Abstract
Correlations of the North Atlantic Oscillation (NAO) with climate variables in Europe are strong in the north and the south of Europe but there is a zone of statistically not significant correlations in between. This paper presents evidence that the impact of NAO can be ascertained more clearly by decomposing it into its constituent centres of action (COA), the Azores High (AZH) and the Icelandic Low (IL), each of which varies continuously in intensity as well as position. Monthly values of intensity and latitude and longitude positions for each COA are estimated from sea level pressure data. These are used to analyse their impact on interannual variations of winter precipitation and climate of Germany. Correlation of NAO with winter precipitation is weak but statistically significant in north Germany and not significant in south Germany. By separately calculating correlations with the AZH and the IL, it is found that the primary influence on the interannual variations of winter precipitation and surface temperature over both north and south Germany is the zonal migration of the IL. When the IL is situated east of its mean location, there is above normal precipitation and warmer temperatures over Germany and in winters when the IL is situated west of its mean longitudinal position, drier and colder conditions occur over Germany. The impact of this teleconnection is greater in north Germany than in the south, presumably because of the mountainous topography in the south which hinders wind flow.
Highlights
The North Atlantic Oscillation (NAO, Hurrell et al, 2001) is a large-scale meridional oscillation of the atmospheric surface pressure field between the Azores high (AZH) and the Icelandic low (IL)
We find that north Germany precipitation is possibly influenced by the NAO, AZH pressure and longitude and IL latitude and longitude
It should be checked which index reflects the direct contribution to the regional temperature variability so to isolate the effect of mutual correlations between NAO, AZH and IL indices we calculated partial correlations of the temperature with each index by keeping all the other indices fixed
Summary
The North Atlantic Oscillation (NAO, Hurrell et al, 2001) is a large-scale meridional oscillation of the atmospheric surface pressure field between the Azores high (AZH) and the Icelandic low (IL). When the index is negative (NAO-), westerlies are suppressed, northern European areas have cold dry winters because storms tracks shift southwards towards the Mediterranean region This brings increased precipitation to southern Europe and North Africa. If on the other hand, R(t) is precipitation over a northern location, the coefficient of the Azores High will be negligible This example illustrates the physical reason why considering the High and the Low separately (rather than their difference index, the NAO) is likely to give a better physical explanation. The position of the AZH or IL can have higher correlations than pressure of either of them, as is the case in this paper where the longitude position of the IL is found to be the dominant factor for the climate of Germany Another example is provided by the result that the latitude position of the Azores High has a higher correlation with transport of Saharan dust to the Atlantic than the NAO (Riemer et al, 2006).
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