Deep-ocean heat uptake and equilibrium climate response

Abstract

We integrate the coupled climate model ECHAM5/MPIOM to equilibrium under atmospheric CO2 quadrupling. The equilibrium global-mean surface-temperature change is 10.8 K. The surface equilibrates within about 1,200 years, the deep ocean within 5,000 years. The impact of the deep ocean on the equilibrium surface-temperature response is illustrated by the difference between ECHAM5/MPIOM and ECHAM5 coupled with slab ocean model (ECHAM5/SOM). The equilibrium global-mean surface temperature response is 11.1 K in ECHAM5/SOM and is thus 0.3 K higher than in ECHAM5/MPIOM. ECHAM5/MPIOM shows less warming over the northern-hemisphere mid and high latitudes, but larger warming over the tropical ocean and especially over the southern-hemisphere high latitudes. ECHAM5/MPIOM shows similar polar amplification in both the Arctic and the Antarctic, in contrast to ECHAM5/SOM, which shows stronger polar amplification in the northern hemisphere. The southern polar warming in ECHAM5/MPIOM is greatly delayed by Antarctic deep-ocean warming due to convective and isopycnal mixing. The equilibrium ocean temperature warming under CO2 quadrupling is around 8.0 K and is near-uniform with depth. The global-mean steric sea-level rise is 5.8 m in equilibrium; of this, 2.3 m are due to the deep-ocean warming after the surface temperature has almost equilibrated. This result suggests that the surface temperature change is a poor predictor for steric sea-level change in the long term. The effective climate response method described in Gregory et al. (2004) is evaluated with our simulation, which shows that their method to estimate the equilibrium climate response is accurate to within 10 %

Southern Ocean warming delayed by circumpolar upwelling and equatorward transport

“…This implies a multi-millennial timescale for the SO response to GHG forcing, consistent with GCM simulations 6,7 . Though we may not yet know the full mechanisms driving SO trends, these results suggest that the observed SO cooling and sea-ice expansion over recent decades must be interpreted against a background of very gradual GHGinduced warming -instead of the rapid warming seen in the Arctic.…”

Emulation of long-term changes in global climate: application to the late Pliocene and future

“…However, a comparison of the difference in temperature response to upper-and deep-ocean heat uptake and its contribution to the relationship between net radiative flux change (N) and global temperature change ( T ) in Geoffroy et al (2013) indicated that the method of of fitting two separate linear models to the early and subsequent (N, T ) data gives a good approximation of T g eq , F and α as they have been calculated here. A study by Li et al (2013) also found that, using the Gregory plot methodology, T g eq was estimated to within 10 % of its ac- tual value, obtained by running the simulation very close to equilibrium (∼ 6000 years). However, this was using the ECHAM5/MPIOM model, meaning that it is not necessarily also true for HadCM3.…”

Very early warning of next El Niño

“…There have been speculations that the recent hiatus in planetary mean surface temperature rise indicates that the climate system is less CO 2 sensitive than previously thought. On the other hand, new studies have demonstrated that decadal atmospheric warming is considerably masked by equatorial Pacific variability in heat uptake and release (36)(37)(38)(39).…”