Gan Bolan, Yu Jingjie, Wu Lixin, Danabasoglu Gokhan, Small R Justin, Baker Allison H, Jia Fan, Jing Zhao, Ma Xiaohui, Yang Haiyuan, Chen Zhaohui
Frontier Science Center for Deep Ocean Multispheres and Earth System (FDOMES) and Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China.
Laoshan Laboratory, Qingdao 266200, China.
Natl Sci Rev. 2023 May 8;10(9):nwad133. doi: 10.1093/nsr/nwad133. eCollection 2023 Sep.
The North Atlantic Ocean hosts the largest volume of global subtropical mode waters (STMWs) in the world, which serve as heat, carbon and oxygen silos in the ocean interior. STMWs are formed in the Gulf Stream region where thermal fronts are pervasive and result in feedback with the atmosphere. However, their roles in STMW formation have been overlooked. Using eddy-resolving global climate simulations, we find that suppressing local frontal-scale ocean-to-atmosphere (FOA) feedback leads to STMW formation being reduced almost by half. This is because FOA feedback enlarges STMW outcropping, attributable to the mixed layer deepening associated with cumulative excessive latent heat loss due to higher wind speeds and greater air-sea humidity contrast driven by the Gulf Stream fronts. Such enhanced heat loss overshadows the stronger restratification induced by vertical eddies and turbulent heat transport, making STMW colder and heavier. With more realistic representation of FOA feedback, the eddy-present/rich coupled global climate models reproduce the observed STMWs much better than the eddy-free ones. Such improvement in STMW production cannot be achieved, even with the oceanic resolution solely refined but without coupling to the overlying atmosphere in oceanic general circulation models. Our findings highlight the need to resolve FOA feedback to ameliorate the common severe underestimation of STMW and associated heat and carbon uptakes in earth system models.
北大西洋拥有全球最大体积的副热带模态水（STMWs），这些副热带模态水在海洋内部充当热量、碳和氧气的储存库。STMWs在墨西哥湾流区域形成，该区域热锋普遍存在，并与大气形成反馈。然而，它们在STMW形成中的作用一直被忽视。通过采用能解析涡旋的全球气候模拟，我们发现抑制局地锋面尺度的海气（FOA）反馈会导致STMW的形成减少近一半。这是因为FOA反馈扩大了STMW的出露范围，这归因于混合层加深，而混合层加深是由墨西哥湾流锋面驱动的更高风速和更大海气湿度差导致的累积过多潜热损失所致。这种增强的热损失掩盖了垂直涡旋和湍流传热引起的更强的重分层作用，使得STMW更冷、更重。由于能更真实地体现FOA反馈，包含涡旋/丰富涡旋的耦合全球气候模型比无涡旋模型能更好地再现观测到的STMWs。即使仅提高海洋分辨率但在海洋环流模型中不与上层大气耦合，也无法实现STMW生成的这种改进。我们的研究结果凸显了解析FOA反馈对于改善地球系统模型中普遍严重低估的STMW以及相关热量和碳吸收情况的必要性。
