The Dominant Contribution of Southern Ocean Heat Uptake to Time-Evolving Radiative Feedback in CESM

In most fully coupled climate models, the net radiative feedback magnitude decreases with time after abruptly quadrupling CO2. Hypotheses have been raised to explain the time dependence of radiative feedbacks, including the influence from surface warming pattern and ocean heat uptake (OHU) pattern. By using the Green's Function derived from pairs of simulations in the atmospheric model (CAM5) coupled with a slab-ocean, with each simulation being forced by a localized surface heat flux anomaly, we evaluate the influences of regional OHU on transient surface warming pattern, accounting for the changes in radiative feedbacks. The attribution of the time-evolving net radiative feedback highlights the remote impact from OHU over the Southern Ocean on tropical sea surface temperature. The time-dependent weakening of OHU over the Southern Ocean gives rise to increasingly enhanced surface warming in southeastern Pacific, which leads to decreasing tropospheric stability and more positive cloud feedback decades after quadrupling CO2. Plain Language Summary Climate sensitivity, defined as surface temperature increase to doubling of carbon dioxide (CO2) concentration, is a broadly used metric of anthropogenic climate change. However, it has spanned a wide range for decades due to the uncertainty in radiative forcing and feedback. The time evolution in radiative feedback, for example, adds challenges for evaluating climate sensitivity via simulations with limited length and for comparing model simulations with observational records. In this study, we investigate how ocean heat uptake influences the time evolution of radiative feedback. More specifically, we quantify the dependence of radiative feedback on regional ocean heat uptake in response to an abrupt increase in CO2 concentration. Results show that the surface warming due to weakened ocean heat uptake over the Southern Ocean decades after CO2 increase is not locally confined, but has far-field impacts on tropical clouds via remote influences on sea surface temperature and atmospheric stability in the tropics. The tropical sea surface temperature patterns have been shown to be key for understanding transient evolution of radiative feedbacks in previous studies; our findings further suggest that Southern Ocean heat uptake could be a potential root cause for these evolutions. Key Points The increase in cloud feedback in CESM can be mostly attributed to the ocean heat uptake evolution in the Southern Ocean The increasingly weakening of ocean heat uptake in the Southern Ocean leads to increasingly enhanced warming locally and remotely The remote impact on the tropical surface temperature pattern leads to an increase in cloud feedback by decreasing tropospheric stability

Automated summary

This study investigates how ocean heat uptake influences the time evolution of radiative feedback, highlighting the remote impact of Southern Ocean heat uptake on tropical sea surface temperature and cloud feedback. The weakening of ocean heat uptake in the Southern Ocean leads to enhanced warming locally and remotely, affecting tropospheric stability and cloud feedback.