spotless sun quiet sun
© Solar Dynamics Observatory
The sun on July 2, 2018
Two solar physicists, Robert Leamon from NASA Goddard Space Flight Center, and Scott McIntosh from the High Altitude Observatory at Boulder, CO, have made an interesting observation that links changes in solar activity with changes in the El Niño Southern Oscillation (ENSO).

As they reported at the AGU 2017 Fall Meeting, the termination of the solar magnetic activity bands at the solar equator that mark the end of the Hale cycle coincides since the 1960's with a shift from El Niño to La Niña conditions in the Pacific.

Predicting the La Niña of 2020-21: Termination of Solar Cycles and Correlated Variance in Solar and Atmospheric Variability
"We look at the particulate and radiative implications of these termination points, their temporal recurrence and signature, from the Sun to the Earth, and show the correlated signature of solar cycle termination events and major oceanic oscillations that extend back many decades. A combined one-two punch of reduced particulate forcing and increased radiative forcing that result from the termination of one solar cycle and rapid blossoming of another correlates strongly with a shift from El Niño to La Niña conditions in the Pacific Ocean."
More information is available at the talk they gave at the last SORCE Meeting:

Terminators: The Death of Solar Cycles and La Niña 2020

As they say in the talk, the probability that the pattern is due to chance is very low. Particularly since the termination of the magnetic activity bands at the equator coincides quite precisely with the El Niño-La Niña shift.

Analysis of the ONI (Oceanic Niño Index) data from NOAA, and sunspot number from SIDC shows
the following pattern:
el nino nina pattern graph

Figure: Top: Six-month smoothed monthly sunspot number from SIDC. Bottom: Oceanic El Niño Index from NOAA. Red and blue boxes mark the El Niño and La Niña periods in the repeating pattern.
Since the 1960's the early solar minimum is associated with La Niña conditions, the late solar minimum is associated with El Niño conditions, and the rapid increase from minimum to maximum is associated to La Niña conditions again. As the authors note, this pattern did not take place in the 1954 minimum, although the rise in activity was also associated with La Niña conditions then. It is unclear why the ENSO system responded differently at that time, but it is clear that solar activity was not the only factor affecting ENSO.

The pattern appears to be repeating again this minimum. The early minimum has been associated to La Niña conditions and, as we move towards the late minimum, an El Niño is being forecasted for late 2018. The authors made their claim for a 2020 La Niña before the 2018 El Niño was forecasted. Now a repetition of the pattern looks even more probable and we should expect a La Niña when solar activity increases in late 2020 to 2021.