The Influence of Warming Patterns on Passive Ocean Heat Uptake

Abstract

The climate's response to forcing depends on how efficiently heat is absorbed by the ocean. Much, if not most, of this ocean heat uptake results from the passive transport of warm surface waters into the ocean's interior. Here we examine how geographic patterns of surface warming influence the efficiency of this passive heat uptake process. We show that the average pattern of surface warming in CMIP5 damps passive ocean heat uptake efficiency by nearly 25%, as compared to homogeneous surface warming. This "pattern effect" occurs because strong ventilation and weak surface warming are robustly colocated, particularly in the Southern Ocean. However, variations in warming patterns across CMIP5 do not drive significant ensemble spread in passive ocean heat uptake efficiency. This spread is likely linked to intermodel differences in ocean circulation, which our idealized results suggest may be dominated by differences in Southern Ocean and subtropical ventilation processes. Here, N, F, and T are the top-of-atmosphere (TOA) radiative imbalance perturbation, radiative forcing perturbation, and surface temperature anomaly, all defined with reference to an initial steady state, and the overbar denotes a global mean. The parameter represents the strength of global radiative feedback. Since the great majority of this radiative imbalance is absorbed by the ocean, it is common to equate N with the global mean rate of ocean heat uptake (OHU, where its global mean is OHU) (Cubasch et al., 2001; Raper et al., 2002), emphasizing the role OHU plays in pacing surface warming. Ocean heat uptake is the result of many dynamic and interactive processes such as advection, diffusion, and vertical mixing (Gregory, 2000). "Passive" ocean heat uptake (henceforth OHU p) refers to the transport of surface temperature anomalies into the interior by the climatological (time mean) of these transport processes (and is sometimes called "added heat" Bouttes et al., 2014). Passive ocean heat uptake thus describes the component of OHU in which heat acts like a passive tracer. In truth, heat is not a passive tracer-its uptake, along with other processes such as wind changes, alters advection and mixing and redistributes existing background temperature gradients. This "redistributed" heat (Garuba & Klinger, 2016) can significantly influence the spatial distribution of ocean warming and, indirectly, OHU (e.g., Banks & Gregory, 2006), particularly through changes in the North Atlantic (

Ocean Model Formulation Influences Transient Climate Response

“…This is in line with investigations of variability in a pre-industrial control state which is stronger than the variability in a transient state with increasing greenhouse gas concentrations (Brierley et al, 2009). Newsom et al (2020) argue that differences in surface warming and its influences on ventilation especially in the Southern Ocean and subtropical areas are important for the heat uptake of the ocean.…”

Projecting Global Mean Sea‐Level Change Using CMIP6 Models

“…Given the increase in GSAT change, our results suggests that the average OHU efficiency (e.g., Kuhlbrodt & Gregory, 2012) in CMIP6 is lower than in CMIP5. While the reasons for this are unclear (it could reflect increases in mean ocean stratification or changes in the patterns of projected surface warming), the intermodel differences in OHU efficiency may be dominated by the intermodel differences in the representation of ocean circulation processes (Newsom et al., 2020). Also, larger differences in GTE may only emerge after 2100, given the slow response of the deep ocean (Geoffroy & Saint‐Martin, 2013; Held et al., 2010).…”

Projecting Global Mean Sea-Level Change Using CMIP6 Models

“…Given the increase in GSAT change, our results suggests that the average OHU efficiency (e.g., Kuhlbrodt & Gregory, 2012) in CMIP6 is lower than in CMIP5. While the reasons for this are unclear (it could reflect increases in mean ocean stratification or changes in the patterns of projected surface warming), the intermodel differences in OHU efficiency may be dominated by the intermodel differences in the representation of ocean circulation processes (Newsom et al, 2020). Also, larger differences in GTE may only emerge after 2100, given the slow response of the deep ocean (Geoffroy & Saint-Martin, 2013;Held et al, 2010).…”