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K.M. Sayanagi (Univ. Arizona, Department of Physics), A.P. Showman (Univ. Arizona, Lunar and Planetary Lab), E.R. Kursinski (Univ. Arizona, Inst. Atmospheric Physics)
Voyager observations showed that Saturn's equatorial jet blew eastward at 450 m/s in 1980-81. However, subsequent Earth observations between 1996-2002 showed the equatorial jet speed to be approximately 350 m/s (Sanchez-Lavega et al., 2003). Sanchez-Lavega et al. interpreted this difference to be a true slowdown, but uncertainties in cloud altitudes combined with Cassini CIRS observations of vertical wind shear raise questions about whether the difference represents a real slowdown or simply changes in the altitudes of the observed clouds. A major event that could have influenced the winds occurred in 1990, when a huge convective storm, called a Great White Spot (GWS), erupted in the equatorial region of Saturn (Sanchez-Lavega, 1994). Here, we test the hypothesis that a GWS-like convective event can cause a slowdown of Saturn's equatorial jet like that observed. Mechanisms for slowing the winds include (1) potential vorticity mixing caused by such a storm, which would transport eastward momentum to higher latitudes, (2) momentum transport away from the jet by storm-induced atmospheric waves and (3) direct vertical mixing with a hypothetical source of deep, slow-moving air. We present order-of-magnitude estimates and fully nonlinear numerical simulations of the atmospheric flow using the EPIC atmosphere model. We envision that GWS-like convective events transport moist air from the deep troposphere to the neutrally buoyant level near the tropopause. Accordingly, we added localized mass-pulses to the equatorial upper troposphere to represent the effects of a GWS in the EPIC model. Our estimates and preliminary simulations suggest that GWS-like events can only cause wind-speed variations exceeding 50 m/s when the storm mass is unrealistically large. To elucidate the dynamical mechanisms involved, we performed a range of simulations which showed that the background wind profiles and other parameters strongly influence the dynamical response to a GWS. We will summarize these simulations in our presentation. This study is supported by NSF grant AST-0206269.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.