The Modal Evolution of the Southern Oscillation

Abstract

The implementation of a progressive Fourier transform, applied to time series representative of the El Niño?Southern Oscillation (ENSO), provides a visual review of the frequency modes that have dominated throughout the available time series record. The modal evolution maps, constructed from the progressive Fourier transform, demonstrate that the ENSO is largely composed of a discrete number of low-frequency modes and that these dominant modes demonstrate a localized influence, in a temporal sense, persisting for periods of years before showing deviations in characteristic frequency. In support of other research, significant energy is maintained in the quasi-biennial and quasi-quadrennial period modes throughout the length of available Southern Oscillation index (SOI) record. Evident through the most recent record of both the SOI and Nino region sea surface temperature (SST) time series is the emergence of energy in the quasi-decadal period modes. The presence of a quasi-decadal mode has also been noted in the North Pacific over the same period and has been related to the warming SSTs near Alaska. During the period in which the quasi-decadal mode has been present, the most severe and the longest-lasting El Niños on instrument record in 1982?83 and 1991?93, respectively, have occurred. This suggests that the superposition of the quasi-decadal mode with the well-recognized quasi-biennial and quasi-quadrennial modes can lead to more severe and longer-lasting climate anomalies. Despite the absence of significant energy in the lower-frequency modes in the earlier instrument-recorded SOI, which corresponds to moderate ENSO activity, the existence of such a mode is suggested by an analysis of a paleoclimatic reconstruction of seasonal SOI presented in this paper and other similar analyses found in scientific literature. The last period of very strong ENSO activity occurred during the period 1870?1930. This same period corresponded to the presence of significant energy in the low-frequency modes of both the 7?8-yr and 35-yr periods, together with significant energy in the quasi-biennial and quasi-quadrennial modes. This enhances the proposition that severe, sustained climate anomalies in the Pacific region are dependent on these modes and that these modes, combined with the increase in amplitude in the quasi-biennial and quasi-quadrennial modes, can be induced by natural variability and cannot be assumed to be directly attributable to anthropogenic effects. The quasi-decadal mode is closely linked to the deepening of the Aleutian low in the North Pacific and will move to a positive phase over the next 5?7 yr, countering the effects of the bias over this period. The ENSO activity over the next 5?10 yr will largely depend on the interaction of the quasi-biennial and quasi-quadrennial modes, which appear to be declining in strength but are still significant, leading to moderate to strong ENSO events of both phases.