Evidence of nonlinear dynamics in the eastward shift of the NAO

Hilmer and Jung [2000] have identified an eastward shift in the sea level pressure (SLP) pattern associated with interannual variability of the North Atlantic Oscillation (NAO) that took place around 1980. We investigate the nature of the eastward shift using a primitive equation, dry atmospheric model driven by diabatic forcing diagnosed from observations. The model results reveal the nonlinear dependence of the spatial pattern of the NAO on the NAO index, the pattern being shifted to the east (west) for high (low) NAO index. General agreement is found between the model and observations. We suggest that the eastward shift noted by Hilmer and Jung [2000] is a consequence of the trend towards higher NAO index during the last several decades of the 20th century.