Insights & News

A Slowing AMOC: Climate Change Mitigator?

3rd March 2021

On Thu 25th Feb 2021, Nature Geoscience published a new research paper titled ‘Current Atlantic Meridional Overturning Circulation weakest in last millennium’. It reports that a major oceanic circulation called the Atlantic Meridional Overturning Circulation (AMOC) is the weakest it’s been in over 1,000 years.

Reactions have been mixed among the scientific community (good compilation here). There’s some question over the reliability of using ‘proxy’ indicators to study the AMOC. On the other hand, multiple proxies were used, which suggests at least some slowdown is probably occurring.

We’ve also seen the emergence of the ‘North Atlantic Warming Hole’ in recent years. It’s an area to the south of Greenland that’s actually cooled slightly in the past decade or so – starkly against the global trend.

Inspired by the new publication, I’ve explored what a weakening AMOC might mean for climates around the North Atlantic in the coming decades. The answer may surprise you.

What’s AMOC?

Right across the Atlantic Ocean, from south to north, spans a vast marine conveyer. At shallow depths, this transports warm water northward, while it moves cold water southward in the deep ocean (see below).

Left-hand diagram is based on one found here and applies to the North Atlantic only. Right-hand diagram was found here.

The AMOC is part of a global circulation that keeps the oceans from stagnating, distributing nutrients, minerals and heat energy.

Between the tropics and north-eastern North Atlantic, the heat transport is especially strong. It provides an important cooling service to the Atlantic tropics and a substantial warming one to Europe.

For example, western parts of Europe are several °C warmer (on average, annually) than they would be if the AMOC didn’t exist. Just how much warmer is a matter of ongoing research, but it’s likely that freezing winter weather would be common there in a no-AMOC world.

Does the Slowdown Mean Europe’s Going to Cool?

The short answer is: Probably not.

The long answer is: Although a slowing of the AMOC reduces heat provision to Europe, it would likely take a very strong slowdown or total collapse to outweigh the climate warming being forced by other factors.

Such an outcome is generally considered unlikely. So, a slowed rate of warming is the most likely outcome for the next few decades in Western and perhaps Central Europe.

On the face of it, this sounds beneficial: A slower increase in the frequency of severe summer heatwaves. Unfortunately, the atmosphere has a habit of complicating matters and this is probably no exception.

Summers Drier in UK & Even Hotter in Western Europe

Believe it or not, that’s the suggestion of a 2015 research paper into the impacts of a slowing AMOC on model simulations of European climate in the coming decades.

They connect a stronger ‘warming hole’ in the North Atlantic with an increase in typical sea-level pressure (SLP) over and around the UK. High SLP areas tend to bring clearer, drier weather, which in summer also tends to be hotter, especially by day.

Two maps centred just north of the UK, showing mean sea level pressure and surface air temperature responses to the 'North Atlantic Warming Hole' in model simulations.

Adapted from the 2015 research paper linked at the beginning of this section. In the left-hand image, orange shading corresponds to increased  SLP, blue reduced. In the right-hand image, orange shading corresponds to raised temperatures, blue shading lowered.

They have clockwise air circulations in the Northern Hemisphere (opposite in Southern). This leads to a secondary impact on Western Europe’s mainland: Increased easterly flow.

Easterlies there have a long overland fetch – land that’s often strongly heated by the sun in summer. Contrast that with westerlies, which haven’t travelled far from the relatively cool, moisture-laden North Atlantic.

So, increased easterlies equate to higher typical summertime temperatures and more erratic, generally lower rainfall. This would superimpose onto the broader-scale climate trend to increase heatwave and drought frequency. More intense downpours, as and when it does rain, may also have to be endured.

We can’t take the increased easterlies as a given, though. The research paper expresses much uncertainty in how accurately the model simulations resolve the ocean-atmosphere processes responsible for the SLP change. In recent summers, the ‘warming hole’ has been strong but only 2018 has aligned much with the model projections.

It remains to be seen whether a stronger ‘warming hole’ manifests in the coming years and forces a more consistent increase in summer SLP over the UK.

What About the Other North Atlantic Neighbours?

A slowing of the AMOC was also reported in a June 2020 paper studying the potential climate impacts in regions ranging from the tropics to the Arctic.

Again, the North Atlantic ‘warming hole’ (NAWH) is tied into the slowing AMOC. Model simulations that include a slowing AMOC produce the NAWH to some extent or other, but when the AMOC is held steady, it doesn’t emerge.

Reorganised Atlantic Rainfall

As climate change increases the globally averaged sea surface temperature, the NAWH region is ‘left behind’. This has a similar effect on temperatures in the lower atmosphere – but the upper part still warms faster.

The net result is a more stable atmosphere above the NAWH. That suppresses the formation of tall clouds capable of producing heavy rainfall. So, it turns drier there in the model simulations.

Reduced downwind moisture supply in turn mitigates predicted rainfall increases across areas from Northern Europe to Northeast Asia. In this respect, the AMOC slowdown may prove beneficial for much of the Northern Hemisphere between 50 and 80°N.

The left-hand plot shows how a model simulation of future climate change with a weakening AMOC (right-hand plot) compares with one that has a steady AMOC. Stippling indicates significant changes at the 95% confidence level. Sourced from Figure 2 of the June 2020 paper linked earlier.

A notable exception is the Southeast region of the USA. Here, a slowing AMOC could enhance an increasing trend in rainfall and associated flood risk.

Meanwhile, at the low latitudes, a striking feature of climate change projections is a southward shift in the inter-tropical convergence zone (ITCZ). That’s a band of moist, unstable air that spans large sections of the tropics at a time. It appears that a slowing AMOC may add to that shift.

This would cause wet and dry seasons adjust their timing and duration across much of the tropics. That would be highly disruptive to affected populations (and not just human ones), perhaps catastrophic for some.

Meanwhile, over the Atlantic Ocean…

South-Shifted ITCZ = Fewer Atlantic Tropical Cyclones?

In the Atlantic basin, tropical cyclones (TCs) developing in the Caribbean and Main Development Region (MDR; between the Caribbean and Africa) often draw upon the ITCZ to aid development. The further north that is, the more readily this can happen.

So, if it shifts south, TC development in these regions may become less frequent in the coming decades.

But this doesn’t necessarily mean an overall reduction in TC frequency across the Atlantic basin. Sea temperatures are expected to continue warming in both the tropics and subtropics. That may be enough to offset reduced TC frequency in the MDR; warmer seas facilitate a moister, more unstable atmosphere.

A Little More Time for the Atlantic-Side Arctic

Alongside mitigated mid-latitude Northern Hemisphere rainfall increases, there could be another appreciable benefit to a slowing AMOC.

Reduced heat transport to the far-northern Atlantic likely gives the sea ice there a bit more ‘breathing room’ as the climate warms.

Which is to say, it could be several years longer before sea ice in regions neighbouring the Atlantic tends to melt out entirely for part of the year. The axe still falls, but more slowly than before.

In winter, this mainly benefits the Labrador, Greenland, and Barents seas. Good news for local communities that depend on the seasonal presence of sea ice for their living.

During summer, those seas will tend to lose much or all sea ice regardless, so the benefit shifts to the Central Arctic Basin. Specifically, the Atlantic side. This delays the time at which a ‘blue ocean’ event (less than 1 million square km of sea ice coverage) becomes a regularity.

So… How Should We Feel About a Slowing AMOC?

It really depends on your perspective.

If you consider the entire world then, on balance, there could be a net positive to its impact. A feedback mechanism that mitigates more than it accelerates the negative trends of climate change.

It’s an example of how Earth’s atmosphere-ocean system attempts to maintain balance: A negative feedback.

BUT it’s one with big exceptions in highly populated regions (western Europe, Southeast USA, central parts of Africa, and southern Asia to name a few). I imagine the apparent overall benefit of a slowing AMOC vanishes when considering how many people experience the positives versus negatives.

Even where the slowdown mitigates climate change, the models predict significant changes in the coming decades. The net effect of increased greenhouse gas concentrations outweighs those of a slowing AMOC.

That’s how powerful we have become as a species. Our cumulative actions carry a greater weight than those of planet Earth’s natural balancing forces.

 

James Peacock MSc
Head Meteorologist at MetSwift

Cover Photo by Unknown Author is licensed under CC BY

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