The Impact of Natural External Forcing on Ocean Heat Uptake Efficiency Since the 1980s

We investigate the temporal evolution of ocean heat uptake efficiency (OHUE) using observations and large ensemble model simulations. OHUE, defined as the ratio of ocean heat uptake rate to changes in global mean surface temperature anomalies, has exhibited significant variability over recent decades. We found a relatively low OHUE in the late 1980s, a peak around 2000, and a subsequent decline. A key finding is the significant influence of volcanic eruptions, particularly the 1991 Mount Pinatubo eruption, which caused abrupt cooling followed by a gradual recovery, leading recent variation of OHUE. Moreover, the Southern Ocean is becoming less efficient at absorbing heat into interior ocean in recent period associated with increased stratification. These results highlight the role of natural external forcing in climate dynamics and the importance of mid-latitudes in global OHUE variability, underscoring the need for idealized experiments to quantify the potential effects of future volcanic eruptions on OHUE.