Ocean circulation, nutrient cycling and atmospheric carbon dioxide

海洋的物理循环、海洋中营养物质的循环和深海中碳的储存之间的联系，是理解温室气体大气二氧化碳（CO2）和地球气候过去和未来变化的基础。然而，目前尚不确定海洋养分循环和大气二氧化碳对全球海洋环流模式的变化有多敏感。营养物质通过海洋环流提供给海洋表面阳光照射的区域，生物体利用这些营养物质生长，将碳储存在软组织中。其中一些生物会死亡，从表层海洋中掉出来，或者被其他生物吃掉，这些生物排出的残骸也会从表层海洋中沉下去。当这种物质下沉时，它被细菌消耗，细菌“溶解”它，将它所含的碳和营养物质释放回深海。这种生物软组织从海洋表面到深海的运输，有助于将碳与大气隔绝，使大气中的二氧化碳浓度保持在较低水平。特别令人感兴趣的是海洋表层的一些区域，这些区域提供的营养物质仍未被生物利用，这意味着深海中储存的碳较少。这样的区域中最大的是南大洋，它在南极洲以北形成了一个环。如果全球海洋环流发生变化，使得更多的营养物质通过南大洋的表层水域循环，那么总的来说，营养物质的生物利用效率将降低，深海中储存的碳将减少，大气中的二氧化碳将增加。相反，如果海洋环流发生变化，通过南大洋表层水域循环的营养物质减少，那么更多的营养物质将被利用，更多的碳将被储存在深海中，大气中的二氧化碳将下降。我们建议量化全球生物养分利用、深海碳储量和大气二氧化碳对海洋环流变化的敏感性（例如，对不同水团形成速率的敏感性）。我们将执行和分析数值三维海洋模型的多次运行，以调查和形式化海洋循环，营养循环和通过下降的生物材料储存在深海中的碳量之间的联系。然后，这些发现将用于研究：(1)海洋环流的变化如何在冰期周期中对过去大气二氧化碳的变化做出贡献；(2)未来海洋环流的变化如何通过加速或减缓大气二氧化碳的增加来放大或抑制当前人为的气候强迫；(3)通过“地球工程”建议，试图提高海洋表面部分地区的生产力，可以封存多少碳。

The link between the physical circulation of the ocean, the cycling of nutrients in the ocean, and the storage of carbon in the deep ocean, is fundamental to the understanding of past and future changes in the greenhouse gas atmospheric carbon dioxide (CO2) and the Earth's climate. However, it is currently uncertain how sensitive ocean nutrient cycling and atmospheric CO2 are to changes in global ocean circulation patterns. Nutrients are supplied to the sunlit regions of the surface ocean by ocean circulation, living organisms then use these nutrients to grow, storing carbon within their soft tissues. Some of these organisms then die and fall out of the surface ocean, or are eaten by other organisms, which excrete their remains that also sink out of the surface ocean. As this material sinks it is consumed by bacteria which 'dissolve' it, releasing the carbon and nutrients it contains back into the deep ocean. This transport of biological soft tissue, from the surface ocean to depth, helps to isolate carbon away from the atmosphere, keeping atmospheric carbon dioxide concentrations low. Of particular interest are areas of the surface ocean in which supplied nutrients remain unutilised by living organisms, meaning less carbon is stored in the deep ocean. The largest such area is the Southern Ocean, which forms a ring around the planet north of Antarctica. If global ocean circulation changes such that more nutrients are circulated through the surface waters of the Southern Ocean, then overall there will be less efficient biological utilisation of nutrients, less carbon will be stored in the deep ocean, and atmospheric CO2 will increase. Conversely, if ocean circulation changes such that fewer nutrients are circulated through the Southern Ocean surface waters, then more nutrients will be utilised, more carbon will be stored in the deep ocean and atmospheric CO2 will fall. We propose to quantify the sensitivity of global biological nutrient utilisation, deep ocean carbon storage and atmospheric CO2 to changes in ocean circulation (for example, to the rates of formation of different water masses). We shall perform and analyse multiple runs of a numerical 3D ocean model to investigate and formalise the link between ocean circulation, nutrient cycling and the amount of carbon stored in the deep ocean by falling biological material. These findings will then be used to examine: (i) How ocean circulation changes could have contributed to past changes in atmospheric CO2 during ice age cycles (ii) How ocean circulation changes in the future might amplify or dampen current anthropogenic climate forcing by accelerating or decelerating the increase in atmospheric CO2, (iii) How much carbon could be sequestered by 'geo-engineering' proposals to try and boost productivity in parts of the surface ocean.