A complex storm system in Saturn’s north polar atmosphere in 2018

Saturn’s convective stormsusually 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 Great White Spots, planetary-scale giant stormsten times larger than the mid-sized ones, which disturb a full latitudeband, enduring several months, and have been observed only seven times since 1876. Here we report a new intermediate type, observed in 2018 in the north polar region. Four large stormswith east–west lengths ∼4,000–8,000 km (the first one lasting longer than 200 days) formed sequentially in close latitudes, experiencing mutual encounters and leading to zonal disturbances affecting a full latitudeband ∼8,000 km wide, during at least eight months. Dynamical simulations indicate that each stormrequired energies around ten times larger than mid-sized stormsbut ∼100 times smaller than those necessary for a Great White Spot. This event occurred at about the same latitudeand season as the Great White Spotin 1960, in close correspondence with the cycle of approximately 60 years hypothesized for equatorial Great White Spots. A series of four stormsappeared on Saturn’s northern polar region in 2018, unusually close to each other in space and time. By their dimension and the energy needed to form them, they appear to be a hitherto unobserved kind of stormat Saturn, intermediate between the regional- and the global-sized ones.