Eruptions and Climate: The influence of major volcanic eruptions on the European climate.

In an article in the journal Nature in July, relayed by the News of the CNRS-INSU, climatologists have pointed to an almost systematic response of the North Atlantic Oscillation - NAO / North Atlantic Oscillation - after major volcanic eruptions.
 

Main eruptive phase of Pinatubo for the basic Clark / Philippines - Doc Nasa 12.06.1991

What is the North Atlantic Oscillation ?

This is an atmospheric and oceanic phenomenon, mainly concerning the North Atlantic, characterized by an up-and-coming air masses, north-south over the Arctic and Icelandic regions towards the subtropical belt, the Azores and the Iberian Peninsula. These air mass movements affects the pressure changes on the ground, changes in the westerly winds and the climate around the Atlantic basin.

It is characterized by an index, the NAO index.

The NAO index is calculated each year from the pressure difference between two cities: Lisbon, in Portugal and Reykjavik, in Iceland, taking the average deviation from January to March.

A positive NAO index means that the pressure during the winter is higher than average in Lisbon, and lower than average in Reykjavik ... with the result, a more powerful Azores anticyclone and a hollow Icelandic depression.

A negative NAO index is the inverse to the Azores anticyclone lower than normal in winter, and an Icelandic depression barely growing.

These pressure changes influence the climate of the northern hemisphere, particularly the European winter climate.

When the pressure difference between the Azores high (A) and the Icelandic Low (D) is lower than usual (NAO -), the storm track moves to the south of Europe - When the pressure difference between the Azores high (A) and the Icelandic low (D) is stronger than usual (NAO +), the storm track moves to Northern Europe. © Pablo Ortega - one click to enlarge

Positive and negative NAO - Source: Heinz Wanner, climate Institute of Geography and Meteorology, University of Bern

Influence of volcanic eruptions on the European winter climate:

The study highlighted the fact that two years after each of the eleven best known eruptions of the last millennium, the NAO index is almost always positive. The last eruption of Mount Pinatubo in 1991 led to this kind of observation.

The mechanism is not fully understood to date, but is a forecast track to the consequences of a major volcanic eruption on the European winter climate.

Sources :

- Actualités du CNRS-INSU" - Prévoir les hivers européens en décryptant 1 000 ans d’histoire climatique - link

- Nature - A model-tested North Atlantic Oscillation reconstruction for the past millenium - by Pablo Ortega, Flavio Lehner, Didier Swingedouw, Valerie Masson-Delmotte, Christoph C. Raible & al.

Nature 523, 71–74 (01 July 2015) | doi:10.1038/nature14518 - link

- Ifremer - l'Oscillation Nord Atlantique - link