We define, calculate and analyze irregularity indices λISSN of daily series of the International Sunspot Number ISSN as a function of increasing smoothing from N = 162 to 648 days. The irregularity indices λ are computed within 4-year sliding windows, with embedding dimensions m= 1 and 2. λISSN displays Schwabe cycles with ∼5.5-year variations ("half Schwabe variations" HSV). The mean of λISSN undergoes a downward step and the amplitude of its variations strongly decreases around 1930. We observe changes in the ratio R of the mean amplitude of λ peaks at solar cycle minima with respect to peaks at solar maxima as a function of date, embedding dimension and, importantly, smoothing parameter N. We identify two distinct regimes, called Q1 and Q2, defined mainly by the evolution of R as a function of N: Q1, with increasing HSV behavior and R value as N is increased, occurs before 1915-1930; and Q2, with decreasing HSV behavior and R value as N is increased, occurs after ∼1975. We attempt to account for these observations with an autoregressive (order 1) model with Poissonian noise and a mean modulated by two sine waves of periods T 1 and T 2 (T 1 = 11 years, and intermediate T 2 is tuned to mimic quasibiennial oscillations QBO). The model can generate both Q1 and Q2 regimes. When m= 1, HSV appears in the absence of T 2 variations. When m= 2, Q1 occurs when T 2 variations are present, whereas Q2 occurs when T 2 variations are suppressed. We propose that the HSV behavior of the irregularity index of ISSN may be linked to the presence of strong QBO before 1915-1930, a transition and their disappearance around 1975, corresponding to a change in regime of solar activity. © Author(s) 2014.
CITATION STYLE
Shapoval, A., Le Mouël, J. L., Shnirman, M., & Courtillot, V. (2014). Can irregularities of solar proxies help understand quasi-biennial solar variations? Nonlinear Processes in Geophysics, 21(4), 797–813. https://doi.org/10.5194/npg-21-797-2014
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