Shallow Water Simulations of the Latitudinal Drift of Saturn’s Anticyclonic Vortex (AV)

The AV is an anticyclonic vortex on Saturn that formed at approximately 40°N following the Great White Spot (GWS) of 2010 and has remained active since then. During its first year, the vortex migrated northward to 44°N and then returned to 43°N, where it has since exhibited latitudinal oscillations (Hueso et al., 2020). This migration is theoretically attributed to the β-drift effect, which is driven by vorticity gradients and controlled by the barotropic beta parameter (β_b), i.e., the sum of the relative and planetary vorticity gradients. For anticyclones, a poleward migration occurs when β_b < 0, while the vortex moves equatorward when β_b > 0. Analyzing the zonal wind profiles retrieved with Cassini during 2004—2009 (García-Melendo et al., 2011), Trammell et al. (2016) calculated the relative vorticity and presented a latitudinal profile. From it, they found that the AV formed at a region where β_b < 0 and identified 43°N as an equilibrium point (β_b = 0). In this study, we employed a Shallow Water model to simulate the positional evolution of the AV after its formation for 100—150 days, limited by numerical dissipation. We retrieved the northward drift rate and compared it to the observed 2011 migration rate (Hueso et al., 2020). The simulations provide further insight into the influence of the zonal wind profile shape on vortex drifting. Furthermore, the retrieved migration velocity serves as a validation benchmark for the numerical model.

References

García-Melendo, E., Pérez-Hoyos, S., Sánchez-Lavega, A., & Hueso, R. (2011). Saturn’s zonal wind profile in 2004–2009 from Cassini ISS images and its long-term variability. Icarus, 215(1), 62–74. https://doi.org/10.1016/j.icarus.2011.07.005

Hueso, R., Sánchez-Lavega, A., Rojas, J. F., Simon, A. A., Barry, T., del Río-Gaztelurrutia, T., Antuñano, A., Sayanagi, K. M., Delcroix, M., Fletcher, L. N., García-Melendo, E., Pérez-Hoyos, S., Blalock, J., Colas, F., Gómez-Forrellad, J. M., Gunnarson, J. L., Peach, D., & Wong, M. H. (2020). Saturn atmospheric dynamics one year after Cassini: Long-lived features and time variations in the drift of the Hexagon. Icarus, 336, 113429. https://doi.org/10.1016/j.icarus.2019.113429

Trammell, H. J., Li, L., Jiang, X., Pan, Y., Smith, M. A., Bering III, E. A., Hörst, S. M., Vasavada, A. R., Ingersoll, A. P., Janssen, M. A., West, R. A., Porco, C. C., Li, C., Simon, A. A., & Baines, K. H. (2016). Vortices in Saturn's Northern Hemisphere (2008–2015) observed by Cassini ISS. Journal of Geophysical Research: Planets, 121(9), 1814–1826. https://doi.org/10.1002/2016JE005122