Icelandic scientists say they have discovered a new overturning site, where cold, dense, deep water is formed and transported through a separate route towards the Denmark Strait and further south into the Atlantic Ocean.
This process is essential for the Atlantic Meridional Overturning Circulation, or AMOC, one of the ‘engines’ of our ocean’s thermohaline circulation, which in the Atlantic is best known for one of its components: the Gulf Stream, which transports warm surface waters from the Gulf of Mexico all the way towards the Norwegian coast and the southern fringes of the Arctic Ocean.
This warm water cools down, whereby its weight increases and it sinks. It is then carried back south into the Atlantic, over the ocean floor.
Eastern deep water current
The Denmark Strait, between Greenland and Iceland plays a key role in directing the deep current, of which, as the new research shows, not only a western (the East Greenland Current), but also an eastern component (named ‘North Icelandic Jet’ by its discoverers) exists.
The role of the North Icelandic Jet for the AMOC could even be more important than the Greenland Current, the researchers write in their publication in Nature Geoscience. They estimate it constitutes more than half of the overturned water and nearly all of the densest, so deepest [and most important as driving force to the AMOC] water flowing south through Denmark Strait.
Variations in the Atlantic Meridional Overturning Circulation are natural, probably even on decadal scales. New Dutch research has shown for instance the overturning has been relatively weak in recent years [which means cold water has accumulated close to the surface instead of sinking to deeper waters, one of two reasons why there has been a lull in upper ocean warming].
Apart from such variations there are however also concerns about instability, notably a risk of a weakening trend in AMOC, which could affect the entire thermohaline circulation and for instance slow down [or theoretically stop] the Gulf Stream. When this happens large parts of the oceans could be starved from ocean floor nutrients, threatening the base of the marine food chain.
Also the temperature difference between the poles and equator could increase dramatically and especially Europe could see temperatures plummet. Scientists aren’t certain about this subject yet, but a slight weakening of AMOC and the Gulf Stream has been suggested as the driving force behind the Little Ice Age.
Why good news?
Paradoxically it could be climate warming that sets the stage for such European cooling events. Especially the role of Greenland melting has been hypothesized – as the influx of fresh melting water could decrease the density of Atlantic surface waters, preventing it from sinking [remember it is called thermohaline circulation – it is driven not just by temperature differences, but also by differences in salinity, as that too determines the density of the water].
The fact that the North Icelandic Jet forms further to the east perhaps makes it (a little) less susceptible to the influence of Greenland melting water. Also the fact that AMOC apparently has two deep currents at its disposal may increase general stability, because perhaps the one can compensate for periods of smaller capacity flow of the other.
Why not good news?
From a European perspective it’s not really sure whether continued warm water transport to the north would actually count as good news. In the northwest for instance, climatic warming already happens at around twice average speeds. Besides, there is also a possibility that the Gulf Stream could even increase in temperature over years to come – adding a cumulative effect to continued atmospheric warming.
We can of course stretch this perspective to the Arctic Ocean and to the Arctic tundra, both of which could do with a little extra temperature isolation. In turn the entire Earth’s climate could benefit from increased albedo and prevented melting of methane clathrates.
If only AMOC had a nice, delicate volume control switch. Allow us to use it and no doubt we’d do something really stupid.
© Rolf Schuttenhelm | www.bitsofscience.org