Key Highlights
- Two major theories to play role in overturning circulations
- The circulation to observe the distribution traces and surface forces.
- Density surfaces to play a leading role in ventilating the shadow zone.
Ocean Biogeochemistry Implications
Nature Communications once published a report of important implications for ocean biogeochemistry by Associate Professor Mark Holzer from UNSW Science’s School of Mathematics & Statistics.
Assistant professor Holzer said in a statement that, “The deep North Pacific is a vast reservoir of remineralized nutrients and respired carbon that has accumulated over centuries.”
When these deep waters are returned to the surface, their nutrients support biological production and their dissolved CO2 can be released into the atmosphere. As such, the deep Pacific plays a key role in the earth’s climate system.
Two Circulation Theories
At present, the question is what are the pathways of the ocean circulation that supply newly ventilated surface water to the deep Pacific? And, how and where does this old water eventually return to the surface?
There are two theories that play a role in overturning circulation,
- The Standard Conveyor: envisions broad overturning with Antarctic Bottom Water upwelling to around 1.5 km depth before flowing back south to the Southern Ocean
- The Shadowed Conveyor: argues that the overturning is compressed to lie below about 2.5 km with a largely stagnant “shadow zone” above it.
With these two major theories, the shadowed conveyor perfectly captures vertically compressed overturning beneath a shadow zone, while the standard view broadly interprets in term of water paths diffusing through the shadow zone.
Large-Scale Deep Circulation
Novel mathematical analyses were implemented to a state-of-the-art ocean circulation model which fits the circulation to observe the traces of distribution and surfacing forcing.
The authors were now able to describe in detail the pathways and timescales of the shadow zone which exchanges the water with the surface ocean.
The analysis allows a new schematic of the large-scale deep circulation in the Pacific. This showed the diffusive transport both along and across density surfaces which plays a leading role in ventilating the shadow zone.
Understanding the Shadow Zone
The wide view of Pacific deep waters follows the density surfaces to upgrade the Southern Ocean; later it was found that about half of the water in the shadow zone follows this route.
Whereas the other half return to the surface in low latitudes and in the subarctic Pacific helps to explain the high biological production.
The scientists have said that this is the new understanding of the deep Pacific circulation and transport pathways, which helps to interpret distributing traces and biogeochemical processes.
Assistant professor Holzer commented, “An exciting direction for future research is to understand how the shadow zone, already low in oxygen and sensitive to increased oxygen demand, shapes the response of the ocean’s biological pump to climate change.”
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