From University of New South Wales (AU) via : “Scientists get to the bottom of deep Pacific ventilation”

U NSW bloc

From University of New South Wales (AU)


July 16, 2021

Credit: CC0 Public Domain.

Recent findings, with important implications for ocean biogeochemistry and climate science, have been published by Nature Communications in a paper by Associate Professor Mark Holzer from UNSW Science’s School of Mathematics & Statistics, with co-authors Tim DeVries (University of California-Santa Barbara (US)) and Casimir de Lavergne (LOCEAN: Laboratory of Oceanography and Climatology |[GIS Climat Environnement Société] (FR) (CNRS) (CEA)).

“The deep North Pacific is a vast reservoir of remineralized nutrients and respired carbon that have accumulated over centuries,” says Holzer. “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.”

But 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? To date, there were two competing theories for the role that the overturning circulation plays in this.

One theory—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 other theory—the ‘shadowed conveyor’—argues that the overturning is compressed to lie below about 2.5 km with a largely stagnant “shadow zone” above it.

“Our work reconciles these two theories: the shadowed conveyor correctly captures vertically compressed overturning beneath a shadow zone, while the standard view must be broadly interpreted in terms of water paths diffusing through the shadow zone. Because the shadow zone is largely shielded from the overturning circulation the question becomes how exactly does water get into and out of it,” Holzer says.

Using novel mathematical analyses applied to a state-of-the-art ocean circulation model that optimally fits the circulation to observed tracer distributions and surface forcings, the authors were able to quantify in detail the pathways and timescales with which the shadow zone exchanges water with the surface ocean.

“Our analyses allowed us to come up with a new schematic of the large-scale deep circulation in the Pacific. We find that diffusive transport both along and across density surfaces plays a leading role in ventilating the shadow zone.”

Contrary to the widely held view that Pacific deep waters exclusively follow density surfaces to upwell in the Southern Ocean, the authors found that only about half of the water in the shadow zone follows this route, with the other half returning to the surface in low latitudes and in the subarctic Pacific, helping to explain the high biological production there.

The scientists say this new understanding of the deep Pacific circulation and transport pathways will help interpret observed tracer distributions and biogeochemical processes.

“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,” Holzer says.

See the full article here .


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U NSW Campus

The University of New South Wales is an Australian public university with its largest campus in the Sydney suburb of Kensington.

Established in 1949, UNSW is a research university, ranked 44th in the world in the 2021 QS World University Rankings and 67th in the world in the 2021 Times Higher Education World University Rankings. UNSW is one of the founding members of the Group of Eight, a coalition of Australian research-intensive universities, and of Universitas 21, a global network of research universities. It has international exchange and research partnerships with over 200 universities around the world.

According to the 2021 QS World University Rankings by Subject, UNSW is ranked top 20 in the world for Law, Accounting and Finance, and 1st in Australia for Mathematics, Engineering and Technology. UNSW also leads Australia in Medicine, where the median ATAR (Australian university entrance examination results) of its Medical School students is higher than any other Australian medical school. UNSW enrolls the highest number of Australia’s top 500 high school students academically, and produces more millionaire graduates than any other Australian university.

The university comprises seven faculties, through which it offers bachelor’s, master’s and doctoral degrees. The main campus is in the Sydney suburb of Kensington, 7 kilometres (4.3 mi) from the Sydney CBD. The creative arts faculty, UNSW Art & Design, is located in Paddington, and subcampuses are located in the Sydney CBD as well as several other suburbs, including Randwick and Coogee. Research stations are located throughout the state of New South Wales.

The university’s second largest campus, known as UNSW Canberra at ADFA (formerly known as UNSW at ADFA), is situated in Canberra, in the Australian Capital Territory (ACT). ADFA is the military academy of the Australian Defense Force, and UNSW Canberra is the only national academic institution with a defense focus.

Research centres

The university has a number of purpose-built research facilities, including:

UNSW Lowy Cancer Research Centre is Australia’s first facility bringing together researchers in childhood and adult cancers, as well as one of the country’s largest cancer-research facilities, housing up to 400 researchers.
The Mark Wainwright Analytical Centre is a centre for the faculties of science, medicine, and engineering. It is used to study the structure and composition of biological, chemical, and physical materials.
UNSW Canberra Cyber is a cyber-security research and teaching centre.
The Sino-Australian Research Centre for Coastal Management (SARCCM) has a multidisciplinary focus, and works collaboratively with the Ocean University of China [中國海洋大學](CN) in coastal management research.