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

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