On the importance of midlatitude oceanic frontal zones for the mean state and dominant variability in the tropospheric circulation

Observations indicate that midlatitude weather systems are organized into storm tracks'' near oceanic frontal zones with pronounced sea-surface temperature (SST) gradients. A pair of atmospheric general circulation model experiments with zonally uniform SST profiles prescribed show that their observed collocation is not fortuitous. In one experiment, a storm track is anchored around a midlatitude SST front that maintains near-surface thermal gradients and energizes eddies. Westerly momentum transport by eddies produces a well-defined polar-front jet along the front, even in winter when a subtropical jet stream intensifies. In the other experiment, removal of the SST front leads to a substantial weakening in eddy activity and the PFJ especially in winter. It also leads to a weakening of the annular mode-the dominant mode of westerly-jet variability-and its notable structural distortion in winter. Though idealized, our experiments suggest the importance of midlatitude oceanic fronts for the tropospheric circulation and its variability.