A possible mechanism for the eastward shift of interannual NAO action centers in last three decades

A weakly nonlinear North Atlantic Oscillation (NAO) model proposed recently by Luo et al. (2006) is used to examine the dynamical processes that dominate the eastward shift of the NAO action centers of interannual variability. It is shown that in a weak mean westerly flow the NAO pattern can exhibit a westward shift due to the eddy forcing from high-frequency transient eddies. Such an eddy-induced westward shift can become rather pronounced if the transient eddies are rather strong. However, if the mean westerly flow is enhanced considerably the NAO pattern undergoes an eastward shift because the mean flow-induced eastward shift exceeds the westward shift. This result suggests that the eastward shift of the NAO centers of action of interannual variability during the period after 1980 is probably due to an increase in the strength of the zonal mean westerly winds in the North Atlantic region.