Tuesday, 24 April 2018

North Atlantic ocean circulation now the weakest its been in sixteen centuries.

Why a 15 Percent Slow-Down in North Atlantic Ocean Circulation is Seriously Bad News

We know somewhere out there is a tipping point where this current system is likely to break down. We still don’t know how far away or close to this tipping point we might be. … This is uncharted territory.”

23 April, 2018

The North Atlantic ocean circulation (often called AMOC or the Great Ocean Conveyor) is now the weakest its been in sixteen centuries.

Increasing melt from Greenland due to human-forced warming of the atmosphere through the deep ocean is freshening the ocean surface of the far North Atlantic. To the south, higher ocean temperatures are increasing surface salt content through greater rates of evaporation. Fresh water prevents ocean water from sinking in the north and rising salt content generates increased sinking in the south. As a result, the rate at which waters move from the Equator toward the Pole is slowing down. Since the mid 20th Century, this critical ocean circulation has reduced in strength by 15 percent on decadal time-sales.

(Deep water formation in the North Atlantic is driven by the sinking of cold, salty water. Over recent years, this formation, which drives larger ocean circulation and atmospheric weather patterns, has been weakening due to increasing fresh water flows coming from a melting Greenland. Image source: Commons and the NASA Earth Observatory.)

Movement of warm Equatorial waters northward and their subsequent overturning and sinking in the North Atlantic drives a number of key weather and climate features. The first is that it tends to keep Europe warm during winter and to moderate European temperatures during summer. The second impact is that a fast moving current off the U.S. East Coast pulls water away from the shore keeping sea levels lower. The third is that warm water in the North Atlantic during winter time tends to keep the regional jet stream relatively flat. And the fourth is that a more rapid circulation keeps the ocean more highly oxygenated — allowing it to support more life.
A slowing down of ocean circulation in the North Atlantic therefore means that Europe will tend to cool during winter even as it heats up during summer. Sea level rise will accelerate faster for the U.S. East Coast relative to the rest of the world due to a slowing Gulf Stream combined with the effects of melting land glaciers and thermal ocean expansion. The North Atlantic jet stream will tend to become wavier — with deep troughs tending to form over Eastern North America and through parts of Europe. These trough zones will tend to generate far more intense fall and winter weather. Finally, a slowing ocean circulation will tend to increase the number of low-oxygen dead zones.
(Cool pool formation near Greenland juxtaposed by a warming and slowing of the Gulf Stream as it is forced southward is an early indication of ocean circulation slow-down. During recent years, this phenomena — which is related to larger human-forced climate change — has become a prevalent feature of North Atlantic Ocean climate and weather patterns. An indicator that climate change and ocean system changes for this region are already under way. Image source: Earth Nullschool.)

A 15 percent slow down in ocean circulation is not yet a catastrophic event. It is, however, enough to produce odd weather and climate signals. We have tended to see higher rates of sea level rise off the U.S. East Coast, we have tended to see more extreme winter weather across the North Atlantic basin. The long term trend for increasing ocean dead zones is well established. And European weather has become more and more extreme — with hot summers and severe winters.
With rates of Greenland melt increasing, there is a risk that the historic observed North Atlantic circulation weakening will increase further and more radically — producing still more profound results than we see today. In the event of large melt outflows coming from Greenland during abnormally warm summers or due to warming deep water melting glaciers from below — a possibility that rises with each 0.1 C of global temperature increase — we could see a very rapid weakening of ocean circulation above and beyond that which has already been recorded.

(Like Antarctica, Greenland features a number of below sea level locations directly beneath its largest ice masses. This feature makes Greenland more vulnerable to rapid ice loss and large melt outflows. Image source: NASA JPL.)

If such a tipping point event is breached — and there is increased risk for it as global temperatures enter a range of 1.5 to 2.5 C above 1880s averages during the 2020s through the 2040s — then we can expect far more profound weather and climate disruptions than those we have already experienced.

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