Study of the mutual influence of the El Niño-Southern Oscillation processes and the North Atlantic and Arctic Oscillations

Abstract

On the basis of the nonlinear techniques for the estimation of coupling between oscillatory systems from time series, we investigate the dynamics of climatic modes characterizing global and Northern Hemi- sphere (NH) processes. The North Atlantic Oscillation (NAO) and Arctic Oscillation indices and the El Nino- Southern Oscillation (ENSO) indices are analyzed in terms of the most reliable data from 1950 through 2004 and earlier data since the 19th century. These indices characterize changes in NH atmospheric pressure (specif- ically, sea-level pressure in the North Atlantic and NH extratropical latitudes as a whole) and in equatorial Pacific sea-surface temperature and sea-level pressure to which the strongest variations of global surface tem- perature and global climate on interannual time scales and of regional climatic anomalies in the NH are linked. The methods used are based on phase-dynamics modeling and nonlinear prediction models (a nonlinear version of Granger causality ). From both methods and various ENSO indices, the inference about the ENSO effect on the NAO during the latter half of the 20th century and in the early 21st century is made with confidence prob- ability of at least 0.95. The influence is characterized by a time delay of about two years. No inverse influence is found with a similar degree of reliability. Results of estimating the coupling between the ENSO and the NAO depend on the type of index that is used to describe the NAO. The ENSO effect on the NAO is detected with sufficient confidence when the NAO index is chosen to be a larger scale characteristic. However, when a more local index of the NAO is used, no statistically significant coupling to the ENSO is found. Increasing the length of the analyzed ENSO and NAO series (over more than 100 yr) does not lead to any more reliable detection of coupling. Analysis of the data for different time intervals during 1950-2004 has revealed a strengthening of the ENSO effect on the NAO, although this inference is not reliable.