Seamounts impact ocean circulation and climate in surprising ways

Massive undersea mountains, or seamounts, rise to heights unimaginable for those of us who live above water, reaching elevations thousands of meters high.

Seamounts, while breathtakingly majestic, are also vital players in stirring up our ocean’s currents, thus significantly impacting how our ocean stores heat and carbon.

Diving deeper into seamounts

Dr Ali Mashayek, a leading scientist from the University of Cambridge’s Department of Earth Science, spearheaded an international team in shining a light on these undersea giants.

Using numerical modeling, the team began to understand how the turbulence surrounding these mountains influences ocean circulation.

Dr Mashayek explains, “The intense turbulence around seamounts makes them a major contributor to ocean mixing at a global scale, but we don’t have that process represented in climate models.”

These findings finally acknowledge the role of seamounts, and as a result, may improve future model forecasts concerning how the ocean will react to global warming.

Ocean’s deep-churning conveyor belt

Picture our ocean as a vast, ceaselessly moving conveyor belt. Warm water from the tropical regions slowly drifts towards the poles, where it cools and plunges thousands of meters into the ocean’s abyss, taking vital carbon, heat and nutrients along with it.

Ocean circulation refers to the perpetual movement of seawater throughout the global oceans, orchestrated by wind, variations in water density, and tidal forces.

Surface currents, strategically driven by the wind, navigate the upper layers of the ocean, while the profound deep currents, termed thermohaline circulation, are governed by disparities in water temperature and salinity.

These interconnected flows play a crucial role in regulating the Earth’s climate. By redistributing heat, transporting essential nutrients, and impacting weather patterns, they ensure a delicate balance within our planet’s intricate environmental systems.

This cold, heavy water must resurface so the ocean doesn’t become overloaded with frigid water. But the mighty question has been — where does the power for this return flow come from?

This new study uncovers some answers, highlighting how seamounts lend a helping hand (or peak) to ocean circulation.

Guardians of deep sea currents

Tens of thousands of seamounts reside at the ocean’s bottom, and this number is believed to be a mere quarter of the actual figure, given only a fragment of the seabed has been mapped.

These undersea mountains don’t just silently watch the currents pass — they intervene, churning the waters around them.

Water charges over their sharp slopes, generating swirling wake vortices that ferry water towards the surface.

Dr Mashayek describes the scene, saying, “The deep waters around a seamount are chaotic and turbulent…the turbulence churns up the ocean just like stirring milk into your coffee.”

This stirring action helps yank the heavy water to the surface, completing a cycle that keeps the ocean’s conveyor belt running.

Seamounts impact on ocean mixing

Despite previous measurements of deep-sea turbulence around seamounts, the science community remained unsure about its role in ocean circulation on a global scale.

However, according to Dr Mashayek and his team, the stirring around seamounts accounts for approximately one-third of ocean mixing worldwide, reaching up to 40% in the Pacific Ocean due to its higher seamount count.

These massive stirring rods could potentially speed up climate change, especially in large carbon stores like the Pacific.

According to co-author Dr Laura Cimoli, “If seamounts are enhancing mixing…then the timescale of storage could be shorter and if carbon is released sooner that could speed climate change.”

Seamounts, circulation, and climate change

The esteemed oceanographer Walter Munk theorized back in the 1960s that seamounts might be “the stirring rods of the ocean”.

It’s taken us this long to measure its significance on a global scale, but our growing knowledge of the seafloor has finally allowed us to test this theory.

“The only reason we’ve been able to put this to the test now is that we only recently had enough of the seafloor mapped,” explains co-author Professor Alberto Naveira Garabato from the University of Southampton.

The number of seamounts is likely to be even larger, so our estimates of their importance in mixing are still conservative. Going forward, the team intends to integrate the physics of seamount-induced turbulence into climate models.

With these improvements, we are one step closer to having a precise representation of deep ocean circulation, and thus, a better understanding of how the ocean will adapt to climate change.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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