From Tony N2MFT:

THE CENTENNIAL GLEISSBERG CYCLE: You've heard of the 11-year sunspot cycle But what about the Centennial Gleissberg Cycle? The Gleissberg Cycle is a slow modulation of the solar cycle, which suppresses sunspot numbers every 80 to 100 years. It may have been responsible for the remarkable weakness of Solar Cycle 24 in 2012-2013. New research published in the journal Space Weather suggests that the minimum of the Gleissberg Cycle has just passed. If so, solar cycles for the next 50 years could become increasingly intense. Read the paper here.

https://www.spaceweather.com/