Impact of the Atlantic meridional overturning circulation on ocean heat storage and transient climate change

Abstract

We propose here that the Atlantic meridional overturning circulation (AMOC) plays an important role in setting the effective heat capacity of the World Ocean and thus impacts the pace of transient climate change. The depth and strength of AMOC are shown to be strongly correlated with the depth of heat storage across a suite of state‐of‐the‐art general circulation models (GCMs). In those models with a deeper and stronger AMOC, a smaller portion of the heat anomaly remains in the ocean mixed layer, and consequently, the surface temperature response is delayed. Representations of AMOC differ vastly across the GCMs, providing a major source of intermodel spread in the sea surface temperature (SST) response. A two‐layer model fit to the GCMs is used to demonstrate that the intermodel spread in SSTs due to variations in the ocean's effective heat capacity is significant but smaller than the spread due to climate feedbacks.

Shortwave and longwave radiative contributions to global warming under increasing CO 2

“…Although C increases over time (Fig. 2B) and there are, of course, multiple timescales of climate response (12,15), accounting for these details (e.g., by representing a deep ocean heat capacity) makes no substantive changes to our results and conclusions. Indeed, surface temperature increases quickly after a CO 2 perturbation-much of the equilibrium temperature response is realized within the first few decades in all of the GCMs ( Fig.…”

“…It has long been recognized that there is no single heat capacity (or characteristic relaxation time) of the climate system (11). Indeed, C increases with time as heat penetrates below the surface mixed layer and into the ocean interior (12)(13)(14)(15). For the CMIP5 GCMs, C corresponds to an equivalent ocean depth of 50 m in the first decade after 4× CO 2 and increases over time, reaching an equivalent depth of several hundred meters after a century (Fig.…”

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

“…Hansen et al (2011) argue that after 100 years a global-average response of between 60 and 90 % encompasses the real world response, with 90 % considered fast and 60 % slow. These curves can be rather readily fit by analytical Green's functions obtained from a two-layer model (see, e.g., Geoffroy et al 2013a, b;Kostov et al 2014). Their form depends on both and the efficiency of ocean heat uptake, as encapsulated in our ocean model.…”

The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) contribution to CMIP6: investigation of sea-level and ocean climate change in response to CO<sub>2</sub> forcing

“…The FAFMIP experiments will provide information on the sensitivity of the AMOC to buoyancy forcing of the magnitude and pattern of that predicted for CO 2 forcing, and will support investigation of the correlation between ocean heat uptake efficiency and the magnitude of the AMOC (Rugenstein et al, 2013;Winton et al, 2014;Kostov et al, 2014). The application of common perturbations to surface fluxes in FAFMIP will provide information about the ocean's role in determining patterns of sea-surface temperature change worldwide (of relevance to the Grand Challenge on clouds, circulation and climate sensitivity).…”