土星极区的位涡：季节性和不稳定性
Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities
摘要：
我们分析了土星极地地区的位涡，因为它是一个基本的动态示踪剂，使我们能够提高我们对这些地区的动态及其季节性变化的理解。特别是，我们提出了纬向平均准地转位涡图之间的68°行星纬度和两极之间的高度在500毫巴和1毫巴为三个不同的时代：㈠ 2013年6月（北半球初夏）对于北极地区，㈡ 2008年12月（北半球冬季后期）和（三）2006年10月（南方的夏天）对于南方来说，使用从卡西尼复合红外光谱仪（CIRS）检索的温度剖面计算数据和风廓线从卡西尼号的成像科学子系统（ISS）获得。结果表明，准地转位涡图是非常相似的，在所有的研究时期，显示在北部和南部的负涡度，表明在所有纬度的科里奥利参数2 $Ωsinϕ$的优势，除了极附近。准地转位涡的纬向梯度表明，在78°N的六边形两侧、73.9°S的急流两侧和两极急流的赤道一侧，动力不稳定主要由正压项引起。两个半球之间的位涡梯度没有差异，这可以解释为什么六边形在北部而不是南部形成。尽管大气温度随时间发生显著变化，但没有发现位涡及其纬向梯度的季节变化。
英文摘要：
We analyse the potential vorticity of Saturn's polar regions, as it is a fundamental dynamical tracer that enables us to improve our understanding of the dynamics of these regions and their seasonal variability. In particular, we present zonally averaged quasi-geostrophic potential vorticity maps between 68° planetographic latitude and the poles at altitudes between 500 mbar and 1mbar for three different epochs: (i) June 2013 (early northern summer) for the north polar region, (ii) December 2008 (late northern winter) for both polar regions and (iii) October 2006 (southern summer) for the south, computed using temperature profiles retrieved from Cassini Composite Infrared Spectrometer (CIRS) data and wind profiles obtained from Cassini's Imaging Science Subsystem (ISS). The results show that quasi-geostrophic potential vorticity maps are very similar at all the studied epochs, showing positive vorticities at the north and negative at the south, indicative of the dominance of the Coriolis parameter 2$Ωsinϕ$ at all latitudes, except near the pole. The meridional gradients of the quasi-geostrophic potential vorticity show that dynamical instabilities, mainly due to the barotropic term, could develop at the flanks of the Hexagon at 78°N, the jet at 73.9°S and on the equatorward flank of both polar jets. There are no differences in potential vorticity gradients between the two hemispheres that could explain why a hexagon forms in the north and not in the south. No seasonal variability of the potential vorticity and its meridional gradient has been found, despite significant changes in the atmospheric temperatures over time.
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