Influence of a mid-latitude oceanic frontal zone on the mean state of the atmospheric general circulation and its annular variability

A close association among a mid-latitude storm-track, a westerly polar-front jet (PFJ) and an underlying oceanic frontal zone is observed most typically in the situation where a subtropical jet (STJ) is relatively weak, as in the South Indian Ocean or in the North Atlantic. Along a near-surface baroclinic zone that tends to be anchored around a frontal zone, enhanced storm-track activity maintains a well-defined PFJ with strong surface westerlies. It is this eddy-driven jet whose axial fluctuations are manifested as the annular mode. In order to assess the particular importance of a mid-latitude oceanic frontal zone in the mean state of a storm-track and PFJ and in their low-frequency variability, a pair of perpetual “aqua-planet experiments” was conducted with a relatively high-resolution AGCM. In the experiment where frontal SST gradient was prescribed at 45° latitude, the main storm-track is anchored firmly around the frontal zone and a well-defined PFJ form slightly poleward, both in the model winter and summer hemispheres. The surface baroclinicity is restored effectively via differential heat supply from the ocean across the frontal zone. In that experiment, the most dominant mode of variability in the extra-tropical zonal-mean zonal wind exhibits certain resemblance to the observed Southern Hemisphere annular mode. In the other experiment where the frontal SST gradient is eliminated, the mean intensities of the storm-track and PFJ both weaken substantially. In addition, the amplitude of the model annular mode is also reduced substantially, and its meridional structure is apparently distorted especially in the presence of the intensified wintertime STJ, with certain implications to the troposphere-stratosphere coupled climate variability. Though idealized, our AGCM experiments suggest that the extra-tropical general circulation and its annular variability can be better understood from the viewpoint of air-sea interactions associated with the mid-latitude oceanic frontal zone, storm-track and PFJ.