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

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.