Are solar cycles predictable?

TL;DR

This paper examines various solar cycle prediction methods, highlighting how cycle overlap and Waldmeier's rule create correlations that explain predictive skill without requiring direct physical links.

Contribution

It provides a physical explanation for the predictive skill of precursor methods based on cycle overlap and amplitude-dependent rise times.

Findings

Cycle overlap and Waldmeier's rule induce correlations in sunspot records.

Precursor method skill can be explained without direct physical links.

Data assimilation with dynamo models shows high predictive correlations.

Abstract

Various methods (or recipes) have been proposed to predict future solar activity levels - with mixed success. Among these, some precursor methods based upon quantities determined around or a few years before solar minimum have provided rather high correlations with the strength of the following cycles. Recently, data assimilation with an advection-dominated (flux-transport) dynamo model has been proposed as a predictive tool, yielding remarkably high correlation coefficients. After discussing the potential implications of these results and the criticism that has been raised, we study the possible physical origin(s) of the predictive skill provided by precursor and other methods. It is found that the combination of the overlap of solar cycles and their amplitude-dependent rise time (Waldmeier's rule) introduces correlations in the sunspot number (or area) record, which account for the predictive skill of many precursor methods. This explanation requires no direct physical relation between the precursor quantity and the dynamo mechanism (in the sense of the Babcock-Leighton scheme or otherwise).