Large-scale interaction among storm tracks, polar-front jets and midlatitude oceanic frontal zones

A close association among a midlatitude storm track, a westerly polar-front jet stream and an underlying oceanic frontal zone is observed most typically in the situation where a subtropical jet stream is weak, as in the summertime Southern Hemisphere or in the North Atlantic. Along a near-surface baroclinic zone that tends to be anchored around a frontal zone, storm track activity is enhanced in a well-defined polar-front jet with modest core velocity. This eddy-driven jet exhibits a deep structure with the strong surface westerlies maintained mainly through a poleward eddy heat flux. The westerly wind stress exerted along the frontal zone acts to maintain it by driving the oceanic current system, suggestive of a feedback loop via midlatitude atmosphere-ocean interaction. It is argued that the context of this feedback must be included in interpreting the tropospheric general circulation. In fact, in a series of “aqua-planet experiments” with a relatively high-resolution AGCM, the organization of atmospheric baroclinic disturbances into a storm track and the associated organization of a polar-front jet are found sensitive to the prescribed midlatitude SST gradient. The simulated sensitivity is consistent with the high positive correlation between a poleward eddy heat flux and near-surface baroclinicity, which is observed in the course of the seasonal march along the core region of the Southern Hemisphere storm track anchored over the intense Antarctic Polar Frontal Zone. The sensitivity is also consistent with a tendency for the preference observed for low-level storm tracks to stay above midlatitude oceanic frontal zones. Over the North and South Pacific, the association is disturbed in winter by an intensified subtropical jet that traps eddy activity into its sharp core. The trapping impairs baroclinic interaction of upper-level eddies with the surface baroclinicity along a midlatitude oceanic front, leading to the suppression of eddy activity as observed in midwinter over the North Pacific.