Volcanic eruptions, large scale modes in the northern hemisphere, and the El Nino ̃- Southern Oscillation

• Main Author: Bo Christiansen
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Arctic; Pacific; North Atlantic; North Atlantic oscillation
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.570.6149; http://web.dmi.dk/fsweb/solar-terrestrial/staff/boc/volcan_rev.pdf

We analyze the impact of 13 major stratospheric-aerosol producing vol-canic eruptions since 1870 on the large scale variability modes of sea level pressure in the northern hemisphere winter. We focus on the Arctic Oscil-lation (AO) and the North Atlantic Oscillation (NAO) to address the ques-tion about the physical nature of these modes. The hypothesis that the phase of the El Nino ̃- Southern Oscillation (ENSO) may control the geographi-cal extent of the dominant mode in the northern hemisphere is also inves-tigated as well as the related possibility that the impact of the eruptions may be different according to the phase of ENSO. We find that both the AO and the NAO are excited in the first winter af-ter the eruptions with statistical significance at the 95 % level. Both the sig-nal and the significance is larger for the NAO than for the AO. The excita-tion of the AO and the NAO is connected with the excitation of a secondary mode which resembles an augmented Pacific-North American Pattern. This mode has opposite polarity in the Atlantic and the Pacific and interferes neg-atively with the AO in the Pacific and positively in the Atlantic in the first winter after the eruptions giving the superposition a strong NAO resemblance. We find some evidence that the correlations between the Atlantic and Pa-cific is stronger in the negative ENSO phase than in the positive phase al-though this difference is not statistical significant when all data since 1870 are considered. We do not find any evidence for that the impact of the vol-canic eruptions is more hemispheric in the negative than in the positive ENSO phase.