[P-PS04] Advancing the science of Venus in the golden age of exploration

[PPS04-11] Quasi-periodic variation of the equatorial jet induced by planetary-scale waves in the Venusian atmosphere

★Invited Papers

*Masahiro Takagi¹, Hiroki Ando¹, Masataka Imai¹, Norihiko Sugimoto², Yoshihisa Matsuda³ (1.Kyoto Sangyo University, 2.Keio University, 3.Tokyo Gakugei University)

Keywords:Venus, equatorial jet, atmospheric wave

Planetary-scale waves with a zonal wavenumber of 1 and periods of 4 and 5 days have been observed at the cloud top (approximately 70 km) in the Venusian atmosphere (Del Genio & Rossow, 1990; Kouyama et al., 2013; Imai et al., 2019). Since the zonal phase velocity of 4-day (5-day) wave is faster (slower) than the zonal-mean zonal wind, it is called a Kelvin (Rossby) wave. Radio occultation observations in the Venus Express mission showed periodic temperature variations in the polar region, which could be caused by neutral Rossby waves (Ando et al., 2017). Akatsuki LIR observations also detected multiple waves with periods of 3.6, 5.0, 5.4, and 6.1 days (Kajiwara et al., 2021). These results indicate that there are many types of waves in the Venusian atmosphere and they could affect atmospheric general circulation and material transport. However, their structures and/or excitation mechanisms have not been fully investigated. Recently, we showed that the 4-day wave observed at the cloud top was excited by the Rossby-Kelvin (RK) instability (Iga & Matsuda, 2005) and that the 5-day wave was also excited by the RK instability in which an equatorial Kelvin mode at 40-55 km, a mid-latitude Rossby mode at 50-80 km, and a high-latitude Rossby mode at 40-65 km were coupled (Takagi et al., 2021). In the present study, we investigate another planetary-scale wave with a period of 7 days (7-day wave) found in our GCM simulation. The result shows that the 7-day wave is excited at the lower cloud layer and it is closely related with the equatorial jet formed by the equatorward angular momentum (AM) transport induced by the 5-day wave. Because the 7-day wave induces the poleward AM transport in low latitudes, the equatorial jet becomes weaker as the 7-day wave grows. As a result, a quasi-periodic variation of the equatorial jet could be caused by the 5-day and 7-day waves in the lower cloud layer. Akatsuki IR2 observations revealed that the equatorial jet exists in the lower to middle cloud layer and varies temporally (Horinouchi et al., 2017). The present result could explain the mechanism of the equatorial jet and its temporal variation.