SCIENCE FACT: The RAPID/MOCHA project

www.rsmas.miami.edu/users/mocha/mocha_files/SJ08_video_0002.wmv

Check out the video!

The RAPID project is an amazing intercontinental project with US and Europe heavily involved.
The big goal is the continuous measurement of the meridional overturning circulation strength at 26.5°N. To do so, a whole array of measuring buoys were lowered down to the Atlantic ocean floor, each of the buoys longer than several times the Eiffel Tower!

Since then, a scientific crew drives out there, locating the buoys with high accuracy gps and collect the measurements.

The project made possible a whole new understanding of the meridional overturning circulation in the Atlantic.

Before, only point-in-time measurements existed about circulation strength. According to those, the AMOC or THC must have weakened. But now, scientists were able to notice the extremely large variability of the AMOC. This was totally unexpected outcome!

Still, many scientists see a reduction, even in the longterm data (see below). Whether that reduction is significant or just part of the multi-year variability is still unknown.

Still, the project has proven to be reliable in measuring the AMOC over time! Hence, it has been extended twice already since 2004. Let's see whether they extended again!.....

http://www.sams.ac.uk/stuart-cunningham/rapic-moc-mocha

http://www.rapid.ac.uk/rapidmoc/

http://www.rsmas.miami.edu/users/mocha/

Newest findings: reduction of AMOC flow strength after all?

The last post illustrates the uncertainty in the current understanding of the weakening in AMOC flow strength. Does a warmer world with more CO2 really lead to a weakening or even to a strengthening of the AMOC over time?

Due to the large variability found in the AMOC data since 2004 (Meinen et al., 2006; Atkinson et al., 2010; Johns et al., 2011) it is questioned whether the AMOC is actually going to decline in the near future. The following graph is the longest RAPID record analyzed so far by Smeed et al. (2014).

Figure from Smeed et al. (2014) showing gulf stream (blue), overturning circulation (red), ekman transport (green) and upper-mid ocean transport (magenta) for 1. April 2004 until 1. October 2012. Positive direction means flow to the north.

In their newly published paper this year, Smeed and colleagues show a decline in AMOC flow strength since 2004 of -0.54 Sv/year (1 Sv = 1 mio. M3/s).  Bryden et al. (2014) present an especially dramatic decline of 30% between 2009 and 2010, which is also apparent in the Smeed figure. Already in 2005, Bryden et al. suggested a decline of 30% in flow strength between 1957 and 2004 using one-time data collections in 1957, 1981, 1992 and the new one in 2004. At first, these results strongly suggest s decrease in ocean circulation after all, most likely due to global warming. 

However, Smeed et al. (2014) noticed their reduction rate of -0.54 Sv/year with climate modeled ones and concluded that theirs is ten times higher. Thus, they concluded that their measured reduction rate cannot be a response to warmer temperatures, but must be part of the internal variability of the AMOC. Smeed et al. (2014) were not able to prove their thesis, since 8.5 years of direct AMOC flow strength measurements are too short to define the inter-decadal variability.

Similar problems occur with Bryden and colleagues’ studies. A reduction of 30% from 1957-2004 should be read with caution, since it has been shown in the last post that the variability within a year of the AMOC is exceptionally large. The one-time measurements done in 1957, 1981 and 1992 are thus completely irrelevant, since the spring/autumn maxima or the summer/winter minima can neither be used as a yearly average. Their second study focuses on one year (09/10), which was also characterized by McCarthy et al. (2012) to have been a highly abnormal year. Thus, it cannot be taken as proof for a slowly decreasing AMOC.

Figure from McCarthy et al. (2012) showing flow anomalies from 2004 to 2011. Notice year 2010 to be highly anomalous.

Still, it can be useful as a short term study to see the actual impacts of such a reduction, which was mainly done in Bryden et al. (2014). In 09/10, the AMOC flow strength was 30% lower than the long term average for more than 14 months. This lead to a reduction of 0.4 PW (Peta-Watt =1x10¹⁵ W) in heat transport (Bryden et al., 2014). In comparison, the whole of the UK consumed “only” 15 TW (Terra-Watts = 1x10¹² W) in 2012 (http://data.london.gov.uk/dataset/total-energy-consumption-borough/resource/c73d0109-67f2-4345-89dc-78248420f184). Thus, the weaker Gulf Stream transported significantly less heat to Europe and induced an especially cold winter that year. In addition it, influences the strength of the North Atlantic Oscillation (NAO), making cold northern winds more likely and intensifying winter conditions (Bryden et al., 2014). In turn, the tropical and southern Atlantic observed a slight warming with intensified summers and storms (Bryden et al., 2014).

It is obvious that a reduction in the flow strength produces not only severe impacts in models, but actually in observed records. Whether the AMOC or THC really weakens is still unclear. The RAPID measurement project has greatly helped in understanding the natural variability of the Atlantic circulation. Hence, it has been extended twice already. Still, the observation data is too short to answer all questions. Much more has to be done, to produce a sure statement on the response of ocean circulation to a warming world.

https://www.papermasters.com/climate-change.html

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?