My GCM runs faster than yours

Abstract

A dry global primitive equation model has beed adapted to perform as a simple GCM by adding empirically derived forcing terms which are constant in time. When integrated to a statistical equilibrium, this model gives an excellent simulation of the observed climate in terms of both time-mean quantities and transient eddy activity. This approach delivers a fast running model, which is easily adapted to run as a linear perturbation model and for which the transient eddy forcing terms are easily diagnosed.

Additional forcing is added to simulate the effect of anomalous heating in the tropical Pacific arising from El-Nino or La-Nina type disturbances. The resulting atmospheric flow over the mid-latitude Pacific and North America resembles the observed `PNA' response to these phenomena. The dynamical mechanisms involved, particularly the role of nonlinearity, the time dependence of the background state and the additional forcing due to transient eddies are investigated by use of a linear perturbation model and a number of ensemble forecast experiments.

An anomaly in the mid-latitude Pacific sea surface temperature gives a quite different model response. Depending on the strength and position of the anomaly relative to the Pacific jet, the response can either resemble the linear response or project strongly onto the model's natural modes of low frequency variability. Initial value ensemble experiments will be presented to asses the potential impact of knowledge of the midlatitude SST on forecast skill.