Oceanic Geostrophic Turbulence Inferred From Vertical Structure Observations

人们认识到，海洋中充满了各种尺度的涡流，这一认识始于通常所说的中尺度涡流的发现：水平尺度在几十到大约100公里之间的涡流。这些涡旋是地转的(一种来自密度梯度的气压梯度力与地球自转的科里奥利效应相平衡的状态)，与龙卷风这样的气象涡旋形成鲜明对比，后者明显是非地转的。因为它们是海洋运动最具能量的特征，所以了解这些地转涡是如何工作的，对于理解和预测海洋中的环流、性质分布和生物量是关键。测试地转湍流理论将刺激新的工作，以更好地了解过去的海洋状态并预测它们的未来。半个世纪前，人们在理论上提出了地转湍流所隐含的能量波数谱的依赖规律。它们分别是将能量传递到较大尺度的逆级联和将拟能(旋转运动的能量)传递到较短尺度的正向级联。虽然在对流层的风观测光谱中已经确定了涡旋级联的存在，但在海洋中支持这种级联的证据很少，而且相互矛盾。最近对百慕大大西洋时间序列(BATS)站点海流和垂直位移的深度结构的观测似乎证实了对波数谱的逆能量和正向拟能级联部分的标度预测。这些初步观测将通过采样四个具有不同涡旋能量和纬度制度的新站点来扩展和概括。该项目将培训一名研究生使用新的滑翔机技术，并加入正在接受使用自主平台进行海洋研究的年轻科学家队伍的培训。在百慕大大西洋时间序列(BATS)站点的初步观测显示，在光谱的较陡峭部分，势能比动能高出一个数量级以上，这意味着涡旋拉伸在涡旋涡度波动中占主导地位。几周尺度上的时间变化表明，两个斜率依赖关系之间的转换波数变化为垂直波数的反立方根，这几乎保持了拟能，而与涡动能量无关。该项目的目标是测试：1)地转湍流波数谱的能量和辐合传输部分是否是海洋涡旋的一般特征；2)在海洋辐合级联波数范围内势能始终超过动能；以及3)地转涡的辐合自限于适度的Rossby数。为了实现这些目标，将在高能量的西北大西洋和东北太平洋阿拉斯加湾的涡流沙漠进行明显不同的涡流能量和纬度区域的重复调查任务。特派团将依靠租用的小船，或在一种情况下，依靠辅助使用大型船只。每个地点的方法是在一年内每三周左右沿着一条共同的轨道，从表面到底部调查大约100公里乘100公里的海洋区域。通过比较BATS和四个新站点的估计，至少可以创建一个关于地转涡逆能量和正向拟能级联的初步气候学。这些计算将为新的海洋理论和数值模型提供丰富的目标。

Appreciation that the ocean is full of eddies on a vast array of scales began with the discovery of what are commonly referred to as mesoscale eddies: those with horizontal scales of order tens to about 100 km. These eddies are geostrophic (a state where the pressure gradient force from density gradients is in balance with the Coriolis effect from the rotation of the earth) in contrast to meteorological eddies like tornadoes which are significantly ageostrophic. Because they are the most energetic features of oceanic motion, understanding how these geostrophic eddies work is key to understanding and predicting circulation, the distribution of properties, and biomass in the ocean. Testing theories of geostrophic turbulence will stimulate new work to better understand oceanic states in the past and to predict their future. Laws for the dependence of energy wavenumber spectra implied by geostrophic turbulence were proposed on theoretical grounds half a century ago. These distinguished between an inverse cascade transferring energy to larger scale and a forward cascade transferring enstrophy (the energy of rotating motions) to shorter scale. While the existence of an enstrophy cascade has been identified in spectra of trophospheric wind observations, evidence supporting such a cascade in the ocean is scant and contradictory. Recent observations of the depth structure of current and vertical displacement at the Bermuda Atlantic Time Series (BATS) site seem to confirm the scaling predictions for both the inverse energy and forward enstrophy cascade portions of the wavenumber spectrum. These preliminary observations will be extended and generalized by sampling four new sites with distinct eddy energy and latitude regimes. This project will train a graduate student in the use of the new glider technology and join the cadre of young scientists being trained to use autonomous platforms for oceanographic research.The initial observations at the Bermuda Atlantic Time Series (BATS) site show potential energy exceeds kinetic energy by more than an order of magnitude in the steeper portion of the spectra, implying vortex stretching dominates relative vorticity in potential vorticity fluctuations in the enstrophy cascade. Temporal variations on scales of several weeks suggest that the transition wavenumber between the two slope dependences varies as the inverse cube root of vertical wavenumber which nearly preserves enstrophy regardless of eddy energy. The goal of this project is to test if: 1) energy and enstrophy transfer portions of the wavenumber spectrum of geostrophic turbulence are a general feature of ocean eddies; 2) potential energy consistently exceeds kinetic energy in the oceanic enstrophy cascade wavenumber range; and 3) enstrophy for geostrophic eddies is self-limited to modest Rossby number. To accomplish these goals, Deepglider repeat survey missions in distinctly different eddy energy and latitude regimes in both the highly energetic Northwest Atlantic and the eddy desert of the Gulf of Alaska in the Northeast Pacific will be conducted. Missions will rely on chartered small boats or, in one case, on ancillary use of a large vessel. The approach at each site is to survey from surface to bottom a roughly 100 km by 100 km region of ocean along a common track every three weeks or so for a year. By comparing estimates from BATS and four new sites, at least a preliminary climatology of the geostrophic eddy inverse energy and forward enstrophy cascades will be created. These calculations will provide rich targets for new theories and numerical models of the ocean.