According to the authors,
"Periods of strong or frequent El Niño tended to occur during peaks in solar activity and during extended droughts in the United States Great Plains linked to La Niña. These changing modes of ENSO activity at millennial and multi-centennial timescales may have been caused by variations in the seasonal receipts of solar radiation associated with the precession of the equinoxes and/or changes in solar activity, respectively.
El Niño and La Niña events are coupled in the Holocene
Intensification of both ENSO phases [El Nino & La Nina] broadly coincided with peaks in solar activity.The paper joins many others in the scientific literature finding solar activity is the main driver of ENSO, as well as other ocean oscillations. Thus, solar influence on ENSO, "the largest perturbation to the climate system on an inter-annual time scale," is another of many solar amplification mechanisms described in the literature.
Our data from the core of the ENSO region thus calls into question earlier studies that reported a lack of El Niño activity in the early Holocene. In agreement with other proxy evidence from the tropical Pacific, the mid-Holocene (5600–3500 yr BP) was a time of consistently weak El Niño activity, as were the Early Middle Ages (∼1000–1500 yr BP). El Niño activity was moderate to high during the remainder of the last 3500 years."