Cyclones: Extratropical

Extratropical cyclones (ECs) are organized synoptic-scale low-pressure (cyclonic) weather systems that govern a considerable proportion of midlatitude (∼30°-60°) weather patterns (also commonly referred to as midlatitude cyclones). ECs ultimately form in response to the global circulation and energy balance. Given that the equator-to-pole temperature gradient is greatest during the transition and cool seasons, the magnitude and frequency of ECs is also greatest during these times. Part of the Earth's attempt to balance the surplus and deficit of net energy at the equator and the poles, respectively, involves large-scale equatorward and poleward air mass advections of relatively homogeneous thermal and moisture characteristics. The interaction along the fronts of these air masses is a key trait in the development and maintenance of ECs.

Satellites have enabled us to observe the typical comma-cloud shape of ECs, which typically have horizontal extents >1,000 kilometers, with a life cycle of several days (see photo).

The resulting mesoscale weather from a passing EC comprises a vast spectrum of events, including light rain or snow, torrential downpours, blizzards, damaging winds, destructive hail, tornadoes, as well as just overcast or clear skies. Given the areal extent, duration, frequency, and resultant sensible weather from ECs, these midlatitude weather systems often have a profound effect on many lives. Thus, our understanding of the characteristics and behavior of ECs and the role they play in weather and climate is of critical importance.

Polar Front Theory

The polar front theory (PFT) serves as the modern conceptual framework for the characteristic life cycle of ECs. The PFT, otherwise known as the Norwegian Cyclone Model, was established during World War I by the Norwegian physicists and meteorologists Jacob “Jack” and Vilhelm Bjerknes (son and father, respectively) and Halvor Solberg and the Swedish meteorologist Tor Bergeron. Together, these scientists pioneered the meteorological school of thought known as the Bergen school of meteorology, of which the PFT was the [Page 651]foundation. In short, the PFT states that wave cyclones develop along frontal boundaries of contrasting air masses (e.g., continental polar and maritime tropical) and are driven by thermal advections (baroclinic instabilities) along the polar front. However, it is important to note that tropical cyclones (TCs), that is, hurricanes, may evolve into ECs. This transition commonly occurs when a TC makes landfall in the middle latitudes, losing the forcing from the latent heat source of the warm tropical ocean waters but using the baroclinic energy to help sustain the cyclonic weather system.

Visible satellite image showing the comma-cloud shape of an extratropical cyclone over the North-Central United States and South-Central Canada

Source: National Aeronautics and Space Administration (NASA).