The Atlantic Meridional Overturning Circulation ocean current plays a key role in the climate of the UK, and it’s shifting.
What exactly is the Atlantic Meridional Overturning Circulation (AMOC)?
The simplest idea of it is just these very large-scale ocean currents in the top thousand metres of the Atlantic. That’s kind of a system of currents that transports warm, salty water northwards throughout the Atlantic and then at high latitude, in the subpolar North Atlantic, the water gets cold, becomes dense and it sinks down to below a thousand metres and it flows back southwards. It’s a loop of warm, salty water travelling north in the upper kilometre of the Atlantic.
It’s matched by a transport of deep, dense water southwards throughout Atlantic Ocean. It’s part of a global conveyor belt system because the Southern Ocean links the Atlantic to all the other major ocean basins. You can think of it as a conveyor belt in the sense of those kind of loops of rubber that you have on the supermarket checkout.
What influence does this conveyor belt system have?
It’s all about heat, it’s like a giant radiator system, if you like. The North Atlantic Ocean is transporting a huge amount of heat northwards, from the tropics and subtropics into the subpolar ocean, the area between the UK, Iceland, Greenland and Canada. The waters around the UK would be much colder if you didn’t have this heat through the Atlantic in this overturning circulation. You can think of it as sort of a battery for the atmosphere. It is incredibly important for the weather and the climate that we receive.
One nice statistic that is quite useful to reflect on is that the the heat held in the top one metre of the ocean contains as much heat energy as the whole of the atmosphere. It has this capacity for storing heat. It’s this movement of the heat around the globe that keeps our climate nice for us to live in. If all the heat was just concentrated in the tropics, they’d be too hot to live in and everywhere else would be too cold to live in. That’s the importance to us, the way it modulates our climate and our weather.
How has the AMOC been changing?
How it’s going to change in the future is the critical question really, because it’s so important for our climate and weather that we have in the UK that we really need to know how it’s going to change. The bad news is that we don’t have very high confidence in those predictions, but all climate models show a slowing of the overturning circulation over the next few decades. With the low emissions scenario it takes longer to get to a very low point but in the high emissions scenarios it might only be another 50 years before it’s reached a very low point.
Why is the shifting of ocean heat important for the climate?
In the model projections our climate is going to change over time with our carbon emissions. It’s quite difficult to separate out the effects of a slowing of the overturning circulation from all the other effects – the ocean, the atmosphere. They interact with each other. There are all kinds of processes going on. So actually being able to pick out what a slowdown in the overturning circulation causes is quite difficult.
But what we think is that it has the effect of cooling parts of the ocean west of the UK because less heat is being carried northward through the Atlantic Ocean. So that distribution of heat from the tropics to the subpolar areas slows down a bit. That part of the ocean gets a bit colder and doesn’t warm as fast as the rest of the world.
That has a direct impact on our weather. In the UK weather is getting wetter, it’s getting warmer and there’s more extreme weather. But with the slowdown in the AMOC, some of that is counteracted a little bit. And that’s because if there’s not as much warming in the Atlantic Ocean then that has an impact on things like where the jet stream sits.
If it moves north of the of the UK, which is something that could happen as the AMOC slows down, then that makes all that makes our weather drier, because all of the movements of the storms that bring our rain might run north of the UK. But it’s unclear specifically what a weakening of the AMOC does.
In terms of the International Panel on Climate Change (IPCC)projections for what’s going to happen to the UK it is basically going to get warmer, get wetter and we’ll have more extremes in our weather…