Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities
Potential Vorticity of Saturn's Polar Regions: Seasonality and Instabilities
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DOI：
10.1029/2018je005764
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发表时间：
2019-01-01
影响因子：
4.8
通讯作者：
Fletcher, Leigh N.
中科院分区：
文献类型：
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作者：
Antunano, Arrate;del Rio-Gaztelurrutia, Teresa;Fletcher, Leigh N.
我们分析了土星极地地区的潜在涡度，因为它是一种基本的动力学示踪剂，使我们能够提高我们对这些地区的动力学及其季节变化的理解。特别地，我们提出了纬68度行星纬度和海拔500 - 1mbar两极之间三个不同时期的纬向平均准地转位涡图：(i) 2013年6月（北方初夏）为北极地区，（ii） 2008年12月（北方冬末）为两极地区，（iii） 2006年10月（南方夏季）为南方地区，计算使用卡西尼号复合红外光谱仪数据检索的温度曲线和卡西尼号成像科学子系统获得的风廓线。结果表明，准地转位涡图在所有研究时期都非常相似，北为正，南为负，表明科里奥利参数2sin在除极附近外的所有纬度都占主导地位。准地转位涡的经向梯度表明，在北纬78度的六边形侧翼、南纬73.9度的急流和两极急流的赤道侧翼，主要由正压项引起的动力不稳定可能发生。两个半球之间的潜在涡度梯度没有差异，这可以解释为什么六边形在北方而不是在南方形成。尽管大气温度随时间有显著变化，但势涡度及其经向梯度没有季节变化。土星极地地区的云层形态多种多样，其中一些显然是土星独有的，比如环绕北极的六边形波。此外，土星的极地地区在29.5年的周期（一个土星太阳轨道）中经历强烈的季节变化，这影响了这些地区的大气动力学。在这项研究中，我们通过计算大气运动和气象学的基本示踪剂，即位涡量，来分析极地大气动力学及其时间变率的行为。为此，我们使用了卡西尼成像科学子系统仪器拍摄的2006年10月、2008年12月和2013年6月的风廓线，以及卡西尼复合红外光谱仪获取的温度。我们的结果表明，土星的极地大气动力学在两个半球表现出相似的行为，没有任何显著的季节变化。我们还表明，在所有研究的时期，动力不稳定性（正压或斜压特征）都可能在六边形急流的侧翼、南部的等效极地急流和强极地气旋的位置发展。我们的结果并没有暗示为什么六边形只存在于北半球。
We analyze 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 degrees planetographic latitude and the poles at altitudes between 500 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 data and wind profiles obtained from Cassini's Imaging Science Subsystem. 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 2sin 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 degrees N, the jet at 73.9 degrees 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.Plain Language Summary Saturn's polar regions are characterized by a wide variety of cloud morphologies, some of them apparently unique to Saturn, such as the Hexagon wave encircling the north pole. Additionally, Saturn's polar regions undergo strong seasonal variations of sunlight over a 29.5-year cycle (one Saturnian solar orbit), which affect the atmospheric dynamics of these regions. In this study, we analyze the behavior of the polar atmospheric dynamics and its temporal variability, by computing a fundamental tracer of atmospheric motions and meteorology, known as potential vorticity. For this purpose, we use wind profiles from October 2006, December 2008, and June 2013, measured from images captured by the Cassini Imaging Science Subsystem Instrument, and temperatures retrieved from the Cassini Composite and Infrared Spectrometer. Our results show that Saturn's polar atmospheric dynamics displays a similar behavior at both hemispheres, without any significant seasonal variability. We also show that at all studied epochs dynamical instabilities (of barotropic or baroclinic character) could develop at the flanks of the Hexagon jet, at the equivalent polar jet in the south, and at the location of the strong polar cyclones. Our results do not show hints of why a Hexagon is only present in the northern hemisphere.