A mixed-flux equilibrium asynchronous coupling scheme for accelerating convergence in ocean-atmosphere models

Abstract

 A coupled atmosphere/mixed-layer ocean energy balance model of intermediate complexity is used to examine the feasibility of a technique to accelerate the model's convergence rate while using equilibrium asynchronous coupling (EAC). EAC consists of an iterative sequence of integrations of an atmospheric model driven to equilibrium with fixed SSTs, the output of which is used to drive an ocean model to equilibrium. In an attempt to accelerate a radiatively perturbed climate to its final equilibrium state, we superimpose on the EAC scheme a mixed-flux condition, in which a portion of the turbulent air-sea heat flux from the most recent atmospheric leg is used in the present oceanic integration. In sensitivity tests using enhanced insolation, this mixed-flux approach strongly hastens the model's convergence rate, whose acceleration is regionally dependent: the tropics show the strongest sensitivity to the mixed-flux condition, while polar regions are least affected. This geographic variation stems from the presence of polar sea-ice, which promotes a temperature inversion and thereby causes a seasonal change in the sign of the total turbulent air-sea heat flux. Potential applications and limitations of this accelerated EAC method are also discussed.