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WeatherBlog 20/2013 | NAO, Easter powder and climate change

What is the NAO Index?

by Lea Hartl 04/02/2013
Since the weather doesn't change much anyway, let's get back to the big picture and suggestions from the WeatherBlog topic requests thread.

Since the weather doesn't change much anyway, let's get back to the big picture and suggestions from the WeatherBlog topic requests thread.NAO stands for North Atlantic Oscillation and the NAO Index describes the pressure conditions over the northern Atlantic. The NAO index expresses the relationship between two semi-permanent pressure formations: the Icelandic low and the Azores high. Air pressure anomalies from Reykjavik and Ponta Delgada are usually used for the calculation. The index is positive if both the Icelandic low and the Azores high are strongly pronounced and the pressure differences are correspondingly large. If the pressure centers are weak, the index is negative.

Influence on the weather

If you now imagine the Azores high swirling clockwise and the Iceland low swirling anti-clockwise, you will notice that the air between them flows from west to east due to the direction of rotation of the two formations. If the NAO index is positive, this westerly flow is very pronounced and Central Europe experiences mild and humid westerly weather. If, as is the case at the moment, the pressure centers are weak or even reversed (high over low situation), the westerly flow weakens or moves south into the Mediterranean region. As a result, Atlantic disturbances tend to pass south of the Alps, while it remains cold and tends to be dry in the north. The ocean surface temperature is primarily used for the long-term forecast of the NAO index, as certain temperature distributions promote the development of high lows over the North Atlantic, for example. This also allows rough seasonal forecasts to be made. A long positive phase indicates snow and rather mild weather on the northern slopes of the Alps, while it tends to remain dry in the south.

As the NAO is difficult to separate from the AO (Antarctic Oscillation) and the oscillation implied by the name is certainly present, but does not obey any recognizable temporal periodicity, the summarizing term Northern Annular Mode (NAM) is increasingly gaining acceptance.

Ice and climate

As with a printer, there are so-called drivers in the Earth-atmosphere system that make certain effects possible or amplify them. The sea ice at the poles, for example, is an important climatic factor, although it seems to have little to do with whether we will be driving powder or sulz tomorrow. The Arctic sea ice is losing area and volume extremely quickly. Large areas of the ocean have recently become ice-free, releasing additional heat and moisture into the atmosphere. Circulation patterns are changing. Various studies suggest that the jet stream is forming more pronounced waves in connection with the reduction in sea ice, i.e. the westward drift in mid-latitudes is less zonal. Blocking layers become more frequent, which in turn favors extreme (hot and cold, wet and dry) weather events. So if you want, you can analyze the current spring, which is definitely worthwhile in terms of skiing, on many levels, from the very small (powder quality at Easter) to the very large (climate change and implications from migration to world hunger).

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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