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.

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