Ocean Heat Uptake Processes: A Model Intercomparison

Abstract

The quasi-equilibrium heat balances, as well as the responses to 4 × CO2 perturbation, are compared among three global climate models with the aim to identify and explain intermodel differences in ocean heat uptake (OHU) processes. It is found that, in quasi equilibrium, convective and mixed layer processes, as well as eddy-related processes, cause cooling of the subsurface ocean. The cooling is balanced by warming caused by advective and diapycnally diffusive processes. It is also found that in the CO2-perturbed climates the largest contribution to OHU comes from changes in vertical mixing processes and the mean circulation, particularly in the extratropics, caused both by changes in wind forcing and by changes in high-latitude buoyancy forcing. There is a substantial warming in the tropics: a significant part of which occurs because of changes in horizontal advection in extratropics. Diapycnal diffusion makes only a weak contribution to the OHU, mainly in the tropics, because of increased stratification. There are important qualitative differences in the contribution of eddy-induced advection and isopycnal diffusion to the OHU among the models. The former is related to the different values of the coefficients used in the corresponding scheme. The latter is related to the different tapering formulations of the isopycnal diffusion scheme. These differences affect the OHU in the deep ocean, which is substantial in two of the models, with the dominant region of deep warming being the Southern Ocean. However, most of the OHU takes place above 2000 m, and the three models are quantitatively similar in their global OHU efficiency and its breakdown among processes and as a function of latitude.

An imperative to monitor Earth's energy imbalance

“…However, the uptake of heat by the ocean acts as a buffer to climate change 20 , slowing the rate of surface warming. Thus, the ocean's ability to store and vertically redistribute large quantities of heat over a decade or so means that trends in GMST are an unreliable indicator of global warming on these timescales ( Fig.…”

Southern Ocean Warming

“…Excess heat eventually accumulates within and north of the ACC in the surface layer and in the relatively shallow layers (upper 1,000 m) that are ventilated within the Antarctic Circumpolar Current (mode and intermediate waters; see Figure 1; Cai et al 2010;Bryan et al, 2014;Marshall and Zanna, 2014;Exarchou et al, 2015;Frölicher et al, 2015;Kuhlbrodt et al, 2015;Morrison et al, 2016). In addition, eddy processes within and north of the Antarctic Circumpolar Current result in southward along-isopycnal heat transport across the Antarctic Circumpolar Current in the mean climatological circulation state (Gregory, 2000;Wolfe et al, 2008).…”

Dependence of global radiative feedbacks on evolving patterns of surface heat fluxes

“…The three representative realistic Q-flux patterns (Figures 2c-2e) feature the "Realistic 1st decade" with a relatively homogeneous heat uptake, the average response of the "Realistic 5th decade", in which the reduction of the Atlantic Meridional Overturning Circulation (AMOC) is strong (red patch in the North Atlantic) and the Southern ocean heat uptake becomes important, and finally, the "Realistic 20th decade", in which the AMOC has restrengthened and the Southern Ocean becomes the dominant heat sink [e.g., Frolicher et al, 2014;Li et al, 2013]. We cannot differentiate which part of the pattern is due to the atmospheric or oceanic influence only [e.g., Stouffer and Manabe, 2003;Xie et al, 2010;Exarchou et al, 2014], since we deduce the Q-flux from the coupled model. We cannot differentiate which part of the pattern is due to the atmospheric or oceanic influence only [e.g., Stouffer and Manabe, 2003;Xie et al, 2010;Exarchou et al, 2014], since we deduce the Q-flux from the coupled model.…”