2013 Volume 91A Pages 243-267
Potential influence of a midlatitude SST frontal zone on the zonally symmetric variability in the extratropical atmosphere is assessed through idealized aqua-planet experiments with a general circulation model. In one experiment with a midlatitude frontal SST gradient as sharp as that observed in the Southwestern Indian Ocean, the annular mode is well reproduced in the model summer hemisphere. As actually observed in the Southern Hemisphere, the model annular mode represents a north-south seesaw in westerly wind speed around the climatological joint axes of a midlatitude westerly jet and storm track, with intensified (weakened) midlatitude westerlies under the enhanced (reduced) eddy momentum transport. This essential feature of the annular mode is retained also over the winter hemisphere, although its structure is somewhat distorted owing to the seasonal intensification of a subtropical jet (STJ). In the other experiment, elimination of the frontal SST gradient results in an equatorward shift of westerly wind anomalies associated with the annular mode in the summer hemisphere, in step with a shift of the mean joint axes of the jet and storm track. More importantly, both the amplitude and persistence of the mode are substantially reduced. In the winter hemisphere, the elimination also results in a marked weakening of the annular variability of midlatitude westerlies. Due to the weakening of the near-surface baroclinicity from lack of the frontal SST gradient, anomalous eddy momentum transport is also reduced markedly. Unlike the observed annular mode, the dominant mode of variability primarily represents STJ variability. Though idealized, these model experiments suggest the potential importance of a midlatitude oceanic frontal zone for the year-round dominance and robustness of the annular mode signal against the wintertime intensification of a STJ, by enhancing storm-track activity.
