Shutdown of northern Atlantic overturning after 2100 following deep mixing collapse in CMIP6 projections
- Hakan Sener
- Oct 15
- 4 min read
Deep convection collapse in northern Atlantic precedes circulation shutdown in 70% of high-emission scenarios—reducing ocean heat transport to 20-40% of current levels and potentially triggering severe European cooling.
A new study led by Sybren Drijfhout (Royal Netherlands Meteorological Institute) analyzes CMIP6 climate models extended to 2300-2500, revealing that the Atlantic meridional overturning circulation (AMOC) evolves toward shutdown in nearly all high-emission scenarios.
The research shows these shutdowns are consistently preceded by mid-21st century collapse of deep ocean mixing in the Labrador, Irminger, and Nordic Seas—a tipping point that pushes the circulation into terminal decline taking 50-100 years to fully unfold.
Key Findings: From Deep Mixing Collapse to Circulation Shutdown
70% of models reach AMOC shutdown under high emissions
Under the SSP585 high-emission scenario, all nine models extended beyond 2100 reach shutdown states with circulation below 6 Sv (compared to 14-26 Sv in the late 20th century). Including models ending at 2100, approximately 70% show characteristics indicating they're en route to shutdown.
Under intermediate emissions (SSP245), 37% of models trend toward shutdown, while 25% do so under the low-emission SSP126 scenario consistent with Paris Agreement targets.
Deep ocean mixing ceases by mid-century
Maximum mixed-layer depths in the Labrador, Irminger, and Nordic Seas—currently reaching 1,000+ meters—collapse to less than 250 meters by 2050-2060 in shutdown scenarios, eliminating the deep convection that drives the circulation.
This mixing collapse precedes AMOC shutdown by approximately 30 years, providing a critical early warning signal. Observational data shows concerning downward trends in mixing depths over the past 5-10 years across all regions.
Ocean heat transport drops to 20-40% of current values
Northward Atlantic heat transport at 26°N—currently monitored by the RAPID array at ~15-20 PW—declines to just 20-40% of present-day values in shutdown states. Heat release to the atmosphere north of 45°N weakens to less than 20% of current levels, with some models showing complete cessation.
Salt Advection Feedback Dominates
Surface freshening triggers irreversible decline
The shutdown mechanism centers on surface salinity decline in convection regions. As the weakening AMOC transports less salt northward from subtropical waters, surface freshening inhibits deep mixing—creating a self-amplifying feedback loop.
Salinity effects dominate density changes regardless of whether warming (high emissions) or cooling (low emissions from reduced heat transport) affects temperature, demonstrating the primacy of the salt-advection feedback over thermal processes.
Warming initiates, advection sustains the collapse
While reduced ocean heat loss to the warming atmosphere appears to trigger initial mixing decline, the salt-advection feedback becomes the dominant factor driving continued weakening. Surface buoyancy flux changes are dominated by declining heat loss, but represent negative feedback that cannot overcome salinity-driven destabilization.
The transition features two key feedbacks: the Welander mechanism (reduced mixing allows freshwater accumulation) and the Stommel feedback (weakening circulation reduces northward salt transport).
Regional Variations and Cascading Effects
Nordic Seas show greater resilience initially
Deep convection in the Nordic Seas proves more resilient than in the subpolar gyre (Labrador and Irminger Seas), possibly related to differences in stratification. However, once subpolar mixing collapses, Nordic Sea convection typically follows within decades.
Models maintaining some Nordic Sea convection can stabilize around 11-12 Sv rather than reaching full shutdown, though still representing dramatic weakening from present-day values.
Circulation shifts to shallow wind-driven mode
Post-shutdown, the maximum overturning shifts from ~1,000m depth to less than 200m, representing pure wind-driven Ekman transport rather than density-driven deep circulation. Northward transport below the mixed layer essentially vanishes, eliminating deep water formation pathways.
Why It Matters for Climate Adaptation
Higher risk than previously assessed: The IPCC's AR6 assigned "medium confidence" that AMOC wouldn't collapse before 2100, but these extended simulations reveal 67% of high-emission scenarios reach shutdown by 2300—no longer qualifying as a "low-likelihood-high-impact event".
Deep mixing collapse provides early warning: The 30-year lag between mixing collapse (mid-century) and full shutdown (post-2100) offers a critical observation window. Current downward trends in mixing depths across all data products warrant urgent attention.
Severe regional cooling likely: Post-shutdown, the subpolar North Atlantic and Northwest Europe could experience severe cooling unless overwhelmed by greenhouse warming. Under low emissions, cooling is amplified by dramatic sea-ice expansion.
Model limitations cut both ways: While models neglect accelerating Greenland meltwater (which would worsen shutdown risk), they also contain biases toward AMOC stability, making net effects on projection reliability uncertain.
An Unfolding Tipping Cascade
This study reveals that AMOC shutdown represents not an abrupt collapse but a multi-decadal tipping cascade: warming triggers mixing decline around 2040-2050, salt-advection feedback sustains the process, and full shutdown emerges 50-100 years later.
The finding that many models currently showing gradual AMOC decline through 2100 are actually en route to eventual shutdown fundamentally changes risk assessment. The 50-100 year shutdown timescale means projections ending at 2100 systematically underestimate long-term risks.
Most critically, the widespread mid-century deep mixing collapse across models—with observational hints of similar trends already emerging—suggests the tipping process may already be underway, warranting immediate intensification of Atlantic deep-water monitoring and fundamental reassessment of AMOC stability in climate risk frameworks.
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