Geomagnetic forecasts driven by thermal wind dynamics in the Earth's core
Geomagnetic forecasts driven by thermal wind dynamics in the Earth's core
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DOI：
10.1093/gji/ggv394
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发表时间：
2015-12-01
影响因子：
2.8
通讯作者：
Aubert, J.
中科院分区：
文献类型：
--
作者：
Aubert, J.
能够准确地预测地球磁场在十年到长期范围内的未来演变，这是一个基本的和实际的兴趣。这项工作使这样的预测相结合的地磁数据与地球一样的对流驱动的流体发电机的数值模型。基本的数据同化框架建立在逆地球发电机建模的最新进展基础上，逆地球发电机建模是一种根据磁场快照及其在地表的瞬时变化率估计地核内部动态结构的方法，并利用观测到的和隐藏的状态变量之间的线性关系和长期相关性。在这里，该方法进一步演变成一个单历元系综卡尔曼滤波器，以初始化在一个给定的历元兼容的观测和代表的隐藏量的估计中的不确定性的状态的系综。的合奏动力学，通过随后的数值积分的预测模型方程，被发现是由热风平衡或浮力，科里奥利力和压力梯度之间的平衡。由此产生的核心流体流动模式是一个准稳定的偏心环流组织在一列平行于地球的旋转轴，在平衡与纵向半球对流密度异常模式。流动为磁场提供感应，磁场也经历实际的扩散量。从过去世纪的数据预测的现在的磁场表明，合奏有一个平均保持良好的一致性与真实的地磁演化和一个可接受的传播以及代表预测误差，至少达到一个长期的范围。因此，地球发电机的可预测性似乎大大超过了以前的理论预期的基础上的混乱分歧的合奏成员。同化一般优于线性数学外推从30年的预测范围向前，在地球表面的误差在长期范围内的40%的改善。在过去的两个世纪中观察到的地磁轴向偶极子衰减预计将继续在下一个世纪以类似的速度，与进一步的损失1.1 +/- 0.3亩T到2115年。预计南大西洋地磁异常的焦点（或最小强度）将在下个世纪进入南太平洋地区，异常本身将进一步加深和扩大。到2065年，最小强度预计将在地球表面减少1.46 +/- 0.4 mu T，焦点将向西移动12.8 +/- 1.4度，并略有向北的分量。这相当于0.26 deg yr（-1）的漂移率，类似于过去四个世纪观察到的典型地磁向西漂移。同样的漂移率也预测到2115年，强度进一步下降（但更不确定）。
There exists a fundamental as well as practical interest in being able to accurately forecast the future evolution of Earth's magnetic field at decadal to secular ranges. This work enables such forecasts by combining geomagnetic data with an Earth-like numerical model of a convection-driven fluid dynamo. The underlying data assimilation framework builds on recent progress in inverse geodynamo modelling, a method which estimates an internal dynamic structure for Earth's core from a snapshot of the magnetic field and its instantaneous rate of change at the surface, and takes advantage of linear relationships and long-range correlations between observed and hidden state variables. Here the method is further evolved into a single-epoch ensemble Kalman filter, in order to initialise at a given epoch an ensemble of states compatible with the observations and representative of the uncertainties in the estimation of hidden quantities. The ensemble dynamics, obtained by subsequent numerical integration of the prognostic model equations, are found to be governed by a thermal wind balance or equilibrium between buoyancy forces, the Coriolis force and the pressure gradient. The resulting core fluid flow pattern is a quasi-steady eccentric gyre organised in a column parallel to Earth's rotation axis, in equilibrium with a longitudinal hemispheric convective density anomaly pattern. The flow provides induction for the magnetic field, which also undergoes a realistic amount of diffusion. Predictions of the present magnetic field from data taken within the past century show that the ensemble has an average retaining good consistency with the true geomagnetic evolution and an acceptable spread well representative of prediction errors, up to at least a secular range. The predictability of the geodynamo thus appears to significantly exceed previous theoretical expectations based on the chaotic divergence of ensemble members. The assimilation generally outperforms the linear mathematical extrapolations from a 30-yr prediction range onwards, with a 40 per cent improvement in Earth-surface error at a secular range. The geomagnetic axial dipole decay observed over the past two centuries is predicted to continue at a similar pace in the next century, with a further loss of 1.1 +/- 0.3 mu T by year 2115. The focal (or minimum intensity) point of the South Atlantic geomagnetic anomaly is predicted to enter the South Pacific region in the next century, with the anomaly itself further deepening and widening. By year 2065, the minimum intensity is predicted to decrease by 1.46 +/- 0.4 mu T at the Earth surface and the focal point to move 12.8 +/- 1.4 deg westwards with a slight northward component. This corresponds to a drift rate of 0.26 deg yr(-1), similar to the typical geomagnetic westward drift observed over the past four centuries. The same drift rate is also predicted until 2115 with a further (but more uncertain) intensity decrease.