A Complex Storm System in Saturn's North Polar Atmosphere in 2018

Saturn’s convective storms usually fall in two categories. One consists of mid-sized storms ~ 2,000 km wide, appearing as irregular bright cloud systems that evolve rapidly, on scales of a few days. The other includes the exceptional Great White Spots (GWS), planetary-scale giant storms that disturb a full latitude band, and which have been observed only seven times. Here we report a new intermediate type, observed in 2018 in the North Polar Region. Four large storms (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters, and leading to zonal disturbances affecting a full latitude band ~ 8,000 km wide, during at least 8 months. Dynamical simulations indicate that each storm required energies ~ 100 times smaller than those necessary for a GWS. This event occurred at about the same latitude and season as the GWS in 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial GWSs.