Zonal wave number 2 baroclinic outbreak episodes in winter

Abstract

Long-lasting episodes, so-called baroclinic outbreaks (BO), with extra-ordinary baroclinic conversions favouring one ultralong wave with zonal wavenumber m were studied in a previous paper (Schilling, 1988). This paper is concentrated on m = 2 winter BO's with durations of more than 7 days. In addition, almost hemispherically-averaged energy parameters (10° N ≤ φ ≤ 90° N) and a two-dimensional spectral representation (ñ is the meridional mode index) are used. From 10 years (1967–1976) of data, we find 15 ultra-persistent winter cases, but only 11 are considered for further investigation, since 4 episodes fail to show sufficient growth of kinetic energy of m = 2 (K₂) despite large input C(A₂A₂). During m = 2 BO's in winter, the meridional mode ñ ≅ 2 (one node between southern boundary at 10° N and pole) exhibits large amplification and K₂ concentrates dramatically in precisely this mode. Mainly, the available potential energy of the super-rotational mode of the zonal mean vertical wind shear supplies that energy inflow. During the first stages of m = 2 BO's, the kinetic energy and baroclinic activity of synoptic waves (m = 5–8) appear to be significantly suppressed. An unusual distribution of modal eddy kinetic energy in 500 mb results so that approximately 25–30% of it is concentrated in only one wave mode (m= 2, ñ = 2).