Abstract
Abstract. A number of studies have shown that 5-day planetary waves modulate noctilucent clouds and the closely related Polar Mesosphere Summer Echoes (PMSE) at the summer mesopause. Summer stratospheric winds should inhibit wave propagation through the stratosphere and, although some numerical models (Geisler and Dickinson, 1976) do show a possibility for upward wave propagation, it has also been suggested that the upward propagation may in practice be confined to the winter hemisphere with horizontal propagation of the wave from the winter to the summer hemisphere at mesosphere heights causing the effects observed at the summer mesopause. It has further been proposed (Garcia et al., 2005) that 5-day planetary waves observed in the summer mesosphere could be excited in-situ by baroclinic instability in the upper mesosphere. In this study, we first extract and analyze 5-day planetary wave characteristics on a global scale in the middle atmosphere (up to 54 km in temperature, and up to 68 km in ozone concentration) using measurements by the Odin satellite for selected days during northern hemisphere summer from 2003, 2004, 2005 and 2007. Second, we show that 5-day temperature fluctuations consistent with westward-traveling 5-day waves are present at the summer mesopause, using local ground-based meteor-radar observations. Finally we examine whether any of three possible sources of the detected temperature fluctuations at the summer mesopause can be excluded: upward propagation from the stratosphere in the summer-hemisphere, horizontal propagation from the winter-hemisphere or in-situ excitation as a result of the baroclinic instability. We find that in one case, far from solstice, the baroclinic instability is unlikely to be involved. In one further case, close to solstice, upward propagation in the same hemisphere seems to be ruled out. In all other cases, all or any of the three proposed mechanisms are consistent with the observations.
Highlights
Large scale Rossby waves, planetary waves with horizontal wavelengths of thousands of kilometers and with periods up to several days, form a well-known class of atmospheric waves
Summer stratospheric winds should inhibit wave propagation through the stratosphere and, some numerical models (Geisler and Dickinson, 1976) do show a possibility for upward wave propagation, it has been suggested that the upward propagation may in practice be confined to the winter hemisphere with horizontal propagation of the wave from the winter to the summer hemisphere at mesosphere heights causing the effects observed at the summer mesopause
We show that 5-day temperature fluctuations consistent with westward-traveling 5-day waves are present at the summer mesopause, using local ground-based meteor-radar observations
Summary
Large scale Rossby waves, planetary waves with horizontal wavelengths of thousands of kilometers and with periods up to several days, form a well-known class of atmospheric waves. Theoretical predictions of the 5-day planetary wave characteristics, for example, by Geisler and Dickinson (1976) and Salby (1981a, b), have later been confirmed by both ground based and satellite observations (Hirota and Hirooka, 1984; Hirooka, 2000; Lawrence and Jarvis, 2003; Garcia et al, 2005; Riggin et al, 2006). The wave is not symmetric about the equator in the mesosphere and wave amplitudes may be greater in the summer mesosphere (Geisler and Dickinson, 1976; Salby, 1981b). Results based on the space observations from Nimbus-6 (Prata, 1989) indicate a period closer to 6 days in spring and autumn (equinox conditions) and about 5.2 days in summer and winter (solstice conditions)
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