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The Atlantic Meridional Overturning Circulation, referred to as AMOC, is an ocean current system that is vital in regulating Earth’s climate by moving heat from the Southern to the Northern Hemisphere. Restricted to the Atlantic basin, the AMOC influences regional weather patterns — from temperate summers in Europe to monsoon periods in Africa and India.

For a long time, climate models have suggested that global warming will lead to a deterioration of the AMOC, with some forecasts indicating a substantial near-collapse compared to the current strength of the AMOC. However, recent research from a group that included the University of Washington indicates that the AMOC is expected to weaken to a considerably lesser degree than current predictions indicate. This study was released on May 29 in Nature Geoscience.

A significant reduction would have extensive ramifications, encompassing shifts in regional sea level rise and notable changes in local climate, such as cooler conditions in northern Europe and drier environments in parts of the Amazon and West Africa.

“Our findings suggest that, instead of a dramatic reduction, the AMOC is more likely to undergo a limited decline throughout the 21^st century — still some weakening, but less severe than earlier forecasts indicate,” states David Bonan, the principal author of the research and a UW postdoctoral research fellow in the Cooperative Institute for Climate, Ocean and Ecosystem Studies.

The researchers devised a simplified physical model grounded in essential ocean circulation principles — particularly the relation between sea water density variations and the depth of the overturning circulation — which also incorporates genuine measurements of the ocean current’s strength. This real-world data was gathered over 20 years using monitoring arrays and various observations of the Atlantic basin.

Results indicate that the AMOC will likely weaken by approximately 18-43% by the conclusion of the 21^st century. While this represents some deterioration, it does not equate to the near-collapse suggested by more extreme climate models.

Paleoclimate records, such as ocean sediments that capture past climatic conditions, indicate that the AMOC has weakened historically. One instance is during the last ice age 20,000 years ago, leading to significant climatic fluctuations that impacted North America and Europe.

Current climate models show considerable variability in 21^st century forecasts of AMOC weakening. This study sought to address these inconsistencies by gaining a better understanding of the physical processes governing AMOC behavior in climate models. Through this research, scientists highlight a previously unresolved characteristic of climate models: the connection between the current and future strength of the AMOC.

Climate models that replicate a stronger present-day AMOC generally forecast greater weakening in response to climate change. The researchers discovered that this correlation arises from the depth of the AMOC. A stronger AMOC typically extends to greater depths, allowing alterations in surface water temperature and salinity — caused by global warming and freshwater influx — to penetrate more deeply into the ocean, driving greater weakening.

A climate model exhibiting a stronger and deeper AMOC is less resilient to surface changes and encounters proportionally more weakening than one with a shallower current. Climate models featuring a shallower present-day AMOC still depict weakening due to climate change, but to a lesser degree than those with a deeper current.

The researchers employed ocean observations to demonstrate that the real-world AMOC is relatively shallow compared to most climate models. The findings imply that the AMOC will only face limited weakening, even under aggressive emissions scenarios. The study also proposes that much of the previous uncertainty and extreme weakening projections arose from biases in how climate models portray the ocean’s current state, especially regarding its density stratification.

“There is immense worth in conducting foundational research,” remarked Bonan. “It can provide a clearer indication of what the future might entail, as our study illustrates.”

Bonan underscored the necessity to explore higher-resolution climate models that also encompass more advanced processes. Higher-resolution models might yield deeper insights into AMOC behavior and refine forecasts of its forthcoming changes. The study offers a framework for probing and assessing more intricate models.

Kyle Armour, a professor of atmospheric & climate science and oceanography at the UW, was a co-author. Other co-authors include Tapio Schneider and Andrew Thompson from the California Institute of Technology, Laure Zanna from New York University, and Shantong Sun from Laoshan Laboratory in Qingdao, China. The research was financed by the National Science Foundation, the David and Lucile Packard Foundation, and Schmidt Sciences LLC.

This narrative is adapted from a release by the California Institute of Technology.

For more details, reach out to Bonan at [email protected].

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