Chaos and order in athmospheric dynamics. Part 2. Interannual rhythms of the El Niño - Southern oscillation

Processes of the El Niño - Southern Oscillation (ENSO) are investigated based on the mathematical theory of the so-called the strange nonchaotic attractor (SNA) in the quasi-periodically forced dynamic systems, and using the sea surface temperature and the atmospheric sea-level pressure data for the 1870-2014 year period. It is found that ENSO is influenced not only by the annual Sun-induced periodic heating of the climate system, but also by the three more other external forces which periods are incommensurable to the annual period. These forces are induced by the 18.6-year Luni-Solar nutation of the Earth’s rotation axis, the 11year cycle of the solar activity and the Chandler wobble in the Earth’s pole motion (the period 1.2 years). Because of the reciprocal incommensurability of the periods of these forces, all of them affect the climatic system in «improper» time moments. As a result, the dynamics of the indices representing the ENSO processes look to be very complex (strange in mathematical terms), but not chaotic. It is shown that power spectra of the ENSO indices have some bands of the increased spectral density located on sub-and superharmonics of above-mentioned periods. On the basis of some special considerations of structure of the power spectra of the ENSO indices the evidence of the discreteness of these spectra, i.e. the spectra being nonchaoticity, is received. Nobody assumed this circumstance earlier. Despite complexity of the ENSO processes, the dynamics of the temporal variations of these process indices reveals an internal ordering similar to that internal order which is known to be inherent to the SNA dynamics. This ordering reveals itself in the existence of spectral density peaks in the ENSO power spectra, and some rhythms corresponding to these peaks in the temporal variations of the ENSO indices. Acceptance of the SNA model for ENSO means that there are no predictability limits for ENSO in principle. In practice, it opens an opportunity to predict ENSO for several years ahead.