Formation Mechanism of a Western Hemisphere Circulation Pattern during Boreal Winter

The formation mechanism of a recurrent, low frequent flow regime—the Western Hemisphere (WH) atmospheric circulation pattern—is investigated in this study. Patterns in observed streamfunction, Rossby wave activity flux and sea surface temperature that are related to the observed WH pattern in 500-hPa height field are defined through the regression and composite analysis. These patterns are used with the GFDL AM2.1 model and a barotropic model to demonstrate the role of tropical Pacific forcing and the Pacific subtropical jet on the excitation and propagation of the wave train associated with the WH pattern. AGCM experiments confirm the dominant tropical west Pacific SST forcing combined with the tropical East Pacific cooling play a key role in driving the WH pattern through Rossby wave propagation. The distinctive response to the tropical forcing is also due to the divergent outflow in regions of strong vorticity gradients associated with the climatological mean subtropical jet. The extratropical eddy-flow feedback helps the wave train extend eastward to the East Atlantic and the western Europe.