Modulation of decadal ENSO-like variation by effective solar radiation

Abstract Prediction of the Pacific sea surface temperature (SST) anomaly in the coming decades is a challenge as the SST anomaly changes over time due to natural and anthropogenic climate forcing. The climate changes in the mid-1970s and late-1990s were related to the decadal Pacific SST variability. The changes in the mid-1970s were associated with the positive phase of decadal El Nino-Southern Oscillation (ENSO)-like SST variation, and the changes in the late-1990s were related to its negative phase. However, it is not clear whether this decadal SST variability is related to any external forcing. Here, we show that the effective solar radiation (ESR), which includes the net solar radiation and the effects of volcanic eruption, has modulated this decadal ENSO-like oscillation. The eastern Pacific warming (cooling) associated with this decadal ENSO-like oscillation over the past 139 years is significantly related to weak (strong) ESR. The weak ESR with strong volcanic eruption is found to strengthen the El Nino, resulting in an El Nino-like SST anomaly on the decadal time scale. The strong eruptions of the El Chicho’n (1982) and Pinatubo (1991) volcanoes reduced the ESR during the 1980s and 1990s, respectively. The radiation reduction weakened the Walker circulation due to the “ocean thermostat” mechanism that generates eastern Pacific warming associated with a decadal El Nino-like SST anomaly. This mechanism has been confirmed by the millennium run of ECHO-G model, in which the positive eastward gradient of SST over the equatorial Pacific was simulated under the weak ESR forcing on the decadal time scale. We now experience a reversal of the trend in the ESR. The strong solar radiation and lack of strong volcanic eruptions over the past 15 years have resulted in strong ESR, which should enhance the Walker circulation, leading to a La Nina-like SST anomaly.