The Origin and Fate of Subantarctic Mode Water and Antarctic Intermediate Water in the Southern Ocean

Mr. Zhi Li, PhD, ARC Australian Centre for Excellence in Antarctic Science, University of New South Wales, Sydney, NSW, Australia, Matthew H England, Centre for Marine Science and Innovation, University of New South Wales, Sydney, NSW, Australia, Sjoerd Groeskamp, Royal Netherlands Institute for Sea Research, Ocean Systems, Den Burg, Netherlands, Yiyong Luo, Ocean University of China, Physical Oceanography Laboratory/CIMST, Qingdao, China and Ivana Cerovecki, Scripps Institution of Oceanography, United States
Subantarctic Mode Water (SAMW) forms in deep mixed layers just north of the Antarctic Circumpolar Current during wintertime, playing a fundamental role in the uptake of heat and carbon dioxide by the oceans. Several methods have been used to estimate the formation of SAMW. Examples include a water mass transformation framework diagnosing buoyancy changes due to air-sea interaction, as well as subduction calculations invoking Lagrangian advection across the mixed layer base. However, few methods have combined their results and also included the related volume change of SAMW. In this study, we combine SAMW formation estimates over seasonal to interannual time scales with SAMW volume changes derived from gridded Argo hydrographic data to estimate contributions from air-sea interaction and subduction to the volume change of SAMW. We further separate SAMW volume changes into components within and below the mixed layer. We find that changes in the mixed-layer component of SAMW coincide with SAMW formation due to air-sea interaction, occurring two months before the subduction takes place. In contrast, changes in SAMW beneath the mixed layer occurs in phase with changes in subduction. We therefore hypothesize a step-wise process that determines the formation, circulation and destruction of SAMW. Only by quantifying, understanding and combining all these processes, can we truly understand the origin and fate of Subantarctic Mode Water.