On forced and free atmospheric oscillations near the 27-day periodicity

Abstract

The Whole Atmosphere Community Climate Model was used to investigate the influences of solar fluctuations on zonal wind oscillations. Two simulations were conducted with short-term solar forcing (<35 days) on and off. We found that a 27-day wave is an inherent feature of the atmosphere when the short-term solar forcing is inactive. This internal 27-day oscillation comes along with other periods of the extra-long period wave band (20-40 days) and cannot be linked to the Sun's rotation period. When the short-term solar variability is part of the forcing, including the solar 27-day periodicity, it affects a wide range of the spectrum of zonal wind. At mid-latitudes, a 10-day wave emerges by the short-term solar forcing, which suggests that indirect and nonlinear interactions are involved. Solar short-term variability seems to generate atmospheric perturbations that interact with modes of the internal wave spectrum or the background mean flow. A robust and clear solar interpretation of these wind oscillations is challenging. However, dynamical responses to short-term solar variability exist and need further investigation.

Solar 27-day signatures in standard phase height measurements above central Europe

“…We also determined power spectra (not shown) of CMAM GH over the Eifel mountain region at pressure levels between 1 and 0.01 hPa, and significant spectral power for periods between 24 and 30 days was found, in agreement with the results presented by Schanz et al (2016).…”

Signature of the 27-day oscillation in the MLT tides and its relation with solar radiation at low latitudes

“…Furthermore, the prominent 27-day variability in the MLT zonal wind can be due to tropospheric convective activities (Huang et al 2015) or a manifestation of a normal mode inherent in the atmosphere (Andrews et al 1987;Schanz et al 2016). In the present study, weak correlation between the solar UV flux variability and MLT tidal modulation is also found at certain intervals.…”

“…A major portion of the solar UV flux is absorbed in the middle atmosphere. The short-term solar UV flux variability can be as large as 25% during solar active condition (Schanz et al 2016). The solar 11-year oscillation was reported mostly in the stratospheric ozone as the photochemical processes associated to ozone production are very sensitive to the solar UV radiation (Chandra and McPeters 1994;Hood 1997).…”

“…Using Whole Atmosphere Community Climate Model (WACCM), Schanz et al (2016) investigated the effect of solar irradiance on the zonal wind variability. They concluded that the solar short-term variability driven atmospheric perturbations might interact with atmospheric internal oscillation modes or background mean flow to cause associated changes in the ambient dynamical condition.…”

“…Furthermore, different signatures of the 27-day signal in the MLT were obtained by the past investigators. Previous studies interpreted that the concerned oscillation in the middle atmosphere was (i) linked with the solar rotation (Pancheva et al 1991;Luo et al 2001;Guharay et al 2017), (ii) a component of the lower atmospheric intraseasonal oscillations (Huang et al 2015) and (iii) an inherent mode of the atmosphere (Schanz et al 2016). In the present work, an extensive investigation related to the imprint of the 27-day signal on the dominant MLT tides and its relationship with the solar rotation is carried out using meteor radar wind observations from three low-latitude Southern hemispheric stations, São João do Cariri (7.4° S, 36.5° W) (CA), Cachoeira Paulista (22.7° S, 45° W) (CP) and Santa Maria (29.7° S, 53.7° W) (SM).…”