The stratospheric 4-day wave in NMC data

The observed characteristics of the 4-day eastward-moving wave 1 in the Southern Hemisphere polar winter stratosphere are surveyed using ten years of National Meteorological Center (NMC) geopotential height data. The 4-day wave is shown to be an ubiquitous feature in the Southern Hemisphere polar winter stratosphere and is usually prominent during July and August. Growth of the 4-day wave is characterized by two types of structures. Some episodes exhibit NW to SE phase tilts and a single high-latitude maximum. Others show NE to SW phase tilts during growth and a high-latitude maximum out of phase with a secondary lower-latitude maximum. Stability analyses show that all characteristics of the first type of episode are consistent with barotropic instability of the stratospheric polar night jet. Analyses of climatological fields suggest that characteristics of the second type of episode may be consistent with barotropic instability of the double-peaked mesospheric jet.