Observed Associations Among Storm Tracks, Jet Streams and Midlatitude Oceanic Fronts

Abstract

An association is discussed among a midlatitude storm track, a westerly polar-front jet stream and an underlying oceanic frontal zone. Their close association is observed when a subtropical jet stream is weak, as in the Southern Hemisphere summer 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 within 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 and its variability. In fact, decadal-scale sea-surface temperature anomalies observed in the North Pacific subarctic frontal zone controlled the anomalous heat release to the atmosphere. Seemingly, the local storm track responded consistently to the decadal-scale shift of the frontal axis, acting to reinforce basin-scale flow anomalies. 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.