The two proxies that carry the water in the Trouet NAO reconstruction are the Baker speleothem in Scotland and Esper’s tree ring chronology in Morocco.
The briefest examination of the Scotland speleothem shows that the version used in Trouet et al had been previously adjusted through detrending from the MWP to the present. In the original article (Proctor et al 2000), this is attributed to particularities of the individual stalagmite, but, since only one stalagmite is presented, I don’t see how one can place any confidence on this conclusion. And, if you need to remove the trend from the MWP to the present from your proxy, then I don’t see how you can use this proxy to draw to conclusions on relative MWP-modern levels.
For reference, the following graphic shows the Trouet version:
As Andy Baker explains in his article and in a CA comment, speleo widths are believed to be narrower in warm and wet periods and wider in cold and dry periods (with temperature and precipitation not being independent due to NAO). Based on this, I’ve plotted the original data in an inverted sense (narrower at the top) in the top left. I’ve plotted the full speleothem (Trouet cuts off at AD1075 or so). They transform the data to z-scores even though the data is highly non-normal. The z=score distribution is truncated at an sd of about 1, corresponding to the minimum width of about 0. Also shown are 25-year and 50-year binned averages. As noted before, although Trouet et al say that they used 25-year averages, they actually used 50-year averages.
Figure 1. Versions of SU967 Speleothem.
The bottom left shows the same plot for the “detrended” widths, while the bottom right shows the same plot for the “precipitation reconstruction” which is a re-scaling (linear transformation) of the detrended width series. This is the version that is carried forward into Trouet. The “detrending” (also called “normalisation”) is described in Proctor et al 2000 as follows:
Personally, I find this justification underwhelming. The adjustment leads to important reversals of medieval-modern relationships – all in the direction of enhancing 20th century levels relative to 11th century levels. Even if the adjustment subsequently proves justified (and there are circumstances in which it could come onside), you can’t assign any confidence to any product of this data set without assessing potential errors in the detrending. Unfortunately, Trouet et al neglect this error source:
These individual error terms include (i) dating uncertainties of the speleothem record, (ii) chronology error of the tree-ring record, and (iii) calibration error of the residual record NAO_ms.
Actually the non-detrended Scottish speleothem record reminded me of the Polissar glacier discharge proxies from Venezuela of all places, as shown below (Discussed a couple of years ago at CA), both showing transitions from ‘MCA’ to LIA around 1250-1300. I submit that this is no less “remarkably similar” than any of the Team comparanda. This is not actually inconsistent with the actual language of Trouet et al where a global reorganization of wind fields at the ‘MCA’-LIA transition is posited.
Proctor et al 2000: http://www.barlang.hu/pages/science/angol/CD2000_815.pdf