New insights into today’s AMOC strength through directly measured data: is the slowdown only a myth?

Up until 2004, most of our understanding of the AMOC and its variability were based on model analysis (see last post). Since 2004, the enormous RAPID project provides real-time measurements of the flow strength in the deep Atlantic at 26°N (http://www.rapid.ac.uk/rw/index.php). Since then, very interesting findings were published.  (For more information see SCIENCE FACT on RAPID project)

First results presented by Meinen et al. (2006) showed unexpectedly high variability in the AMOC transport within one year. You can see in the graph that the amount of deep water going south varied between 10 and 90 Sv (1 Sverdrup = 1 mio m3/s) within one year!

Figure from Meinen et al. (2006) showing southwards (blue) and net (red dashed) transport of deep water between the continental shelf and 72°W. LADCP points refer to cruise start and end.

These results strongly question whether the model outcome of a weakening in the AMOC is true or not. If the yearly variability is this high, how high is the interannual variability? Maybe the reduction of strength which appears in all models is only part of natural variability?

With more data, Atkinson et al. (2010) were able to define the yearly variability of the AMOC, with faster flow in spring and autumn, and slower flow in summer and winter. They also identified the upper layer Ekman transport (see post ? for more information) as a potential driver for some of the variability observed at 26.5°N. According to Atkinson et al. (2010), there is a positive linear relationship between the North Atlantic Oscillation (NAO) and the Ekman transport at 26.5°N. This implies that a weaker NAO will weaken the Ekman transport, which in turn will weaken the Ekman induced flow-part of the AMOC. A slower AMOC will then leads to slightly colder sea surface temperatures (SST) in the North Atlantic which makes temperature differences in the North Atlantic region more extreme, and in return strengthens the NAO. This suggests that there might be a negative feedback system which partially leads to the large variability. In 2011, Johns et al. find other influential drivers, such as the Mid-Ocean and the Western Boundary Abaco flow, adding to Atkinson et al.(2010). This means that the AMOC or THC cannot be seen as a pushed water mass that flows north, but as a resulting flow mass from many influencing factors at one point in the ocean.

Figure from Johns et al. (2011) with 5 heat transport curves relative to 0°C at 26.5°N. The black curve is the total of all.

Think back to the first model of ocean circulation: the conveyor belt. Deep water is produced in the North and “pushes” the stream through all oceans. It upwells and flows back to the production centre forming the warm Gulf Stream. But obviously, this view is far too simplified, as shown above. Instead, the flow at any point is controlled by certain factors, such as other currents, winds, and eddy mixing. Some researches go almost as far as to say: there is no current. Every flow of water is just an addition of the upper factors (Lozier,2010; Zhang & Qiu, 2014; Ide&Wiggins, 2015).

This view of ocean circulation might seem very complicated, but this view allows for much more factor inclusion in a model. For example: moisture export from the tropical Atlantic to the Pacific. Remember, the evaporation of lots of water from the tropical Atlantic is greater than all freshwater flowing and raining into the tropical Atlantic, because the easterly trade winds blow all the moisture across Middle America into the Pacific. This leads to saltier water and possible deep water formation in the Arctic. Everyone is concerned about the large freshwater input into the North Atlantic under warming conditions. However, warming conditions also means stronger evaporation in the tropical Atlantic PLUS stronger winds that export more moisture to the Pacific (Richter& Xie, 2010)! This suggests that a warming world might actually stabilize the THC (Richter & Xie, 2010).

With the new insights Matei et al. (2012) managed to remodel the AMOC and predict flow strengths for 4 years after 2012. Their new model underlines the theories listed above: no weakening was projected until 2014, which was the prediction limit at that time.

We have passed the year 2014… were their predictions right?

The Scare of Rapid Climate Change for our near future…how much is true...

Current global warming has been proven to lie outside the range for the earth’s natural variability (IPCC, 2013). So we know that the state of current warming is likely abnormal and that it might have influences on the Atlantic meridional overturning circulation (AMOC), also known as the thermohaline circulation (THC).

You have seen the melting rates in the Arctic on the bottom of the last post… in coupling with warmer temperatures, waters in the North Atlantic will become fresher and warmer, which reduces their density and slows down deep water formation in the North Atlantic. No more water pushes southward, slowing down the AMOC in return.

Scientists were quite worried about the force of global warming on the AMOC. In 2002,Vellinga & Wood used a HadCM3 model (a coupled ocean-atmosphere model) to investigate the global impact that awaits us, if the AMOC were to shut down. The results are frightening. Within only 20 years, Europe would cool by 1-3°C, and the northwest Atlantic up to 8°C! Even North America and Asia would suffer under cooling of 2°C. The numbers might seem small, but Vellinga & Wood(2002) note that a cooling of >1°C has never been observed since 1659 (the onset of direct air temperature measurements in the UK).

If this gives you an unwell feeling, think about the currently projected warming due to anthropogenic greenhouse gases: 4°C by 2100, if we do not cut our emissions quickly (IPCC, 2013). This implies a warming of 1°C in 20 years, similar to the cooling in the little Ice Age.

In response to the “shut down scare”, many studies investigated the likelihood of current AMOC or THC shut down. However, Stouffer et al. (2006) note that global warming may increase the freshwater input to the North Atlantic, but only by the order of 0.14 Sv (see INFO BOX). To shut down the conveyor, at least 1.0 Sv are needed which is highly unlikely to occur. Similarly, Wood et al. (2003) state a shut down to be highly unlikely under current CO₂ projections.

Model studies are helpful in understanding system behavior. Their problem: models are only as good as the understanding of the system during the time the model was written. Anything we do not know, we cannot imply in a model and cannot reprocess. One such variable are thresholds. As the three modes of the THC show, it is likely that ocean circulation presents threshold behavior. Knutti& Stocker (2002) conclude in their ocean model analysis that today’s models are insufficient for finding the AMOC’s threshold points, mostly due to missing information. Thus, they are unable to surely predict the changes in ocean circulation under climate change scenarios.

Paleoclimatology is unable to help in this case, since CO₂ has not been this high for more than 2 million years…

So the only possibility to get a better insight into the AMOCs behavior is to measure it directly...

…and the RAPID program was born!



http://www.rapid.ac.uk/index.php