Asymmetrical meridional expansion of bright clouds from Saturn's 2010 great white storm

Abstract

Saturn's recurring great white storms play an important role in modifying its atmosphere. In 2010, such a storm with clouds encircling the planet occurred in the northern hemisphere. An interesting phenomenon of this storm is that the associated bright clouds expanded asymmetrically with respect to latitude, such that the southern boundary of the bright clouds moved ~2.7 times as far as the northern boundary during an 8-month period. Based on the wind and temperature fields retrieved from the Cassini visible and infrared observations, we explore the mechanism behind this asymmetrical expansion. Our analysis shows that the northern edge, which quickly stopped moving, coincides with the largest meridional gradient of the quasi-geostrophic potential vorticity (PV) in the region of interest, which is coincident with the strongest jet in the region, suggesting this forms an effective barrier to meridional transport, much like a polar vortex. In contrast, the storm's southern edge, which kept moving, passed through weaker PV gradients and jets. For the threshold value of the meridional gradient of PV needed to form an effective barrier to meridional transport in Saturn's mid-latitude upper-troposphere, we estimate lower and upper bounds of ~2.1 × 10−11 m−1 s−1 and ~3.6 × 10−11 m−1 s−1.