Interdecadal Variations of Radiative Feedbacks Associated With the El Niño and Southern Oscillation (ENSO) in CMIP6 Models

Internally generated radiative feedback at interannual timescales can influence climate sensitivity estimates. This study has clarified the mechanisms of variability in radiative feedback associated with the El Nino and Southern Oscillation based on preindustrial control simulations for Coupled Model Intercomparison Project Phase 6. Radiative feedback showed large modulation with a comparable magnitude to the intermodel uncertainty associated with internal changes in the zonal gradient of the ocean thermocline and sea surface temperature (SST) in the equatorial Pacific on interdecadal timescales. When the interdecadal SST anomalies in the eastern and western equatorial Pacific were simulated as positive and negative, respectively, the statistical properties of the interannual variations were also modified with diminished Walker circulation changes per unit of global mean surface temperature rise. Meanwhile, the enhanced local Hadley circulation led to a middle-to upper-level cloud decrease and a low-level cloud increase in the subtropics, causing enhanced negative radiative feedback as a global average. Global warming is usually estimated based on radiative feedbacks: how the climate system responds to external forcing. Even though internal variability can give non-negligible uncertainty in its estimate, radiative feedbacks characterized by internal variability on interannual timescales such as El Nino and Southern Oscillation (ENSO) potentially contribute to its evaluation. This study clarifies the mechanism of variability on internally generated radiative feedbacks. Radiative feedbacks varied on interdecadal timescales associated with the pattern changes for the background sea surface temperature on the equatorial Pacific. Such background shift strengthens ENSO, thereby enhanced local Hadley circulation strengthens negative feedback. Internally generated radiative feedbacks can fluctuate on interdecadal timescales in accordance with the background shiftThe background shift on the equatorial Pacific works to amplify the El Nino and Southern Oscillation (ENSO)Enhanced local Hadley circulation on the equatorial Pacific related to modulated ENSO strengthens negative feedback