Eric's latest and probably last comment on the Stoat blog also confuses matters further, because first he dismisses Rob's critical analysis as "nonsense", but then he takes his time to repeat that "we assume, in effect, that most of the trend in Z200 is "natural variability"."(emphasis mine)
Which is exactly the assumption that many here have expressed serious doubts about.
I think it is worth taking the bolded section above in context:
There is one aspect that might be worth discussing, which is that we assume, in effect, that most of the the trend in z200 is “natural variability”. (We don’t actually assume it — that’s a result of the analysis, but in the end it amounts to the same thing, pretty much). But this doesn’t come out of nowhere! it comes largely from our previous work published in 2014, showing that the trend in z200 is related to tropical forcing.
The 2014 paper is, I believe, the following:
Ding, Q. H. et al. Tropical forcing of the recent rapid Arctic warming in northeastern Canada and Greenland. Nature 509, 209-212 (2014).
From the abstract:
Here we show that the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific. Atmospheric model experiments forced by prescribed tropical sea surface temperatures simulate the observed circulation changes and associated tropospheric and surface warming over northeastern Canada and Greenland. Experiments from the Coupled Model Intercomparison Project Phase 5 (ref. 16) models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming. This suggests that a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability.
I think if we have doubts about an assumption of natural variability in Z200 (I have no idea myself), it might worth looking at this paper, not just the 2017 one. I could not find it at a glance outside of the paid Nature publication however.