The ocean’s role in the transient response of climate to abrupt greenhouse gas forcing

We study the role of the ocean in setting the patterns and timescale of the transient response of the climate to anthropogenic greenhouse gas forcing. A novel framework is set out which involves integration of an ocean-only model in which the anthropogenic temperature signal is forced from the surface by anomalous downwelling heat fluxes and damped at a rate controlled by a ‘climate feedback’ parameter. We observe a broad correspondence between the evolution of the anthropogenic temperature (T₀₍ₓ₇ₑ₎) in our simplified ocean-only model and that of coupled climate models perturbed by a quadrupling of CO₂. This suggests that many of the mechanisms at work in fully coupled models are captured by our idealized ocean-only system. The framework allows us to probe the role of the ocean in delaying warming signals in the Southern Ocean and in the northern North Atlantic, and in amplifying the warming signal in the Arctic. By comparing active and passive temperature-like tracers we assess the degree to which changes in ocean circulation play a role in setting the distribution and evolution of T₀₍ₓ₇ₑ₎. The background ocean circulation strongly influences the large-scale patterns of ocean heat uptake and storage, such that T₀₍ₓ₇ₑ₎ is governed by an advection/diffusion equation and weakly damped to the atmosphere at a rate set by climate feedbacks. Where warming is sufficiently small, for example in the Southern Ocean, changes in ocean circulation play a secondary role. In other regions, most noticeably in the North Atlantic, changes in ocean circulation induced by T₀₍ₓ₇ₑ₎ are central in shaping the response.