降；月平均海气CO2交换通量的分布范围为-13至3 mmol C/m2/day。其中，1、3、4、5、10月份表现为大气CO2的汇，6、7、8、9、11、12月份表现为大气CO2的源；从季节分布看，春、冬北黄海表现为大气CO2的汇，不同月份变化范围分别为-15至0 mmol C/m2/day和-8至0 mmol C/m2/day，夏、秋北黄海表现为大气CO2的源，不同月份的变化范围分别在-3至4 mmol C/m2/day和-6至9 mmol C/m2/day；从控制机制看，温度和生物作用不是控制北黄海pCO2月际变化的主要因素，层化作用对海水pCO2的分布有较显著的影响，温度归一化海水pCO2与层化强度在深水区和浅水区的关系分别为y = -186×log(PEA) + 713和y = -95×log(PEA) + 490；从层化破坏期到层化消亡期，海水pCO2增加了46 μatm，海水CO2交换通量增加了5 mmol C/m2/day；从层化消亡期到层化形成期，海水pCO2减少了113 μatm，海水CO2交换通量减少了13 mmol C/m2/day；从层化形成期到层化稳定期，海水pCO2增加了69 μatm，海水CO2交换通量增加了8 mmol C/m2/day；从层化稳定期到层化破坏期，海水pCO2减少了1 μatm，海水CO2交换通量基本不变；层化破坏期，强层化区域海水pCO2低于弱层化区域；层化消亡期，海水pCO2主要由水团混合控制；层化形成期，海水pCO2主要由生物作用控制；层化稳定期，冲淡水中pCO2收到生物作用，海流传输和海水碳酸盐体系的缓冲作用控制。本项目按照申请书中的研究计划，完成了相应的研究内容和研究目标。研究的成果已经发表于SCI文章以及专著中，此外，基于的研究成果很好的应用于国家应对气候变化领域当中，包括《中国海洋环境状况公报》以及多个十四五规划中。