The Cycles and Dynamical Properties of Convective Outbreaks in Jupiter’s Highest Speed Jet from a 55-Year Study

Jupiter’s zonal wind system has its most intense jet centered at latitude 23.5°N where peak velocities reach 140 to 180 ms-1. One of the most spectacular planetary-scale disturbances takes place in its interior, changing a bright whitish band of clouds into a dark belt, known as the North Temperate Belt(NTB), and the whole phenomenon, as the NTB Disturbance (NTBD). The disturbance begins with the rapid eruption of 1 to 3 “plumes”, bright clouds of convective origin that, after a rapid initial expansion, generate a turbulent wake formed by a turmoil of eddies and filaments that encircles the planet. We have studied the reported cases in the modern literature, starting in 1970 and ending with the last two most recent NTBD eruptions in August-September 2020 and January-February 2025. Our analysis shows the existence of a cycle between plume outbreaks with a mean period of 4.64 years (with a range 3.84 to 4.87 years) for the 10 outbreaks observed between years 1970 and 2025. Interestingly, there was a large period of ∼17 years (from the end of 1990 to early 2007) without eruptions (the “NTBD desert”). During this period, the NTB was a dark belt, populated with large and long-lived anticyclones.

The average velocity of the 17 plumes studied in the 10 eruptions is in the range of 157 - 176 m/ always higher than that of the peak of the jet at the time of their development. Their average latitude was between 22.2ºN and 23.9ºN (north and south of the jet peak). A detailed study of the motions of the 2020 and 2025 plumes using ground-based and Hubble Space Telescope images at visible wavelengths shows that they underwent intense meridional migrations during the first 2-3 days of their life, with speeds of about 7 m/s directed toward the jet's peak. Simultaneously, the plumes accelerated zonally in these first few days, with speed increasing by 10 to 15 m/s with respect to their mean motion. When 2 or 3 plumes occur in the same NTBD, the spatial distance between them is very variable (from 17,000 to 187,000 km) as well as the time separation between plume outbreaks (0.4 to 16 days), suggesting this is an stochastic process. The cyclic behaviour of the NTBD, the 4.6-year period, is probably regulated by the thermodynamics in the water clouds where moist convection initiates (perhaps as for Saturn major storms), favoured or not by the vertical structure of the jet velocity (wind shear).