Boundary Upwelling of Antarctic Bottom Water by Topographic Turbulence

Abstract

Abstract The lower cell of the meridional overturning circulation (MOC) is sourced by dense Antarctic Bottom Waters (AABWs), which form and sink around Antarctica and subsequently fill the abyssal ocean. For the MOC to “overturn,” these dense waters must upwell via mixing with lighter waters above. Here, we investigate the processes underpinning such mixing, and the resulting water mass transformation, using an observationally forced, high‐resolution numerical model of the Drake Passage in the Southern Ocean. In the Drake Passage, the mixing of dense AABW formed in the Weddell Sea with lighter deep waters transported from the Pacific Ocean by the Antarctic Circumpolar Current is catalyzed by energetic flows impinging on rough topography. We find that multiple topographic interaction processes facilitate the mixing of the two water masses, ultimately resulting in the upwelling of waters with neutral density greater than 28.19 kg m −3 , and the downwelling of the lighter waters above. In particular, we identify the role of sharp density interfaces between AABW and overlying waters and find that the dynamics of the interfaces' interaction with topography can modify many of the processes that generate mixing. Such sharp interfaces between water masses have been observed in several parts of the global ocean, but are unresolved and unrepresented in climate‐scale ocean models. We suggest that they are likely to play an important role in abyssal dynamics and mixing, and therefore require further exploration. Plain Language Summary Dense, cold waters are formed near Antarctica, then sink to the bottom and spread northwards through the deep ocean. A global overturning circulation is maintained by the return of these dense waters back toward the surface, referred to as upwelling. This circulation allows the deep ocean to exchange heat and carbon with the upper ocean and atmosphere and is, therefore, a key component of the climate system. The upwelling of the deep waters happens due to turbulence in the deep ocean mixing dense waters with the lighter waters above. This turbulent mixing is often caused by the impingement of currents on rough seafloor topography. The processes that generate the turbulence, and the resulting upwelling, are not currently well understood. In this study, we use a realistic numerical simulation to investigate the processes causing the turbulent mixing of deep waters in an energetic region of the Southern Ocean. We find that there are sharp density gradients between the dense bottom waters and the overlying lighter waters, and that these gradients themselves can play a dynamic role in generating turbulent mixing. The sharp gradients are not resolved or represented in climate models, so their impact on the deep‐ocean circulation requires further exploration. Key Points Sharp density interfaces exist between Antarctic Bottom Water (AABW) and the overlying Circumpolar Deep Water in the Drake Passage The sharpness of the interfaces contributes to rich submesoscale and internal wave dynamics and complex topographic interactions These small‐scale, temporally variable and complex dynamics result in boundary‐focused upwelling of AABW