Global Warming Patterns over the North Pacific: ENSO versus AO

The relationships between the natural variability and CO2-induced response over the Pacific region are investigated in terms of the spatial anomaly pattern of SST, sea level pressure and precipitation by a multi-model intercomparison analysis, based on the 18-model results contributing to the IPCC Fourth Assessment Report. The analysis indicates that the CO2-induced response pattern is related with the model natural variability modes, ENSO and AO. In the tropical Pacific, an ENSO-like global warming pattern is simulated by the majority of the models, with mostly El Niño-like change. In the Arctic region, an AO-like global warming pattern is simulated by many models, with the positive definite AO-phase change, if AO-like. It is suggested that the increase in meridional temperature gradient in the upper troposphere, and the lower stratosphere, provides a preferable condition for the positive AO-like change in the high latitudes by intensifying the subtropical jet, while the increase in the static stability provides a preferable condition for the El Niño-like change in the low latitudes, by reducing the large-scale ambient circulations. However, the sign of the mass (SLP) anomaly is incompatible over the North Pacific, between the positive AO-like change and the El Niño-like change. As a result, the present models cannot fully determine the relative importance between the mechanisms inducing the positive AO-like change and inducing the ENSO-like change, leading to scattering in global warming patterns in regional scales over the North Pacific.