The Global Stationary Wave Response to Climate Change in a Coupled GCM

Abstract

The stationary wave response to global climate change in the Geophysical Fluid Dynamics Laboratory's R30 coupled ocean?atmosphere GCM is studied. An ensemble of climate change simulations that use a standard prescription for time-dependent increases of greenhouse gas and sulfate aerosol concentrations is compared to a multiple-century control simulation with these constituents fixed at preindustrial levels. The primary response to climate change is to zonalize the atmospheric circulation, that is, to reduce the amplitude of the stationary waves in all seasons. This zonalization is particularly strong in the boreal summer over the Tropics. In January, changes in the stationary waves resemble that of an El Niño, and all months exhibit an El Niño?like increase of precipitation in the central tropical Pacific. The dynamics of the stationary wave changes are studied with a linear stationary wave model, which is shown to simulate the stationary wave response to climate change remarkably well. The linear model is used to decompose the response into parts associated with changes to the zonal-mean basic state and with changes to the zonally asymmetric ?forcings? such as diabatic heating and transient eddy fluxes. The decomposition reveals that at least as much of the climate change response is accounted for by the change to the zonal-mean basic state as by the change to the zonally asymmetric forcings. For the January response in the Pacific?North American sector, it is also found that the diabatic heating forcing contribution dominates the climate change response but is significantly cancelled and phase shifted by the transient eddy forcing. The importance of the zonal mean and of the diabatic heating forcing contrasts strongly with previous linear stationary wave models of the El Niño, despite the similarity of the January stationary wave response to El Niño. In particular, in El Niño, changes to the zonal-mean circulation contribute little to the stationary wave response, and the transient eddy forcing dominates. The conclusions from the linear stationary wave model apparently contradict previous findings on the stationary wave response to climate change response in a coarse-resolution version of this model.