Revisiting Hansen & Sutera’s suggestion of bimodality in the Northern Hemisphere midlatitude circulation

Abstract

Inspired by the Charney-and-de Vore hypothesis, Hansen and Sutera in 1986 came forward with the claim of having detected a fingerprint of the presence of two preferred state in the tropospheric circulation at northern Hemisphere mid-latitudes. Shortly after, Nitsche et al. found that this claim was premature, since it may be caused by a insufficient data base. Now, with much more data, both from reanalyses and from extended control runs with earth System models, we have revisited this case, and found no evidence of such bimodality, when using the original metric. Following the principle that lack of evidence is not evidence of lacking reality, it may be possible to detect footprints of multimodality with different metrics, and we are not making any claims of that sort, even if the literature provides little evidence of the success of alternative metrics. Our result has a dual significance, namely first for our understanding of atmospheric dynamics, and secondly of the ubiquitous temptations of the Zeitgeist.