Exploring state dependence of the climate response to radiative forcing using two idealized coupled climate models

Studies of climate sensitivity and feedbacks typically employ a suite of models with similar base climates but different model physics. Such an approach is useful for uncovering how changes to physical processes affect the climate response to changes in radiative forcing, but obscures the dependence of the climate response on the initial state of the climate itself. In order to better understand this dependence, we study the response to radiative forcing of two nearly identical configurations of the Community Earth System Model (CESM) with production-grade physics and resolutions that have dramatically different climates. The first, called Aqua, is completely covered with a uniform-depth ocean except for two 10º-wide polar continents to avoid the polar singularities in the ocean model. The second, Ridge, is identical to Aqua except for the presence of a thin ridge continent connecting the two polar caps. The ridge supports gyres in the ocean and leads to a warm, ice-free climate resembling a global Pacific Ocean, with a warm pool and cold tongue in the tropical ocean connected by a Walker circulation in the atmosphere. In contrast, the mean climate of Aqua is zonally symmetric and dominated by a global cold belt in the ocean driven by vigorous equatorial upwelling. The lack of gyres leads to a deep oceanic thermocline and reduces meridional heat transport, which allows for the development of persistent sea ice at high latitudes.

These two mean climates are perturbed by increasing atmospheric CO₂ concentration at a rate of 1% per year until quadrupling. Aqua initially warms more slowly than Ridge, with the transient climate response (TCR) at doubling 23% smaller for Aqua than Ridge. After doubling, however, Aqua begins to warm faster than Ridge and Aqua’s global mean temperature surpasses Ridge’s at quadrupling. A linear feedback analysis is used to gain insight into the time-evolving responses of these two configurations to increased CO₂ concentration. At all stages, Aqua’s net top-of-the-atmosphere heating is greater than Ridge’s. At early times, this is due to high clouds replacing low clouds in Aqua’s high latitudes, but decreasing surface albedo due to sea-ice loss eventually becomes a dominant factor. Aqua’s deep thermocline supports a higher ocean heat uptake (OHU) efficiency relative to Ridge that initially offsets these positive feedbacks and results in Aqua’s lower TCR. As CO₂ concentration approaches quadrupling, the combined effects of declining OHU efficiency and a strengthening ice-albedo feedback drive Aqua’s warming to temperatures compatible to Ridge. In the century following quadrupling, Aqua warms several Kelvin more than Ridge.

These idealized systems can shed light on the fundamental aspects of Earth’s climate system—such as how the response to radiative forcing depends on the base climate—that might be obscured in more complex configurations.