Wavelet based fractal analysis of El Niño/La Niña episodes

Abstract

The fractal dimension H, which is frequency-dependent within Quasi-Biennial (QB) period, was explored to measure the noise characteristics of Niño 3.4 Sea Surface Temperature (SST), where “noise” specifies the cycles within QB such as the Tropospheric Biennial Oscillation (TBO). The results show that the oscillation pattern of H corresponds mostly to development of El Niño, particularly during two strong Tropical Pacific Decadal Oscillation (TDO) periods of 1894 to 1923 and 1978 to 2000. This represents a stochastic resonance mechanism when a positive-phase noise overlaps a stronger positive-phase of TDO. In this case, SST would exceed a critical value to trigger an El Niño. The mechanism provides a favorable condition by which the onset of El Niño becomes more sensitive to noise. Self-organized criticality (SOC) explains that a small disturbance on an uncertain system will result in an avalanche, including scale-invariance (scaling) and criticality (threshold) features. The results show that strong and medium El Niño events regularly show scaling within QB period especially after the 1970s. Therefore, scaling is a critical state for onset of a strong El Niño and noise modulation by SOC within QB period plays a significant role in the El Niño developments.