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The origin and evolution of Saturn's 2011-2012 stratospheric vortex

The origin and evolution of Saturn's 2011-2012 stratospheric vortex

Publication date: 01 September 2012

Authors: Fletcher, L.N., et al.

Journal: Icarus
Volume: 221
Issue: 2, November-December 2012
Page: 560-586
Year: 2012

Copyright: Elsevier Inc.

Available online 31 August 2012

The planet-encircling springtime storm in Saturn's troposphere (December 2010-July 2011) produced dramatic perturbations to stratospheric temperatures, winds and composition at mbar pressures that persisted long after the tropospheric disturbance had abated. Thermal infrared (IR) spectroscopy from the Cassini Composite Infrared Spectrometer (CIRS), supported by ground-based IR imaging from the VISIR instrument on the Very Large Telescope and the MIRSI instrument on NASA's IRTF, is used to track the evolution of a large, hot stratospheric anticyclone between January 2011 and March 2012. The evolutionary sequence can be divided into three phases: (I) the formation and intensification of two distinct warm airmasses near 0.5 mbar between 25 and 35°N (B1 and B2) between January-April 2011, moving westward with different zonal velocities, B1 residing directly above the convective tropospheric storm head; (II) the merging of the warm airmasses to form the large single 'stratospheric beacon' near 40°N (B0) between April and June 2011, disassociated from the storm head and at a higher pressure (2 mbar) than the original beacons, a downward shift of 1.4 scale heights (approximately 85 km) post-merger; and (III) the mature phase characterised by slow cooling (0.11 ± 0.01 K/day) and longitudinal shrinkage of the anticyclone since July 2011. Peak temperatures of 221.6 ± 1.4 K at 2 mbar were measured on May 5th 2011 immediately after the merger, some 80 K warmer than the quiescent surroundings. From July 2011 to the time of writing, B0 remained as a long-lived stable stratospheric phenomenon at 2 mbar, moving west with a near-constant velocity of 2.70 ± 0.04 deg/day (-24.5 ± 0.4 m/s at 40°N relative to System III longitudes). No perturbations to visible clouds and hazes were detected during this period. [Abstract abbreviated due to character limitations.]

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