Saturn's Stratospheric Response to the 2010 Great White Spot

On December 5, 2010, a large convective storm erupted near 40°N planetographic latitude in Saturn's troposphere. The bright, white clouds produced by the storm then expanded eastward from the storm head, completely encircling Saturn the latitude band between 25°N and 45°N within two months (Sánchez-Lavega et al., 2012, Icarus, 220, 561–576). Observations of electrical activity by the Cassini Radio and Plasma Wave Spectrometer indicated that lighting activity in the storm was an order of magnitude more frequent than in typical Saturnian storms, with a total power output comparable to Saturn's total emitted power (Fischer et al., 2011, Nature, 475, 75–77). Storms of this magnitude, known as Great White Spots (GWS), are rare on Saturn, occurring approximately every 30 Earth years (one Saturnian year), always in the northern hemisphere and always in northern spring. The convective activity lasted for over six months, ending in mid-June 2011 when the convective plume encountered an anticyclonic vortex, which had originally formed to the east of the plume in late December 2010.

Thermal infrared observations taken in January 2011 from the Very Large Telescope (VLT) and the Cassini Composite Infrared Spectrometer (CIRS) showed upper tropospheric temperature contrasts associated with the storm of roughly 5K, with warmer temperatures over the storm head, and colder temperatures associated with the anticyclonic vortex and the wake east of the storm. (Fletcher et al, 2011, Science, 332, 1413–1417). Surprisingly, the thermal observations showed even stronger temperature contrasts in the stratosphere, with a pair of stratospheric hot spots, known colloquially as “beacons”, located on either side of the tropospheric cold region, one of them directly over the tropospheric plume. The zonal temperature contrast was 16K, much larger than usual (<2K) for Saturn's stratosphere.

By March 2011, continuing observations from VLT and CIRS showed that temperatures in the beacons had reached 190K at 0.5 mbar, ~50K warmer than the ambient temperatures at 40°N. One of the beacons remained fixed over the tropospheric plume, drifting westward at 2.7 °/day, while the other drifted westward much more slowly (0.6°/day). At the beginning of May 2011, the two beacons merged, resulting in a single large beacon (80° in longitude) with a peak temperature of 220 K at 2 mbar, 80 K above the quiescent temperature, located about 2 scale heights lower in altitude than the original two beacons, and drifting westward at a speed intermediate between the original beacons, no longer at the location of the tropospheric plume. By July 2011, the single beacon had cooled back to ~190K and accelerated to a westward velocity of 2.7°/day which it has maintained into mid 2012 while slowly cooling and shrinking. By March 2012 the beacon had cooled to 170 K and shrunk to a longitudinal width of 30°. Estimates of the stratospheric wind fields from the thermal wind approximation show that the beacons are coherent anticyclonic vortices, with peak wind speeds of ~150 m/s before the merger, and in excess of 200 m/s after the merger.

Explanations for the origin of the beacons are still speculative. However, it is interesting to note that the latitude at which the storm occurred meets the necessary criteria for barotropic instability (Read et al., 2009, Planet. Space Sci. 57, 1682–1698), and is also the location of a waveguide where quasi-stationary planetary waves can propagate from the troposphere upward into the stratosphere (Achterberg and Flasar, 1996, Icarus, 119, 350–369). Thus it seems likely that the formation of the stratospheric vortices is related to planetary scale stratospheric waves forced by the strong convective plume in the troposphere.