General circulation driven by baroclinic forcing due to cloud layer heating: Significance of planetary rotation and polar eddy heat transport

Abstract

A high significance of planetary rotation and poleward eddy heat fluxes is determined for general circulation driven by baroclinic forcing due to cloud layer heating. In a high-resolution simplified Venus general circulation model, a planetary-scale mixed Rossby-gravity wave with meridional winds across the poles produces strong poleward heat flux and indirect circulation. This strong poleward heat transport induces downward momentum transport of indirect cells in the regions of weak high-latitude jets. It also reduces the meridional temperature gradient and vertical shear of the high-latitude jets in accordance with the thermal wind relation below the cloud layer. In contrast, strong equatorial superrotation and midlatitude jets form in the cloud layer in the absence of polar indirect cells in an experiment involving Titan's rotation. Both the strong midlatitude jet and meridional temperature gradient are maintained in the situation that eddy horizontal heat fluxes are weak. The presence or absence of strong poleward eddy heat flux is one of the important factors determining the slow or fast superrotation state in the cloud layer through the downward angular momentum transport and the thermal wind relation. For fast Earth rotation, a weak global-scale Hadley circulation of the low-density upper atmosphere maintains equatorial superrotation and midlatitude jets above the cloud layer, whereas multiple meridional circulations suppress the zonal wind speed below the cloud layer.

Effects of Polar Indirect Circulation on Superrotation and Multiple Equilibrium in Long‐Term AGCM Experiments With an Idealized Venus‐Like Forcing: Sensitivity to Horizontal Resolution and Initial Condition

“…Figure shows the zonal mean horizontal eddy heat fluxes and their spectra at the level of maximum flux. The structures of the poleward eddy heat fluxes are almost the same among the T42 and T63 experiments of this study and the T106 experiment of Yamamoto and Takahashi (). The eddy heat flux is strong in the lower part of the high‐latitude jet in the experiments of T42 and higher, whereas it is weak in the T21 experiment.…”

“…In contrast, an equatorial zonal wind has a latitudinally constant speed of ~40 m s −1 around the region of thermal forcing with the largest meridional temperature contrast T 1 for T21. The mass stream functions are almost the same among T42, T63, and T106 (Figures b and c of this study and Figure 3a of Yamamoto & Takahashi, ). Different from the low‐resolution experiment of T21, polar indirect circulation cells are formed below the altitude of 10 4 Pa (at altitudes of >10 4 Pa).…”

“…The high S reflects a relatively high zonal wind speed in the lower atmosphere at~10 6 Pa in the high-resolution experiments (Figures 3a and 3b). Because the results of the T63 experiment, shown later in this paper, are very similar to those of the T106 experiment investigated in Yamamoto and Takahashi (2016), the results of the T21, T42, and T63 experiments are the focus of this article.…”

“…The longitudinal mean zonal flows and meridional circulations in the T21, T42, and T63 experiments at 400 Venus solar days are shown in Figures a–c and those in the T106 experiment can be seen in Figure 3a of Yamamoto and Takahashi (). Here 400 Venus solar days equal 46,800 Earth days (i.e., a Venus solar day is 117 Earth days).…”

“…Compared with the low‐resolution experiment, the strong equatorward eddy momentum fluxes emitted from the high‐latitude jets are located at lower altitudes in the high‐resolution experiments, because the high‐latitude jets extend toward the lower altitudes. The equatorward momentum flux is produced by quasistationary Rossby‐type waves (Yamamoto & Takahashi, ). The magnitudes of the E‐P fluxes are almost the same as the eddy momentum fluxes because the horizontal momentum fluxes are relatively large in comparison with the terms of the horizontal eddy heat fluxes.…”