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

Abstract

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 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. KEY WORDS: NAO · Storm tracks · Surface wind · Maximum and minimum temperatures · Precipitation rate · Precipitable waterResale or republication not permitted without written consent of the publisher

Decadal trends in the east Atlantic-west Russia pattern and Mediterranean precipitation

“…The usefulness of the data for climate analyses over the European region has been widely proven (e.g. Trigo et al, 2002). The NNRP data assimilation system includes the T62/28-level global spectral model with horizontal resolution of about 210 km.…”

“…The opposite situation takes place during periods with low NAO index values, when wetter air masses are transported to the western and northern Mediterranean regions (Hurrell, 1995). A significant positive trend of the NAO pattern has been noted over the four last decades of the past century (Hurrell, 1995), and the role of the NAO trend in the weather conditions over Europe has been widely investigated (Hurrell, 1995;Paeth et al, 1999;Ulbrich and Cristoph, 1999;Ben-Gai et al, 2001;Hurrell et al, 2001;Krichak et al, 2002;Trigo et al, 2002).…”

Analyses of extreme flooding in Austria over the period 1951–2006

“…More specifically, the aim is to assess whether the North Atlantic Oscillation (NAO) is a significant predictor in explaining the changes in magnitude and frequency of occurrence of floods. The NAO is considered because of its relation with weather and flooding over Europe (Hurrell, 1995;Hurrell and Van Loon, 1997;Wanner et al, 2001;Schmidli et al, 2002;Trigo et al, 2002;Marković and Koch, 2005;, Boé et al, 2009. Arnell (1997, 1999) found a strong correlation between NAO and European regional flow series.…”

North Atlantic cloud cover response to the North Atlantic oscillation and relationship to surface temperature changes

“…In the latter case, a similar cloud response to the NAO as described here might be expected to result in a negative cloud feedback since the effects of cloud changes would now tend to oppose advectively driven SAT changes. Such a reversal in sign of the cloud feedback could contribute to a seasonal asymmetry in the SAT response to NAO variability analogous to the diurnal asymmetry suggested by Trigo et al [2002] (i.e., relatively larger magnitude SAT anomalies during the polar night when advection and cloud radiative effects reinforce each other).…”

“…Trigo et al [2002] found reduced cloud cover over central Europe and the Iberian Peninsula during high NAO winter months. They also reported decreases in cloudiness during the low phase of the NAO over northern Europe.…”