Global climate modeling of Saturn's atmosphere. Part IV: Stratospheric equatorial oscillation

The Composite InfraRed Spectrometer (CIRS) on board Cassini revealed an equatorial oscillation of strato-spheric temperature, reminiscent of the Earth's Quasi-Biennial Oscillation (QBO), as well as anomalously high temperatures under Saturn's rings. To better understand these predominant features of Saturn's atmospheric circulation in the stratosphere, we have extended to the upper stratosphere the DYNAMICO-Saturn global climate model (GCM), already used in a previous publication to study the tropospheric dynamics, jets formation and planetary-scale waves activity. Firstly, we study the higher model top impact on the tropospheric zonal jets and kinetic energy distribution. Raising the model top prevents energy and enstrophy accumulation at tropopause levels. The reference GCM simulation with 1/2 degrees latitude/longitude resolution and a raised model top exhibits a QBO-like oscillation produced by resolved planetary-scale waves. However, the period is more irregular and the downward propagation faster than observations. Furthermore, compared to the CIRS temperature retrievals, the modeled QBO-like oscillation underestimates by half both the amplitude of temperature anomalies at the equator and the vertical characteristic length of this equatorial oscillation. This QBO-like oscillation is mainly driven by westward-propagating waves; a significant lack of eastward waveforcing explains a fluctuating eastward phase of the QBO-like oscillation. We also show that the seasonal cycle of Saturn is a key parameter of the establishment and the regularity of the equatorial oscillation. At 20 degrees N and 20 degrees S latitudes, the DYNAMICO-Saturn GCM exhibits several strong seasonal eastward jets, alternatively in the northern and southern hemisphere. These jets are correlated with the rings' shadowing. Using a GCM simulation without rings' shadowing, we show its impact on Saturn's stratospheric dynamics. Both residual-mean circulation and eddy forcing are impacted by rings' shadowing. In particular, the QBO-like oscillation is weakened by an increased drag caused by those two changes associated with rings' shadowing.

Automated summary

The study extended the DYNAMICO-Saturn global climate model (GCM) to the upper stratosphere to investigate the equatorial oscillation phenomenon similar to the Earth's Quasi-Biennial Oscillation (QBO) and the seasonal eastward jets correlated with Saturn's rings. The impact of rings' shadowing on Saturn's stratospheric dynamics was also analyzed, showing a weakened QBO-like oscillation due to increased drag caused by changes associated with rings' shadowing.