Question v): Internal variability from the longer time horizon
Climate model simulations that are selected to mimic the observed behaviour of key modes of climate variability (NAO, ENSO, SAM) show considerably less distance to observations in many regions of the planet, and show significant changes in their overturning circulation in response to these modes (Schurer et al., in prep.). Climate model simulations suggests that a further large part of the multidecadal variability of the Antlantic Meridional Overturning Circulation was externally forced, with a role for aerosols from 1900 and likely a dominant role for volcanic eruptions. (Schurer et al., in prep.; Undorf et al., J. Climate 2018).
Longterm trends in the North Atlantic Oscillation have first reduced (to the 1960s), then enhanced (to the 1990s) then again reduced (over the hiatus period) the observed winter warming compared to what it would have been without the NAO tendencies (Iles and Hegerl, 2017).