The role of a reduction in the Atlantic meridional overturning and that of a persistently negative North Atlantic Oscillation in explaining the coldness of the European Little Ice Age (LIA) has been assessed in two sets of numerical experiments.
These experiments are performed using an intermediate complexity climate model and a full complexity GCM.
A persistently negative NAO winter circulation, at NAO-index value −0.5, is imposed using recently developed data-assimilation techniques applicable on paleoclimatic timescales.
The hosing experiments lead to a reduction in oceanic meridional heat transport and cooler sea-surface temperatures.
Next to a direct cooling effect on European climate, the change in ocean surface temperatures feedback on the atmospheric circulation modifying European climate significantly.
The data-assimilation experiments showed a reduction of winter temperatures over parts of Europe, but there is little persistence into the summer season.
The state of the NAO, measured as the difference in sea level pressure (SLP) between the Azores and Iceland, relates to the strength of the westerly circulation that carries warm and wet air from the Atlantic into western Europe.
Reconstructions of the SLP fields back to 1,500 indicate that the negative phase of the NAO has been more active during extreme cold periods of the LIA, leading to reduced influence of the moist and warm zonal flow from the northeastern North Atlantic, which favored cool and dry European winters (Luterbacher et al. In addition, the recurrence of positive SLP anomalies centered over Scandinavia or northern Europe, which caused anomalous advection of cold air towards central and eastern Europe, enhanced the wintertime cooling (Luterbacher et al.
), who showed a relationship between external forcings (i.e. Basically, changes in the stratospheric temperature and wind anomalies modify the mid-latitude planetary waves refraction, which interact with the intensity of the westerly winds, and subsequent the NAO.
The experiment confirmed that regional climate changes associated with the changing state of the NAO can be much stronger than hemisphere wide changes.
The output of all model experiments are compared to reconstructions of winter and summer temperature based on the available temperature data for the LIA period.