AOPP Seminar - Warming simpler worlds: using idealized coupled models to explore the state dependence of the climate response to radiative forcing

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. 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 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, leading to colder polar regions hosting year-round snow on the polar caps.

These two mean climates are perturbed by increasing atmospheric CO2 concentration at a rate of 1% per year until quadrupling. Both worlds warm more slowly than realistic (i.e., CMIP) models and at the same rate for the first ~90 years, despite their different initial climates. After 90 years, Aqua begins to warm more quickly than Ridge and approaches the low-end estimates for realistic models once the CO2 is quadruple its initial concentration. The similar warming rates at early times bely the very different ways in which this warming is achieved. Aqua’s net radiation imbalance is approximately twice that of Ridge with a concomitantly smaller radiative response, indicating that Aqua’s ocean is significantly more efficient at heat uptake than Ridge’s ocean, but that the feedbacks tending to bring Aqua back to equilibrium are weaker than those on Ridge. Decomposing the climate feedback parameter into individual radiative contributions shows that Aqua has a warming feedback due to albedo changes resulting from the loss of year-round snow while Ridge is cooled by the loss of mid-level clouds. Such 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.