由密度分层湍流对流维持的巨行星中的差异旋转
Differential rotation in giant planets maintained by density-stratified turbulent convection
摘要：
巨行星表面的纬向风随纬度而变化。例如，木星和土星就有几条相对于其全球磁场的角速度交替的向东（逆行）和向西（逆行）喷流带。这些表面风廓线可能是这些行星内部液体深处角速度的深度和纬度变化的表现。二十年前，有人提出，这种差异旋转可以通过对流流体柱的涡旋拉伸来维持，这些流体柱从北半球表面到南半球表面跨越这些行星的内部。这一经典机制解释了实验室实验和计算机模拟中所见的差异旋转，最好的情况是旋转的恒定密度流体球体中的弱湍流对流。然而，这些实验和模拟对于巨行星的密度分层强湍流内部是很差的近似。经典地转理论预测的这些行星的细长全球对流柱可能不会发展。在这里，我们提出了一个更强大的机制，以维持差异旋转半径的基础上，当地产生的涡量上升的羽流扩大和下沉的羽流收缩。我们的高分辨率二维计算机模拟演示了这种机制如何在巨型行星的赤道区域维持逆时针或逆行的表面风，这取决于密度尺度高度如何随深度变化。
英文摘要：
The zonal winds on the surfaces of giant planets vary with latitude. Jupiter and Saturn, for example, have several bands of alternating eastward (prograde) and westward (retrograde) jets relative to the angular velocity of their global magnetic fields. These surface wind profiles are likely manifestations of the variations in depth and latitude of angular velocity deep within the liquid interiors of these planets. Two decades ago it was proposed that this differential rotation could be maintained by vortex stretching of convective fluid columns that span the interiors of these planets from the northern hemisphere surface to the southern hemisphere surface. This now classic mechanism explains the differential rotation seen in laboratory experiments and in computer simulations of, at best, weakly turbulent convection in rotating constant-density fluid spheres. However, these experiments and simulations are poor approximations for the density-stratified strongly-turbulent interiors of giant planets. The long thin global convective columns predicted by the classic geostrophic theory for these planets would likely not develop. Here we propose a much more robust mechanism for maintaining differential rotation in radius based on the local generation of vorticity as rising plumes expand and sinking plumes contract. Our high-resolution two-dimensional computer simulations demonstrate how this mechanism could maintain either prograde or retrograde surface winds in the equatorial region of a giant planet depending on how the density scale height varies with depth.
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