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- Title
On the Global Decrease in the Deep and Abyssal Density Stratification Along the Spreading Pathways of Antarctic Bottom Water Since the 1990s.
- Authors
Tan, Shuwen; Thurnherr, Andreas M.
- Abstract
The density stratification in the ocean is directly related to the diapycnal mixing, which drives the abyssal cell of the Meridional Overturning Circulation (MOC). It is important to understand how stratification has been changing in the world's deep and abyssal oceans under climate change. Using repeat hydrographic data obtained since the 1990s, we find a decreasing stratification associated with changes in the source Antarctica Bottom Water (AABW) properties in its formation basins as well as in basins along its dispersal pathways. Averaged south of 60°S, the squared buoyancy frequency N2 shows a negative trend of −6% per decade in waters deeper than 4,000 m. The observed decadal reduction in stratification is associated with large spatial variability, especially in the Southern Ocean basins with multiple AABW sources. Additionally, there are also significant differences between neighboring basins that are related to the blocking effect of topography. Plain Language Summary: Cold and dense Antarctica Bottom Water (AABW) forms near the coastline of Antarctica, sinks to the ocean bottom, moves northward in deep branches of the Meridional Overturning Circulation, becomes lighter through mixing, and eventually upwells to shallower depths. The AABW properties change under the changing climate. Repeat hydrography sections occupied approximately once per decade since the 1990s have revealed continuously warming and freshening of the AABW. In many regions of the ocean, the warming and freshening trends are strongest at the bottom. This causes a decrease in the vertical gradient of density, namely stratification. As the level of mixing is strongly related to stratification, it is important to quantify its trend and understand its spatial structure. We quantify the trend in stratification globally and find a reduction in stratification in the Southern Ocean and along the spreading pathways of the AABW at lower latitudes. Key Points: Three decades of repeat hydrographic sections reveal decreasing density stratification in the deep and abyssal layers of the Southern OceanSouth of 60°S, N2 has reduced by 6% per decade in the ocean below 4,000 m, with a peak reduction rate of 15% per decade at 4,800 mThe decreasing stratification is found along the pathway of Antarctica Bottom Water and its vertical structure depends on the topography of basin boundaries
- Subjects
ANTARCTICA; BOTTOM water (Oceanography); HYDROGRAPHY; SUBGLACIAL lakes; MERIDIONAL overturning circulation; OCEAN bottom; DENSITY; TOPOGRAPHY
- Publication
Geophysical Research Letters, 2023, Vol 50, Issue 11, p1
- ISSN
0094-8276
- Publication type
Article
- DOI
10.1029/2022GL102422