Baroclinic outbreak episodes in wavenumber domain

In contrast to the common definition of anomalous atmospheric flow by persistent and localized geopotential height anomalies, we define significant excursions from normal conditions by persistent anomalies of a single spectral energy conversion on a circumpolar scale. Whereas amplification of transient geopotential height or streamfunction anomalies generally involves a variety of possible processes and complex energetics, our approach defines strongly distorted energetics of a certain kind. With data covering 10 years (1967 – 1976), we study baroclinic energy conversions from zonal mean to zonal wavenumber m available potential energy, C(A_z A_m), when these exceed the long-term average by more than a standard deviation for at least 3 consecutive days. During these episodes, called baroclinic outbreaks (BO), C(A_m K_m) and the kinetic energy of wave m, K_m, are also large in all probability. Although short BO cases are frequent for all m, there is a secondary maximum of frequency at about 7 days for the ultralong wave m = 2. Composite energetics of BO’s are presented, and a profound transformation of the 500 mb flow is shown for a long lasting m = 2 BO event.