Synchronous and Asynchronous Integrations in an Ocean General Circulation Model

Abstract

A basinwide ocean general circulation model of the North Pacific is used to study the difference in distributions of tracers between asynchronous and synchronous integrations. An integration in which equal time steps and no depth acceleration are applied is called a synchronous integration. On the contrary, the integration is called asynchronous integration or acceleration. The acceleration can be divided into tracer acceleration and depth acceleration. Only the tracer acceleration is examined in this study. The values of 48, 24, 12, 6, 3, and 1 for the ratio (α) of tracer time step to momentum time step are used in the model. Results show that a 15-yr integration adjustment is necessary for tracers to reach a new state at which there is a 1% difference from a final state upon a switch to a smaller tracer time step from an equilibrium state. The too large ratio of 48 generally produces an unacceptable departure from synchronous integration. If a difference of about 3% is allowed, the ratio of about 8 can be used. The largest difference mainly appears at the subsurface of the Tropics. This is mainly due to destruction of geostrophic balance at the equator. Clorofluorocarbons (CFCs) are taken as a reference to study the distribution of passive or anthropogenic tracers using different α values. The results from several 10-yr runs show that the largest α value of 48 generates an over 10% error in distributions of CFC-11, compared with the case of α = 1. Decreases in α values reduce the error. When α is less than 12, an asynchronous integration of the CFC model will not generate an over 3% error in CFC distributions.