The North Atlantic Oscillation influence on Europe: climate impacts and associated physical mechanisms

A multivariable analysis of the influence of the North Atlantic Oscillation (NAO) on the climate of the North Atlantic and European sectors is presented using the 40 yr (1958-1997) consistent data set from NCEP. Using high and low NAO index composites, anomaly fields of climate variables are then interpreted based on physical mechanisms associated with the anomalous mean flow (characterised by the surface wind field) and the anomalous eddy activity (characterised by the surface vorticity and the 500 hPa storm track fields). It is shown that NAO-related temperature patterns are mainly controlled by the advection of heat by the anomalous mean flow. However, large asymmetries between minimum and maximum temperatures, and more significantly, between positive and negative phases of NAO imply the importance of a different mechanism, namely, the modulation of short wave and long wave radiation by cloud cover variations associated with the NAO. Furthermore, NAO influence on 2 different precipitation-related variables-precipitation rate and precipitable water-displays different patterns. Precipitable water is shown to be strongly related to the corresponding anomaly fields of temperature while precipitation rate appears to be controlled by the surface vorticity field and associated strength of the tropospheric synoptic activity.