North Atlantic subtropical mode water formation controlled by Gulf Stream fronts

Bolan Gan; Jingjie Yu; Lixin Wu; Gokhan Danabasoglu; R Justin Small; Allison H Baker; Fan Jia; Zhao Jing; Xiaohui Ma; Haiyuan Yang; Zhaohui Chen

doi:10.1093/nsr/nwad133

North Atlantic subtropical mode water formation controlled by Gulf Stream fronts

Natl Sci Rev. 2023 May 8;10(9):nwad133. doi: 10.1093/nsr/nwad133. eCollection 2023 Sep.

Authors

Bolan Gan^{1

2}, Jingjie Yu¹, Lixin Wu^{1

2}, Gokhan Danabasoglu³, R Justin Small³, Allison H Baker³, Fan Jia^{2

4}, Zhao Jing^{1

2}, Xiaohui Ma^{1

2}, Haiyuan Yang^{1

2}, Zhaohui Chen^{1

2}

Affiliations

¹ Frontier Science Center for Deep Ocean Multispheres and Earth System (FDOMES) and Physical Oceanography Laboratory, Ocean University of China, Qingdao 266100, China.
² Laoshan Laboratory, Qingdao 266200, China.
³ National Center for Atmospheric Research, Boulder, CO 80305, USA.
⁴ CAS Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China.

Abstract

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.

Keywords: Gulf Stream thermal fronts; North Atlantic subtropical mode water; eddy-resolving coupled global climate model; frontal-scale ocean-to-atmosphere feedback.