The seasonal evolution of Saturn ’ s polar atmospheric temperatures and hydrocarbon composition is derived from a decade of Cassini Composite Infrared Spectrometer ( CIRS ) 7-16 \mu m thermal infrared spectroscopy .
We construct a near-continuous record of atmospheric variability poleward of 60 ^ { \circ } from northern winter/southern summer ( 2004 , L _ { s } = 293 ^ { \circ } ) through the equinox ( 2009 , L _ { s } = 0 ^ { \circ } ) to northern spring/southern autumn ( 2014 , L _ { s } = 56 ^ { \circ } ) .
The hot tropospheric polar cyclones that are entrained by prograde jets within 2-3 ^ { \circ } of each pole , and the hexagonal shape of the north polar belt , are both persistent features throughout the decade of observations .
The hexagon vertices rotated westward by \approx 30 ^ { \circ } longitude between March 2007 and April 2013 , confirming that they are not stationary in the Voyager-defined System III longitude system as previously thought .
Tropospheric temperature contrasts between the cool polar zones ( near 80-85 ^ { \circ } ) and warm polar belts ( near 75-80 ^ { \circ } ) have varied in both hemispheres , resulting in changes to the vertical windshear on the zonal jets in the upper troposphere and lower stratosphere .
The extended region of south polar stratospheric emission has cooled dramatically poleward of the sharp temperature gradient near 75 ^ { \circ } S ( by approximately -5 K/yr ) , coinciding with a depletion in the abundances of acetylene ( 0.030 \pm 0.005 ppm/yr ) and ethane ( 0.35 \pm 0.1 ppm/yr ) , and suggestive of stratospheric upwelling with vertical wind speeds of w \approx + 0.1 mm/s .
The upwelling appears most intense within 5 ^ { \circ } latitude of the south pole .
This is mirrored by a general warming of the northern polar stratosphere ( +5 K/yr ) and an enhancement in acetylene ( 0.030 \pm 0.003 ppm/yr ) and ethane ( 0.45 \pm 0.1 ppm/yr ) abundances that appears to be most intense poleward of 75 ^ { \circ } N , suggesting subsidence at w \approx - 0.15 mm/s .
However , the sharp gradient in stratospheric emission expected to form near 75 ^ { \circ } N by northern summer solstice ( 2017 , L _ { s } = 90 ^ { \circ } ) has not yet been observed , so we continue to await the development of a northern summer stratospheric vortex .
The peak stratospheric warming in the north occurs at lower pressure levels ( p < 1 mbar ) than the peak stratospheric cooling in the south ( p > 1 mbar ) .
Vertical motions are derived from both the temperature field ( using the measured rates of temperature change and the deviations from the expectations of radiative equilibrium models ) and hydrocarbon distributions ( solving the continuity equation ) .
Vertical velocities tend towards zero in the upper troposphere where seasonal temperature contrasts are smaller , except within the tropospheric polar cyclones where w \approx \pm 0.02 mm/s .
North polar minima in tropospheric and stratospheric temperatures were detected in 2008-2010 ( lagging one season , or 6-8 years , behind winter solstice ) ; south polar maxima appear to have occurred before the start of the Cassini observations ( 1-2 years after summer solstice ) , consistent with the expectations of radiative climate models .
The influence of dynamics implies that the coldest winter temperatures occur in the 75 - 80 ^ { \circ } region in the stratosphere , and in the cool polar zones in the troposphere , rather than at the poles themselves .
In addition to vertical motions , we propose that the UV-absorbent polar stratospheric aerosols entrained within Saturn ’ s vortices contribute significantly to the radiative budget at the poles , adding to the localised enhancement in the south polar cooling and north polar warming poleward of \pm 75 ^ { \circ } .