The El Niño growth spurt in atmospheric CO2 is mostly caused by drought in the tropics. Rainfall that normally falls over tropical landmasses shifts to the oceans during El Niño events. This slows the normal growth of tropical forests and increases forest fires. Indonesia suffered severe fires during the 1997 event and, from recent news, is already being hit hard this year.
The loss of carbon from tropical forests in El Niño years is temporary as the forests tend to regrow in normal years, building back their biomass and sucking CO2 out of the air in the process. But the eventual recovery from this El Niño won’t bring us back below 400 ppm, because its impact will be dwarfed by the global consumption of fossil fuels, pushing CO2 levels ever higher.