Introduction: What is the Meridional Overturning Circulation?
The Meridional Overturning Circulation (MOC) spans the globe and includes surface currents, deep ocean currents, and areas where deep water formation takes place. Deep water formation is the result of warm, salty water getting cooled in cold regions like near Greenland and Svalbard, north of Scandinavia, and in regions around Antarctica. This water is heavier than the surrounding waters owing especially to its salinity, and sinks to the ocean floor; the momentum imparted through this sinking is the main driver of the MOC. The deep water formation areas, as well as the paths of current and deep water currents, are noted in the banner graphic above.
Changes in the MOC: Salinity and seawater density
Variations in the MOC have climate impacts in the North Atlantic and surrounding land masses, and result from changes in deep water formation. Anything that reduces the density of seawater with slow the MOC. The density of seawater increases as its temperature decreases and its salinity increases, as we can see below (red line). Freshwater, on the other hand, reaches as maximum density at 4oC, with density decreasing below that temperature threshold. Note that the seawater is more dense than freshwater because of its salinity.
Anything reducing the density of water involved in deep water formation will slow down the MOC. This would include freshening the water in the area or warming the temperature of that water. What phenomena in the climate system would reduce salinity or warm the water?
One Possible Example of an MOC Slowdown: The Younger Dryas
We were coming out of the most recent glacial about 12,800 years ago when there was an abrupt cooling in the climate in Europe and North America (NAm) back to near-Ice Age conditions: the Younger Dryas (YD). This cold period lasted for about 1200 years, and was marked by southward shifts of cold vegetation types, the loss of megafauna (e.g. woolly mammoths, pictured below), and the disappearance of the Clovis people/civilization in North America. It appears from temperature proxies that northern winter temperatures decreased more than those of northern summer.
Below is a graphic that shows decreased temperature and snow accumulation for Greenland (dark and light blue), cooling in the Cariaco River area in northern Venezuela, and out of phase warming temperature in Antarctica. Freshwater flux from the Laurentide Ice Sheet down the St. Laurence River (about 150,000 m3/sec) in red at the bottom of the graphic.
One theory for the sudden cooling at the start of the YD is a freshening of the salt water south of Greenland by increased meltwater release from the Laurentide Ice Sheet over NAm. This in turn resulted in an abrupt reduction in deep water formation slowing the AMOC and in turn, the Gulf Stream and North Atlantic Drift Current. Those currents warm eastern NAm and Europe, respectively.
Abrupt Climate Change: What do we know?
what do models tell us?
A paper approved for publication in Nature Communications examines the AMOC as simulated by 30 climate models with abrupt CO2 increases at their outset, then running the climate model for 250 simulation years. There is a large difference in the AMOC among these models in the equilibrium climate reached after about 50 years. They’re consistent with each other in that the largest decreases in AMOC are associated with the coolest temperatures in the North Atlantic, as warm surface waters from the tropical and subtropical areas cannot reach that region as quickly.
There are other differences of interest:
- With stronger decreases in AMOC
- Global mean surface temperatures increase less (because of less northward transport of heat by the oceans)
- The jet stream and northern hemisphere storm track shifts further north
- With weaker decreases in AMOC
- Wet areas become more wet
- Dry areas become more dry
- There are smaller changes in the global atmospheric circulation
What else can we conclude? The global climate depends pretty significantly on the AMOC, and we really should have a better handle on how it works. Additionally, the time and spatial resolution of climate models are not fine enough to tell us what we need to know about the abruptness of any climate change.
What do observations and paleoclimatology tell us?
There is enough research evidence over the last couple of decades that abrupt climate change is more frequent than previously thought. For example, some temperature proxies have shown that the YD started with several downward steps in temperature over several decades amounting to 7oC or so, while the climb out of the YD was of similar magnitude but perhaps over only a single decade. Such changes would wreak havoc with an earth system upon which upward of 8,000,000,000 human beings depend.
We do know that the YD started suddenly, which implies a critical threshold in the climate system was crossed. What we don’t know is what in the climate system was critical to the abrupt change, and what critical value was crossed. The AMOC looks like a likely culprit, but we do not know enough yet to know the critical value of, say, heat transport, below which the abrupt climate change on the order of a YD would occur.
It’s clear over the past 30 years that extreme individual events are on the rise, with increasing precipitation and temperature extremes resulting in deadly heat waves, catastrophic floods, record-strong hurricanes, and even cold waves (like last February in the southern Plains, e.g.). That’s with only a global mean temperature change of a bit over 1oC. Changes in the general circulation that might not have been diagnosed with climate models are already wreaking havoc on people, infrastructure, agriculture and more, sooner than we expected.
The only thing I’m sure of, at least, is that a disruption of the AMOC would not result in anything like what we saw in the movie, “The Day After Tomorrow” (great special effects notwithstanding).