Antarctic Bottom Water formation and dynamics in a changing climate

Antarctic Bottom Water (AABW) is an important part of the climate system as it supplies the lower limb of the global overturning circulation. AABW is formed from dense waters on the Antarctic shelf which mix with surrounding waters while overflowing into the abyssal ocean. In recent decades, AABW has warmed, freshened, and declined in volume and AABW formation is also projected to decline in the future. The production and propagation of AABW and how these change are difficult to observe and numerical models still remain an important tool to investigate open questions.For the first project, I used the ocean-sea ice model ACCESS-OM2-01 to investigate the interannual variability of AABW formation. The simulated formation and export of AABW exhibits strong interannual variability which is not correlated between the different formation regions. The main factor controlling years of high AABW formation are weaker upstream easterly winds, which reduce sea ice import into the AABW formation region, leaving increased areas of open water primed for air-sea buoyancy loss and convective overturning. This study highlights the variability of simulated AABW formation in all four formation regions, with potential implications for interpreting trends in observational data using only limited duration and coverage.Modelling the formation and downslope flow of AABW represents an ongoing challenge for ocean and climate models due to the high horizontal resolution required. In my second project, we assessed the formation and export of AABW to the abyss and its sensitivity to horizontal model resolution in a circumpolar ocean-sea ice model available at horizontal resolutions of 1/10°, 1/20° and 1/40°. The AABW transport across the 1000 m isobath of the Antarctic continental slope increases by 27% with 1/20° resolution compared to 1/10°, but there is no further transport increase at 1/40° resolution. The higher AABW export at 1/20° compared to 1/10° resolution is due to formation of denser waters on the continental shelf and less diapycnal mixing during the downslope flow. This has effects downstream in the abyss of the Australian Antarctic Basin which is better ventilated in the 1/20° case.Freshening of Antarctic shelf waters has occurred over the past five decades leading to a reduction of AABW volume. However, since the mid 2010s a rebound in salinity in the Ross Sea has been observed but the mechanisms have not yet been fully quantified. In my third project, we use the high-resolution ocean-sea ice model ACCESS-OM2-01 to isolate the effects of changes in winds and meltwater input on the salinity in the Ross Sea. Decreasing the zonal winds upstream of the Ross Sea by 50% or decreasing the meltwater input in the Amundsen Sea by 50% both increase the bottom salinity by ~0.07 psu in the western Ross Sea. Propagation of salinity anomalies into the Ross Sea occurs both via advection within 2-3 years and baroclinic waves within the first 2-3 months. Both decreasing the winds or decreasing the meltwater leads to a reduction of sea ice transport into the Ross Sea leaving increased areas of open water where dense shelf waters are formed.南极底层水（Antarctic Bottom Water, AABW）是气候系统的重要组成部分，其为全球翻转环流的下层支提供物质支撑。南极底层水由南极陆架上的高密度水体形成，这些水体在溢流进入深海大洋的过程中与周边水体发生混合。近数十年来，南极底层水出现增温、淡化现象，且体积持续缩减；未来其形成过程也预计将进一步减弱。南极底层水的生成与传播过程及其变化机制难以直接观测，因此数值模型仍是探究相关未解问题的重要工具。 在第一项研究中，笔者采用海冰-海洋耦合模型ACCESS-OM2-01，探究了南极底层水生成过程的年际变率。模拟得到的南极底层水生成与输出过程呈现出显著的年际变率，且不同生成区域的变率之间不存在相关性。调控南极底层水高生成年份的主要因素为上游东风的减弱——东风减弱会减少流入南极底层水生成区域的海冰量，使得开阔水域面积增加，进而为海气浮力损失与对流翻转过程提供有利条件。本研究揭示了四个南极底层水生成区域的模拟生成过程均存在变率，这一结果对于仅依靠有限时长与覆盖范围的观测数据解读相关趋势具有潜在参考价值。 由于需要极高的水平分辨率，模拟南极底层水的生成与坡地流动过程仍是海洋与气候模型面临的长期挑战。在第二项研究中，我们依托一套可提供1/10°、1/20°与1/40°三种水平分辨率的环极海冰-海洋耦合模型，评估了南极底层水向深海的生成与输出过程，以及该过程对模型水平分辨率的敏感性。相较于1/10°分辨率，1/20°分辨率下跨南极大陆坡1000米等深线的南极底层水输运量提升27%，但1/40°分辨率下输运量未出现进一步增长。1/20°分辨率下南极底层水输出量高于1/10°分辨率的原因在于：陆架上生成的水体密度更高，且坡地流动过程中的跨等密度面混合更少。这一差异会对澳大利亚南极海盆的深海下游区域产生影响——在1/20°分辨率设置下，该区域的通风效果更佳。 过去五十年来，南极陆架水体持续淡化，导致南极底层水的体积缩减。但自2010年代中期以来，罗斯海的盐度出现回升现象，其背后的机制尚未得到完全量化。在第三项研究中，我们采用高分辨率海冰-海洋耦合模型ACCESS-OM2-01，分离出风速变化与融水输入对罗斯海盐度的影响。将罗斯海上游的纬向风速降低50%，或是将阿蒙森海的融水输入量降低50%，均可使罗斯海西部的底层盐度提升约0.07 psu（实用盐度单位）。盐度异常向罗斯海的传播主要通过两种途径：一是在2~3年内通过平流输送，二是在最初的2~3个月内通过斜压波。降低风速或是减少融水输入，均会减少流入罗斯海的海冰量，使得开阔水域面积增加，进而利于高密度陆架水体的生成。