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

Abstract

AbstractThe 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.