As
an intrinsic mode of variability of the large - scale atmospheric circulation, the NAO requires no external forcing for excitation (e.g., Hurrell and Deser 2009; Branstator and Selten 2009).
One key metric in this debate is the spatial pattern of cooling which may provide a «fingerprint» of the underlying climate change, whether that was externally forced (from solar or volcanic activity) or was part of
an intrinsic mode of variability.