Atlantic Ocean circulation slowdown is likely part of a longer-term cycle

Recent studies have shown that the Atlantic Meridional Overturning Circulation (AMOC) has steadily slowed over the past decade.

Image courtesy NOAA.
<img src="https://vortex.accuweather.com/adc2004/pub/includes/columns/climatewx/2018/590x665_07190047_screen-shot-2018-07-18-at-8.46.56-pm.png"/>

New research from the University of Washington and the Ocean University of China has determined that this slowdown is not caused by global warming but is part of a decades-long cycle that will have an impact on temperatures in the coming decades.

The Atlantic Meridional Overturning Circulation, or AMOC, is a conveyor belt that brings surface water northward in the Atlantic; from there, the heavier salty water sinks and returns at depth from the Labrador and Nordic seas, near the North Pole, all the way south to the Southern Ocean, according to the <a href="http://www.washington.edu/news/2018/07/18/atlantic-ocean-circulation-is-not-collapsing-but-as-it-shifts-gears-global-warming-will-reaccelerate/" target=n>University of Washington News report.</a>

The research team found that through direct measurements the AMOC has declined 10 times more than expected since 2004.
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Below are some excerpts from the UW News report.....</strong>

<em>“Many have focused on the fact that it’s declining very rapidly, and that if the trend continues it will go past a tipping point, bringing a catastrophe such as an ice age. It turns out that none of that is going to happen in the near future. The fast response may instead be part of a natural cycle and there are signs that the decline is already ending," said co-author Ka-Kit Tung, a UW professor of applied mathematics with an adjunct appointment in atmospheric sciences.
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The AMOC's speed has a key role in the amount of surface warming. If the current slows down, then it stores less heat and the Earth will experience a high rate of temperature increase than what we have seen since 2000.

<em>When the current is faster, more of the warm, salty tropical water travels to the North Atlantic. Over years this causes more glaciers to melt, and eventually the freshwater makes the surface water lighter and less likely to sink, slowing the current.</em>

<em>When the AMOC is in a slow phase, the North Atlantic becomes cooler, icemelt slows, and eventually the freshwater melt source dries up and the heavier saltier water can plunge down again, which speeds up the whole circulation.</em>

From 1975 to 1998 the AMOC was in a slow phase, which allowed excess heat to accumulate in the atmosphere. The AMOC shifted to a fast stage at the start of this century, which allowed excess heat to remain trapped in the deep oceans.

In conclusion, the study argues that the AMOC is not collapsing as some earlier studies have suggested, but is currently transitioning from a fast phase to a slower one.

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This study was published in the journal Nature.

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