Abstract
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 ( $$T_{anthro}$$ ) in our simplified ocean-only model and that of coupled climate models perturbed by a quadrupling of $$\hbox {CO}_{2}$$ . 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 $$T_{anthro}$$ . The background ocean circulation strongly influences the large-scale patterns of ocean heat uptake and storage, such that $$T_{anthro}$$ 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 $$T_{anthro}$$ are central in shaping the response.
Highlights
The response of the climate to greenhouse gas (GHG) forcing is not uniform in space and time but instead exhibits considerable structure in both the spatial patterns and timing of the anthropogenically induced temperature signal
In particular we find that delayed warming around Antarctica is likely not attributable to local anomalous ocean heat uptake and storage, changes in the strength of the Atlantic Meridional Overturning Circulation (AMOC) are a major contributor to delayed warming in the Atlantic sector, and poleward advection of anthropogenic temperature across the Arctic circle may play an important role in Arctic amplification
In some regions the anthropogenic temperature (Tanthro) acts nearly like a passive tracer introduced in to the ocean at the surface, weakly damped by climate feedbacks but advected and mixed by climatological currents. This limit is approached over much of the Southern Ocean, where warming is sufficiently small that changes in ocean circulation are small
Summary
The response of the climate to greenhouse gas (GHG) forcing is not uniform in space and time but instead exhibits considerable structure in both the spatial patterns and timing of the anthropogenically induced temperature signal. In the calculations described here we choose Hanthro and to be spatially-uniform over the ocean and constant in time, crudely mimicking enhanced downwelling radiation due to GHG forcing and the large-scale radiative damping of resulting SST anomalies to space. It should be noted that the AMOC is somewhat weak (peaking at 12 Sv) and the mixed layer deepest in the Greenland–Iceland–Norwegian Sea rather than the Labrador Sea. it should be noted that the AMOC is somewhat weak (peaking at 12 Sv) and the mixed layer deepest in the Greenland–Iceland–Norwegian Sea rather than the Labrador Sea This control solution is perturbed with a downwelling flux of magnitude Hanthro = 4 Wm−2, approximating the global downwelling longwave radiative forcing from a doubling of atmospheric CO2 (Myhre et al 1998; Andrews et al 2012), following the procedure outlined in Sect.
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