Ground-based observations of the long-term evolution and death of Saturn’s 2010 Great White Spot

We report on the long-term evolution of Saturn’s sixth Great White Spot (GWS) event that initiated at northern mid-latitudes of the planet on December 5th, 2010 (Fletcher, L. et al. [2011]. Science 332, 1413–1417; Sánchez-Lavega, A. et al. [2011]. Nature 475, 71–74; Fischer, G. et al. [2011]. Nature 475, 75–77). We find from ground-based observations that the GWS formed a planetary-scale disturbance that encircled the planet in 50 days, covering the latitude band between 24.6° and 44.8°N (planetographic) or about 22,000 km in meridional extent and 280,000 km in full zonal circumference length. The head of the GWS was located at an averaged latitude of 40.8 ± 1°N in the peak of a westward jet and showed a mean linear drift in System III longitude of 2.793 deg/day, equivalent to a mean zonal velocity of u = −27.9 m s−1, with maximum speed fluctuations around this mean of −5.3 to +2.7 m s−1. The difference between the undisturbed jet peak velocity and the GWS head was Δu = −12 m s−1. Assuming the GWS has a deep origin at the water cloud a vertical extent of Δz ∼ 250 km is expected and we can derive a vertical shear of the zonal winds ∂u/∂z ∼ 5 × 10−5 s−1. The cloud morphology of the disturbance was sculpted by the winds at this latitude and their latitudinal shears, showing several distinct features: (1) A long-lived Dark Spot (DS, anticyclone vortex) placed at 41.5 ± 1.1°N with a speed u = −11.0 ± 0.1 m s−1 and a size of 7800 km (East–West) per 6000 km (North–South). (2) Two branches of zonally periodic features at both sides of the jet peak, a northern branch at 44.4°N (anticyclonic) and a southern branch at 32°N (cyclonic), with wavelengths in the range ∼ 5000–14,000 km. Precise long-term cloud tracking of disturbance features shows that they moved with speeds close to those of the prevailing winds, although differences up to ∼−45 m s−1 were measured, probably due to wave motion or to real wind changes produced by momentum transfers induced by the disturbance. Vortex DS and the GWS head encountered between the 15th and 19th of June 2011, disappearing within the resolution of our images. We present and discuss two simple hypothesis to explain the nature of this phenomenon. Taking into account our results together with previous historical events, we summarize the mysteries of GWS phenomena: seasonal forcing, occurrence at preferred latitudes only in the Northern hemisphere, no relation of the outbreaks with the wind profile structure and the existence of a continuous deep moist convection source to feed the disturbance.