Assessing the Impact of Surface Energy Inputs on Radiative Feedbacks in Tropical and Extra-tropical Regions: Strength, Evolution, and Timescales

In recent years, radiative feedbacks in the earth system have been strongly tied to the spatial pattern of sea surface temperatures (SSTs). This “pattern effect” has been strongly tied to the strength of cloud radiative feedbacks driven by atmospheric stability changes. SST patch Green’s functions experiments have revealed that the ratio of warming in deep convective tropical regions, versus outside, drives significant changes in atmospheric stability. These Green’s functions can be used to reconstruct feedbacks from given warming patterns. However, it remains unclear how different warming patterns arise. Different Green’s functions, prescribing surface heat fluxes in atmosphere-ocean coupled models instead of temperature changes in fixed SST experiments, may answer this question by showing how energy inputs translate into temperature changes.

Using a simplistic set of patches of applied surface heat fluxes in CESM2-CAM6 and HadCM3, we find that heat input into the tropics results in strongly negative radiative feedbacks from enhanced warm pool warming. This results in a small climate sensitivity to this tropical forcing. Conversely, heat fluxes input into the extratropics cause significantly less negative feedbacks that result in greater climate sensitivity to extratropical forcing.Furthermore, the response to tropical forcing occurs rapidly, with equilibrium roughly achieved within a few years both in slab ocean and fully coupled models. The response to extratropical forcing, by contrast, induces near-zero feedbacks in the first few years, followed by significantly weaker negative feedbacks than seen under tropical forcing, which leave this simulation far from equilibrium after 150 years in the fully coupled model.

These outcomes of forcing, from within the tropics and outside, can be combined to explain the early changes in feedbacks in response to global uniform forcing, or near-uniform global forcings such as from CO₂. Reconstruction of the uniform case by summing the tropical and extra-tropical cases gives a good fit, except for an apparent temperature dependence in CESM2, and shows that extra-tropical component of surface forcing is driving the long-term feedbacks in the uniform forcing scenario.

Understanding the process of how the pattern of forcing results in different temperature change patterns may be key to comprehending future temperature changes, given that the pattern of future forcing evolves with the changing mix of anthropogenic forcing agents. Furthermore, exploring how models vary in their conversion of forcing into temperature change, even within the simple experimental design of this study, may highlight significant model feedback differences and contribute to narrowing the range in model predictions of future warming.