Equilibrium Ocean Thermal Expansion Depends Non-Linearly on the Forcing Level

Abstract:

The ocean dominates the planetary heat budget and takes thousands of years to equilibrate to perturbed
surface conditions. We show two commonly held assumptions to be inaccurate: (a) A temperature perturbation
in the atmosphere translates to a roughly uniform equilibrium ocean temperature anomaly.
(b) Equilibrium global sea level rise due to thermal expansion is proportional to global surface warming.
We analyze a vast range forcing levels and equilibration time scales of up to 10 000 years, for one model of
intermediate complexity and one state-of-the-art global climate model.
The response time scales and regions of anomalous ocean heat storage depend non-linearly on the forcing
level and equilibrium surface warming. The Atlantic Meridional Overturning Circulation is only proportional
to the forcing in its initial response, but not in its centennial to millennial recovery. In the Southern
Ocean, water mass properties and surface air temperature response do not scale linearly with the forcing
level. Interior and deep oceans warm very little compared to the surface layers for small perturbations, but
do so increasingly for higher forcing levels. The deep ocean temperature anomaly does not correspond to
either high or low latitude atmospheric surface temperature anomaly. Depending on where the excess heat
is stored in the long term, the global sea level due to thermal expansion varies. We discuss the scalability of
equilibrium climate sensitivity between these simulations and their relation to different definitions of radiative
forcing.
Two far reaching consequences are: (1) that one cannot deduce long term (centennial to millennial) from
short term (decadal to centennial) behavior of ocean circulation and heat uptake. (2) The explanatory power
of deep sea proxies of past climate change to represent surface temperature perturbations, might be limited
due to the uncertainty of the detailed forcing history.