The Active Role of the Ocean in the Temporal Evolution of Climate Sensitivity

Abstract

The temporal evolution of the effective climate sensitivity is shown to be influenced by the changing pattern of sea surface temperature (SST) and ocean heat uptake (OHU), which in turn have been attributed to ocean circulation changes. A set of novel experiments are performed to isolate the active role of the ocean by comparing a fully coupled CO2 quadrupling community Earth System Model (CESM) simulation against a partially coupled one, where the effect of the ocean circulation change and its impact on surface fluxes are disabled. The active OHU is responsible for the reduced effective climate sensitivity and weaker surface warming response in the fully coupled simulation. The passive OHU excites qualitatively similar feedbacks to CO2 quadrupling in a slab ocean model configuration due to the similar SST spatial pattern response in both experiments. Additionally, the nonunitary forcing efficacy of the active OHU (1.7) explains the very different net feedback parameters in the fully and partially coupled responses.

On the Relative Roles of the Atmosphere and Ocean in the Atlantic Multidecadal Variability

“…Note that both anomaly components are subjects to the Figure 1. Schematic of the partial coupling method (adapted from ; Garuba et al, 2018) oceanic transport or damping (i.e., ⃗ v ⋅ ∇T ′ AF and ⃗ v ⋅ ∇T ′ RF ); however, these damping terms are not part of the forcing for either T ′ AF or T ′ RF .…”

Connecting Direct Effects of CO2 Radiative Forcing to Ocean Heat Uptake and Circulation

“…These models are used to make quantitative projections of future climate and typically project an ocean heat uptake (OHU) that is about 90% of the Earth system's total change in energy content (Stocker et al, ). This OHU can be decomposed into a “passive” response to the altered surface boundary conditions, where the climatological ocean circulation advects the perturbed temperature field, and an “active” response, where the change in ocean circulation due to forcing redistributes the ocean's existing heat content (Garuba et al, ; Marshall et al, ; Winton, Adcroft, et al, ). The active component of OHU is, in part, related to a projected slowing of the Atlantic Meridional Overturning Circulation (AMOC), a robust projection under radiative forcing (Cheng et al, ) that is, in turn, related to changing ocean density gradients (e.g., Butler et al, ; Jansen et al, ).…”

Asymmetric dynamical ocean responses in warming icehouse and cooling greenhouse climates

“…Frölicher et al 2015) or temporally (e.g. Garuba et al 2018). The Southern Ocean is presently a primary region of anthropogenic heat and carbon uptake, whereas anthropogenic carbon storage is spread across lower latitudes, and anthropogenic heat storage is more restricted to the upper ocean, and is greatest in the Southern and Atlantic Oceans (Frölicher et al 2015).…”

“…Similar spatial patterns have been found in natural variability of carbon and heat storage by Thomas et al (2018), who described the variability as a function of convective states in the Weddell Sea. Regional changes in ocean circulation brought about by climate warming might affect the regional ocean heat uptake, resulting in changes to the global atmospheric temperature warming rate (Garuba et al 2018).…”