Large tropical eruptions gave an uncertain El Niño-like response in 1870-2025 SST data after forecasts from the pre-eruption ENSO state; the same test detected tropical cooling

We tested if large tropical volcanic eruptions push the tropical Pacific toward El Niño beyond what the ocean state before the eruption predicts. We defined 16 eruption events from 1872 to 2022 from CMIP7 stratospheric aerosol optical depth (AOD) and matched them to Smithsonian GVP eruptions. Five events had a tropical AOD rise of 0.04 or more (1872 event, Krakatau, Agung, El Chichón, Pinatubo). For each event, we made a cross-validated statistical forecast of SST indices from the month before the AOD rise, with HadISST1 and ERSSTv5. We compared the forecast residuals with residuals from random start months of the same calendar month. A simple composite showed relative Niño3.4 0.48-0.55 K above zero in months 1-12 (p about 0.03). The forecast residual was +0.31 K (p = 0.065) and +0.26 K (p = 0.24), so the pre-eruption state explained 36-53% of the composite. El Chichón gave most of the remaining residual; without it, the mean was +0.13-0.14 K. All 16 events gave a residual near zero (+0.03 and -0.01 K; 80% detection limit 0.26-0.34 K). As a positive control, the ENSO-removed tropical mean SST was 0.08-0.13 K below forecast in months 13-24 (p = 0.040 and 0.013), and this cooling increased with AOD (p = 0.004-0.005). Thus the method detects volcanic cooling, but the instrumental record does not give clear evidence for an El Niño response that the pre-eruption state does not predict. Main limitations: only 5 large events, sparse SST before 1880, an uncertain 1872 eruption match, an SST-only forecast model, and no correction for multiple tests.

ENSO · volcanic forcing · El Niño · stratospheric aerosol · Pinatubo · El Chichón · sea surface temperature · HadISST · ERSSTv5 · superposed epoch analysis · statistical forecast · null result