The North Atlantic Deep Water surfaces in the Southern Ocean and is modified by surface fluxes to become Antarctic Bottom Water (sinking to the ocean floor) and Antarctic Intermediate water (flowing north at about 100m depth). Refer to the figures of (potential) temperature and salinity to answer the following question:

What is the main surface flux that creates Antarctic Bottom water?

Question

The North Atlantic Deep Water surfaces in the Southern Ocean and is modified by surface fluxes to become Antarctic Bottom Water (sinking to the ocean floor) and Antarctic Intermediate water (flowing north at about 100m depth). Refer to the figures of (potential) temperature and salinity to answer the following question:

What is the main surface flux that creates Antarctic Bottom water?

BlackTom AI · Accepted Answer

To approach this question, consider what surface processes change the properties of North Atlantic Deep Water as it saltier, denser water interacts with the Southern Ocean and transitions into Antarctic Bottom Water.
Option 1 – Cooling: While cooling can increase density, in the Southern Ocean the dominant surface modification leading toward bottom-water formation is not cooling alone. The surface warming/cooling pattern varies regionally, but the key driver for the densification that drives AABW formation is more closely tied to salinity changes than temperature changes at the surface in this context.
Option 2 – Warming: Warming would generally reduce density and hinder sinking, not promote the formation of very dense bottom water. Since AABW formation requires becoming sufficiently dense to sink, warming is the opposite of what is needed for the main densification process here.
Option 3 – Gaining salinity: This is the central mechanism. In the Southern Ocean, high-salinity surface water increases density, promoting deep convection and sinking, which converts North Atlantic Deep Water into Antarctic Bottom Water. The salinity signal is the primary surface flux altering density and promoting the formation of AABW, making this choice the correct driver in this scenario.
Option 4 – Freshening: Freshening lowers salinity and density, which would reduce the tendency to sink and would oppose bottom-water formation. In the mechanisms described, freshening would hinder, not enable, the creation of Antarctic Bottom Water, so this option is not supported by the physics of AABW formation.
In summary, the surface flux that most effectively increases water density to drive sinking into the abyss is the gain in salinity, rather than cooling, warming, or freshening, which either have weaker density effects or counteract sinking processes.

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