A comparison of global ocean general circulation model solutions obtained with synchronous and accelerated integration methods

A 10 000-year synchronous control integration using a comprehensive ocean general circulation model (OGCM) subject to realistic, time-dependent forcing in a global domain is performed to quantitatively determine how well its solution is reproduced by two accelerated, but otherwise identical, equilibrium integrations. To our knowledge, this is the first long synchronous integration of such an OGCM. The two accelerated cases use tracer time steps increasing with depth and unequal momentum and tracer time steps, respectively. After accelerated equilibration, these cases are integrated further synchronously to achieve synchronous equilibration. The equilibration time is defined as the time when the annual- and global-mean potential temperature trend is below 10−5 °C year−1. The synchronous control integration achieves equilibrium in about 3500 years. The accelerated equilibration times are 685 and 3000 surface years for the two cases, respectively. Their synchronous extensions require about 100 and 700 surface years. The accelerated solutions do differ from each other and from those of the control experiment. However, for practical purposes, many aspects of the accelerated and control solutions are more similar than otherwise. Because of severe non-conservation issues associated with tracer time-step variations with depth, this technique is not recommended. Instead, unequal momentum and tracer time steps should be used. Compared to the control case, this method of acceleration provides a factor of 2.5 reduction in the computational cost (accelerated + synchronous equilibration) with the possibility of further reductions. Any accelerated integration must be followed by a synchronous extension to recover the correct seasonal cycle and to eliminate any possible oscillatory behavior present in the accelerated phase. If the incremental gains with further synchronous extensions are judged to be minimal or of minor significance, shorter (<700 years) integrations may certainly suffice. Among others, the equilibration times are affected by the choice of surface forcing method.