On the Primary Drivers of Observed Variability in Climate Indices

Atmospheric and oceanic modes indicated by climate indices have inter-annual to multi-decadal variability, which play crucial roles in the non-stationary variation of global climate. In this study, the harmonic relationship among the driving-force signals of climate indices was detected by effectively combining Slow Feature Analysis (SFA) with Wavelet analysis. The result shows that most of the significant peak-scales of climate indices and their driving-force signals are the multiples of five base scales detected in El Niño /Southern Oscillation (ENSO): 0.98 yr, 2.24 yr, 3.77 yr, 6.34 yr, and 10.67 yr. The 0.98 yr is the annual cycle, the scale of 2.24 yr is associated with the Quasi Biennial Oscillation (QBO) signal, the 10.67 yr scale arises from the sunspot cycle (the Schwabe cycle), and the 3.77 yr, 6.34 yr scales are the intrinsic inter-annual variability of ENSO. Other detected statistically significant peak-scales in Pacific Decadal Oscillation (PDO), Atlantic Multi-decadal Oscillation (AMO) and North Atlantic Oscillation (NAO) and corresponding driving force signals can be expressed as the harmonics of the five base scales. The fact that the base scales are all from ENSO signals indicates that ENSO has footprint in PDO, AMO, and NAO. The base scales and their harmonic oscillations can be considered as the key linkages among different climatic modes with synchronous behaviors. In addition, the fact that the harmonic oscillations possess basic scales that relate to ENSO, solar and QBO signals indicate that these three factors are key driving factors in global climate variability.