Antarctic Bottom Water (AABW)

[kassy]

Antarctic Bottom Water (AABW) deserves it´s own topic. Most of the research is rather recent.
Here is a new overview article with lots of data.


​​Observing Antarctic Bottom Water in the Southern Ocean​

Dense, cold waters formed on Antarctic continental shelves descend along the Antarctic continental margin, where they mix with other Southern Ocean waters to form Antarctic Bottom Water (AABW). AABW then spreads into the deepest parts of all major ocean basins, isolating heat and carbon from the atmosphere for centuries. Despite AABW’s key role in regulating Earth’s climate on long time scales and in recording Southern Ocean conditions, AABW remains poorly observed. This lack of observational data is mostly due to two factors. First, AABW originates on the Antarctic continental shelf and slope where in situ measurements are limited and ocean observations by satellites are hampered by persistent sea ice cover and long periods of darkness in winter. Second, north of the Antarctic continental slope, AABW is found below approximately 2 km depth, where in situ observations are also scarce and satellites cannot provide direct measurements. Here, we review progress made during the past decades in observing AABW. We describe 1) long-term monitoring obtained by moorings, by ship-based surveys, and beneath ice shelves through bore holes; 2) the recent development of autonomous observing tools in coastal Antarctic and deep ocean systems; and 3) alternative approaches including data assimilation models and satellite-derived proxies. The variety of approaches is beginning to transform our understanding of AABW, including its formation processes, temporal variability, and contribution to the lower limb of the global ocean meridional overturning circulation. In particular, these observations highlight the key role played by winds, sea ice, and the Antarctic Ice Sheet in AABW-related processes. We conclude by discussing future avenues for observing and understanding AABW, impressing the need for a sustained and coordinated observing system.

https://www.bas.ac.uk/data/our-data/publication/observing-antarctic-bottom-water-in-the-southern-ocean/


1 Introduction
Antarctic Bottom Water (AABW) plays a primary role in the climate system, as it supplies the lower branch of the global Meridional (i.e., north–south) Overturning Circulation (MOC; Lumpkin and Speer, 2007; Talley, 2013). The process of AABW formation near the Antarctic coast and its northward spreading allows ventilation of most of the abyssal (>2 km depth) ocean (Johnson, 2008), supplying oxygen (Gordon, 2013) and storing heat and carbon at depth for centuries (de Lavergne et al., 2017; Holzer et al., 2021). Sinking AABW also carries nutrients that have not been utilized by marine organisms due to local light and iron limitation and thereby affects global primary production and the efficiency of the biological carbon pump (Marinov et al., 2006). Changes in AABW formation and circulation are thus thought to influence atmospheric carbon dioxide, and consequently Earth’s climate, on centennial to millennial time scales (Sigman and Boyle, 2000; Ferrari et al., 2014; Marzocchi and Jansen, 2019).

AABW originates on the Antarctic continental shelf (Figure 1), where extremely cold and salty waters are produced. Seawater that is near the surface freezing point and has absolute salinities higher than 34.6 g/kg is known as high-salinity shelf water (HSSW) and is produced on the shelf as a result of surface heat loss and salt input through brine rejection when sea ice forms. Sea ice formation is enhanced near the Antarctic coast, especially in ice-free coastal polynyas where sea ice is continuously formed and advected away by katabatic winds (see Figure 2B for locations of the main Antarctic coastal polynyas). In some locations (e.g., Ross and Weddell Seas, Prydz Bay), HSSW is further cooled by ice–ocean interaction at the base of ice shelves, producing supercooled water colder than the surface freezing point, known as Ice Shelf Water (ISW) that is typically below −2°C. Once formed, a fraction of HSSW/ISW escapes the continental shelf and cascades into the abyssal Southern Ocean. Dense waters produced on the continental shelf (HSSW and ISW) are usually referred altogether to as Dense Shelf Water (DSW). While sinking as a gravity plume down the continental slope, DSW mixes with other Southern Ocean waters, mostly with warmer (approximately 1°C to 2°C) Circumpolar Deep Water (CDW) and fresher (absolute salinity<34.6 g/kg) Antarctic Surface Water (Orsi et al., 1999; Akhoudas et al., 2021). This mixing process produces AABW, which is water that is colder than 0°C with neutral densities (Jackett and McDougall, 1997) greater than 28.27 kg/m3 (Figure 1). AABW properties and formation rates can also be influenced by offshore polynyas, as the one observed in the 1970s in the Weddell Sea causing convection up to 3,000 m depth (Gordon, 1978). However, such offshore vigorous deep convective events have not been observed since the 1970s, indicating that the present-day main source region of AABW is the Antarctic continental shelf (see Figures 2B, 3).

And much more:
https://www.frontiersin.org/articles/10.3389/fmars.2023.1221701/full

[kassy]

From the Ice Apocalypse thread:

For your consideration:

Li, Q., England, M.H., Hogg, A.M. et al. Abyssal ocean overturning slowdown and warming driven by Antarctic meltwater. Nature 615, 841–847 (2023). https://doi.org/10.1038/s41586-023-05762-w

https://www.nature.com/articles/s41586-023-05762-w

Abstract: “The abyssal ocean circulation is a key component of the global meridional overturning circulation, cycling heat, carbon, oxygen and nutrients throughout the world ocean. The strongest historical trend observed in the abyssal ocean is warming at high southern latitudes, yet it is unclear what processes have driven this warming, and whether this warming is linked to a slowdown in the ocean’s overturning circulation. Furthermore, attributing change to specific drivers is difficult owing to limited measurements, and because coupled climate models exhibit biases in the region. In addition, future change remains uncertain, with the latest coordinated climate model projections not accounting for dynamic ice-sheet melt. Here we use a transient forced high-resolution coupled ocean–sea-ice model to show that under a high-emissions scenario, abyssal warming is set to accelerate over the next 30 years. We find that meltwater input around Antarctica drives a contraction of Antarctic Bottom Water (AABW), opening a pathway that allows warm Circumpolar Deep Water greater access to the continental shelf. The reduction in AABW formation results in warming and ageing of the abyssal ocean, consistent with recent measurements. In contrast, projected wind and thermal forcing has little impact on the properties, age and volume of AABW. These results highlight the critical importance of Antarctic meltwater in setting the abyssal ocean overturning, with implications for global ocean biogeochemistry and climate that could last for centuries."

and

Discussion of the paper:
https://e360.yale.edu/features/climate-change-ocean-circulation-collapse-antarctica

Some quotes:

A new analysis by Australian and American researchers, using new and more detailed modeling of the oceans, predicts that the long-feared turn-off of the circulation will likely occur in the Southern Ocean, as billions of tons of ice melt on the land mass of Antarctica. And rather than being more than a century away, as models predict for the North Atlantic, it could happen within the next three decades.

Leading ocean and climate researchers not involved in the study who were contacted for comment praised the findings. “This is a really important paper,” says Stefan Rahmstorf, an oceanographer and head of earth system analysis at the Potsdam Institute for Climate Impact Research in Germany. “I think the method and model are convincing.”

...

Meanwhile the long-standing concern about a shutdown of the ocean circulation in the North Atlantic sometime in the 21st century appears to be subsiding. A Swiss study published this month found that, contrary to past belief, the circulation did not fail at the end of the last ice age, suggesting, the researchers say, that it was more stable than previously supposed, and less likely to collapse.

...

About 250 trillion tons of salty water sinks in this way around Antarctica each year, subsequently spreading north along the ocean floor into the Indian, Atlantic, and Pacific oceans. Similar volumes spread south from Greenland. The process is known as deep-water formation or ocean overturning, and it has continued largely unchanged for thousands of years.

But for how much longer? As the world warms, less ice is forming in the oceans at the ends of the Earth each year. At the same time, more ice on the nearby great ice sheets of Antarctica and Greenland is melting and releasing fresh water into the ocean.

As a result, surface water in the Southern Ocean and around Greenland is already becoming less salty, less dense, and so less able to sink. Since the 1990s, measurements taken from ships have shown that the water on the ocean floor, below 13,000 feet in depth, has warmed and freshened, with the trend strongest in the Southern Ocean.

...

The most recent assessment of the UN’s Intergovernmental Panel on Climate Change (IPCC) reported “medium confidence” of this occurring later this century. But it said models were not able to quantify the impact of glacial meltwater on how fast or how far this trend might go. Rahmstorf says this failure, which arose because ice dynamics have been poorly understood and not integrated into models of climate change and ocean circulation, is “a long-standing and major shortcoming” of models presented in major IPCC reports.

Until now.

The groundbreaking modeling study published by Australian and American researchers at the end of March for the first time includes a detailed assessment of the likely impact of melting ice, revealing the importance of this past failure. It predicts a 42 percent decline in deep-water formation in the Southern Ocean by 2050. This is more than twice the 19 percent they predict for an equivalent event in the North Atlantic.

...