North Atlantic storminess and Atlantic Meridional Overturning Circulation during the last Millennium: Reconciling contradictory proxy records of NAO variability

Within the last Millennium, the transition between the Medieval Climate Anomaly (MCA; ca. 1000–1300 CE)and the Little Ice Age (LIA; ca. 1400–1800 CE) has been recorded in a global array of climatic and oceano-graphic proxies. In this study, we review proxy evidence for two alternative hypotheses for the effects ofthis shift in the North Atlantic region. One hypothesis postulates that the MCA/LIA transition included aweakening of the Atlantic Meridional Overturning Circulation (AMOC) and a transition to more negativeNorth Atlantic Oscillation (NAO) conditions, resulting in a strong cooling of the North Atlantic region. The al-ternative hypothesis proposes a MCA/LIA shift to an increased number of storms over the North Atlanticlinked to increased mid-latitude cyclogenesis and hence a pervasive positive NAO state. The two sets ofproxy records and thus of the two competing hypotheses are then reconciled based on available resultsfrom climate model simulations of the last Millennium. While an increase in storm frequency implicates pos-itive NAO, increased intensity would be consistent with negative NAO during the LIA. Such an increase in cy-clone intensity could have resulted from the steepening of the meridional temperature gradient as the polescooled more strongly than the Tropics from the MCA into the LIA