Equilibrium Ocean Warming Patterns Depend Non-linearly on Forcing History
Equilibrium Ocean Warming Patterns Depend Non-linearly on Forcing History
Abstract:
The ocean dominates the planetary heat budget and requires 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 of forcing levels and equilibration time scales of up to 30 000 years for a model of
intermediate complexity.
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 the surface air temperature response do not scale linearly with the forcing level.
Interior and deep oceans warm little compared to the surface layers for small perturbations, but do so
increasingly for higher forcing levels. Depending on where the excess heat is stored in the long term, the
thermal expansion varies. We show through pattern scaling that the non-linear dependency of thermal
expansion on surface temperature is stronger than to be expected from the non-linear equation of state.
We conclude that (1) the long-term behavior of ocean circulation and heat uptake cannot be deduced from
short-term (decadal to centennial) behavior, and; (2) the explanatory power of deep sea proxies of surface
temperature to explain past climate change and perturbations might be limited due to the uncertainty of
the detailed forcing history
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 of forcing levels and equilibration time scales of up to 30 000 years for a model of
intermediate complexity.
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 the surface air temperature response do not scale linearly with the forcing level.
Interior and deep oceans warm little compared to the surface layers for small perturbations, but do so
increasingly for higher forcing levels. Depending on where the excess heat is stored in the long term, the
thermal expansion varies. We show through pattern scaling that the non-linear dependency of thermal
expansion on surface temperature is stronger than to be expected from the non-linear equation of state.
We conclude that (1) the long-term behavior of ocean circulation and heat uptake cannot be deduced from
short-term (decadal to centennial) behavior, and; (2) the explanatory power of deep sea proxies of surface
temperature to explain past climate change and perturbations might be limited due to the uncertainty of
the detailed forcing history