Formation and Quasi‐Periodic Variation of Equatorial Jet Caused by Planetary‐Scale Waves in the Venusian Lower Cloud Layer

Abstract

The equatorial jet in the Venusian lower cloud layer (47–55 km altitudes) and its quasi‐periodic variation are found in a general circulation model (GCM). The equatorial jet is produced by the 5.8‐day wave and destroyed by the 7‐day wave, and its quasi‐periodic variation with a timescale of about 280 days is caused by the alternating development of these waves in the GCM. The 5.8‐day wave, which is excited by the Rossby‐Kelvin instability, produces the equatorward angular momentum (AM) flux, and accelerates the zonal‐mean zonal wind in the equatorial region. The 7‐day wave, newly found in the present study, is a planetary‐scale wave antisymmetric about the equator, although it has not yet been observed. It is excited by the coupling among the lower‐altitude equatorial Rossby mode, the mid‐latitude Rossby mode, and the high‐latitude Rossby mode. In the growth period, the 7‐day wave produces the poleward (equatorward) AM flux around the equatorward (poleward) critical latitude. As a result, the zonal‐mean zonal wind is decelerated (accelerated) in the equatorial region and high latitudes (mid‐latitudes). In the regrowth period, the lower‐altitude equatorial Rossby mode disappears due to the disappearance of the equatorial jet, but the high‐latitude Rossby mode is still enhanced by the coupling with the mid‐latitude Rossby mode. These results could provide a possible explanation of the equatorial jet inferred from the recent Akatsuki observation, although it should be investigated by further observations.

Planetary‐Scale Wave Activity in Venus Cloud Layer Simulated by the Venus PCM

“…Lee et al, 2007;Mendonça & Read, 2016;Sugimoto et al, 2014b). The recent GCM work of AFES-Venus (except for special designations, all the AFES-Venus mentioned in this article are in reference to Takagi et al (2022Takagi et al ( , 2023) successfully reproduced Rossby and Kelvin waves that are similar to observations and simulated the lower-cloud equatorial jet and cloudtop mid-latitude jets of SR. In the current work, by comparing Venus GCMs, primarily with AFES-Venus, we aim to elucidate the general mechanisms by which the Rossby waves, Kelvin waves, thermal tides, and MMC maintain the SR AM balance.…”

“…The KE of the wave is divided into the weak lower equatorial Rossby mode at 48-50 km from 20°S to 20°N, the weak higher equatorial Rossby mode at 56-63 km from 20°S to 20°N, and the antisymmetric high-latitude Rossby mode at 48-65 km from 30°S/N to 90°S/N. Apart from the intricate, deep atmospheric waves below 40 km in high latitudes, the distribution pattern of KE for the 8.5-day wave in V-PCM is nearly similar to the 7-day wave in AFES-Venus (see Figure 3 in Takagi et al (2023)). At 48-50 km, the lower equatorial Rossby mode has a geopotential height deviation phase that is in phase with the meridional wind, a temperature deviation phase that is in phase with the meridional wind, and a 2 km vertical wavelength that is due to the critical level.…”

“…This intensifies instability, thereby strengthening the wave. Takagi et al (2023) emphasized the emergence of a lower-cloud equatorial jet due to Rossby-Kelvin instability in AFES-Venus. As the 5.8-day wave grows, there is a noticeable rise in equatorward AM flux at the lower cloud.…”

“…Recently, quantitatively meridional AM transport by thermal tides and transient waves in the cloud top was evaluated, and vertical AM transport by thermal tides and MMC was estimated with a simplified model (Horinouchi et al, 2020). Studies by Venus General Circulation Models (GCMs) indicate that the AM budget of the lower-cloud equatorial jet and cloud-top mid-latitude jets of SR may be significantly influenced by the planetary-scale waves (Takagi et al, 2022(Takagi et al, , 2023.…”

Analyzing the Instabilities in the Venus Atmosphere Using Bred Vectors

“…The figures reveal that the wave amplitudes are symmetric about the equator. According to previous model studies, 4-day and 5-day waves at the Venus cloud top are symmetric about the equator (Takagi et al, 2022), while the 7-day wave is antisymmetric about the equator (Takagi et al, 2023). In our study, we did not decompose the waves with different wave numbers.…”