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Which comes first: the extratropical cyclone or the front? An attempt is presented to disentangle the historic “chicken or egg” relationship that puzzles meteorologists to this day.
BACKGROUND.
The acceptance of two contrasting interpretations of cyclogenesis over the last century “has been antiphonal with the two streams alternating in holding ascendency” (Davies 1997, p. 261). The first perspective emerged as a mainstream theory of cyclogenesis after the Bergen School of Meteorology promoted it in the 1910s-20s. They attributed cyclogenesis to the intrinsic instability of the hemispheric-spanning “polar front” upon which frontal cyclones develop as wavelike disturbances (Bjerknes and Solberg 1922). The key elements of today’s weather charts and visualizations of extratropical cyclones in introductory textbooks build upon the concepts developed in the 1910s, demonstrating the success of the Bergen School’s concept. A contrasting view emerged in the middle of the last century when Hsieh (1949), who studied a North American cyclone, concluded that “this type of surface cyclogenesis is evidently different from the usual type associated with waves on a surface front” (Hsieh 1949, p. 401).
According to the Bergen School’s cyclone model, during the life cycle of the surface cyclone, warm air pushes poleward while cold air advances toward the equator (Bjerknes 1919). The two air masses are separated by narrow transition zones, that is, the warm and cold fronts. Later, the highly idealized concept of the polar front as a discontinuity was replaced by a narrow transition zone (Bjerknes and Palmén 1937). According to this polar front paradigm, the surface front is considered to be the origin of the cyclone development, which occurs as an instability of the preexisting front. The fundamental question that emerges from this paradigm is why and when do fronts become unstable (e.g., Solberg 1928; Kotschin 1932; Orlanski 1968; Kasahara and Rao 1972; Sinton and Mechoso 1984; Sinton and Heise 1993; Parker 1998; Patoux et al. 2005). Schär and Davies (1990) and Joly and Thorpe (1990) identified an intrinsic frontal instability of surface fronts that are either associated with a warm-air precursor or a positive low-level potential vorticity (PV) anomaly, respectively. These studies provided, decades after the Bergen School presented its concepts, a theoretical basis for the primarily observation-based frontal-cyclone model. For more comprehensive literature reviews, the reader may...