Probing climate feedbacks on different time-scales with single frequency forcing simulations

The climate system response to external forcing involves a series of atmospheric and oceanic feedback processes acting on a variety of timescales. These processes cannot be readily identified in standard model simulations where the imposed forcing - be it historical or idealized -acts over a broad continuum of time-scales. Here, we probe the time-scale dependence of feedback mechanisms using single-frequency forcing simulations in a coupled climate model (CESM-CAM4). In a series of experiments, we prescribe radiative forcings (equivalent to doubling CO2) that are uniform in space and vary sinusoidally in time with frequencies ranging from 1/8 months^-1 to 1/100 years^-1. The model response exhibits negligible non-linearity, with the response being largely confined to the forcing frequency. This modeling framework allows us to robustly probe processes acting on different time-scales, which combine to determine the climate system's response to sustained CO2 forcing. We focus here on understanding the time-scale dependence of sea-surface temperature patterns, which play a key role in setting equilibrium climate sensitivity. We find that the response patterns of Pacific sea-surface temperatures exhibit a high degree of time-scale dependence, going from enhanced East-Pacific warming (forcing period of ~2 years) to a pattern of enhanced West-Pacific warming (~8 years) to a uniform warming pattern (~30 years), reflecting the changing activation of ocean-atmosphere feedbacks. In turn, these changing patterns lead to strongly time-scale dependent radiative feedbacks and climate sensitivity. This approach thus allows us to understand and quantify the dependence of radiative feedbacks on the frequency-content of the external forcing.