Very well written article - leaves me where I left off in graduate school in 1988! Back then volcanic eruptions were theorized as a cause of ENSO and in my opinion, they are still in the lead for the most likely cause. I wanted to weigh in with a personal observation. In late May of 1991 I transited the Strait of Malacca and noted that the ITCZ was still over the equator (in my estimate, about a week late in moving into the Southern Hemisphere). From this, I predicted that it would be sunny when we arrived in the Philippines on June 7th, even though the seasonal monsoon shift generally occurs on the first of June. I also predicted that the Monsoon rains would start on June 8th. I was correct on both counts. Pinatubo erupted a few days later. My theory at the time was the causative factor for the ITCZ being out of place was the black smoke from the oil well fires in Kuwait caused by Iraqi forces during Desert Storm.

I also wonder out loud if the Mount St. Helens eruption in 1980 may have played a part in the 82-83 ENSO?