Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Torino, Italy.
PLoS One. 2013 Aug 19;8(8):e71129. doi: 10.1371/journal.pone.0071129. eCollection 2013.
Precipitation is one of the most important meteorological variables for defining the climate dynamics, but the spatial patterns of precipitation have not been fully investigated yet. The complex network theory, which provides a robust tool to investigate the statistical interdependence of many interacting elements, is used here to analyze the spatial dynamics of annual precipitation over seventy years (1941-2010). The precipitation network is built associating a node to a geographical region, which has a temporal distribution of precipitation, and identifying possible links among nodes through the correlation function. The precipitation network reveals significant spatial variability with barely connected regions, as Eastern China and Japan, and highly connected regions, such as the African Sahel, Eastern Australia and, to a lesser extent, Northern Europe. Sahel and Eastern Australia are remarkably dry regions, where low amounts of rainfall are uniformly distributed on continental scales and small-scale extreme events are rare. As a consequence, the precipitation gradient is low, making these regions well connected on a large spatial scale. On the contrary, the Asiatic South-East is often reached by extreme events such as monsoons, tropical cyclones and heat waves, which can all contribute to reduce the correlation to the short-range scale only. Some patterns emerging between mid-latitude and tropical regions suggest a possible impact of the propagation of planetary waves on precipitation at a global scale. Other links can be qualitatively associated to the atmospheric and oceanic circulation. To analyze the sensitivity of the network to the physical closeness of the nodes, short-term connections are broken. The African Sahel, Eastern Australia and Northern Europe regions again appear as the supernodes of the network, confirming furthermore their long-range connection structure. Almost all North-American and Asian nodes vanish, revealing that extreme events can enhance high precipitation gradients, leading to a systematic absence of long-range patterns.
降水是定义气候动态的最重要气象变量之一，但降水的空间格局尚未得到充分研究。复杂网络理论为研究许多相互作用元素的统计相关性提供了一个强大的工具，本文使用该理论来分析 70 年来（1941-2010 年）年降水量的空间动态。通过相关函数，将一个节点与具有时间分布的地理区域相关联，以构建降水网络，并确定节点之间可能的联系。降水网络揭示了显著的空间变异性，几乎没有连接的地区，如东亚和日本，以及高度连接的地区，如非洲萨赫勒地区、澳大利亚东部，在较小程度上还包括北欧。萨赫勒和澳大利亚东部是非常干燥的地区，那里的降雨量很少，呈大陆尺度均匀分布，小尺度极端事件很少发生。因此，降水梯度较低，使这些地区在大的空间尺度上连接良好。相反，亚洲东南部经常受到季风、热带气旋和热浪等极端事件的影响，这些事件都可能导致相关度仅降低到短程尺度。中纬度和热带地区之间出现的一些模式表明，行星波的传播可能对全球尺度的降水产生影响。其他联系可以定性地与大气和海洋环流相关联。为了分析网络对节点物理接近度的敏感性，暂时断开短期连接。非洲萨赫勒地区、澳大利亚东部和北欧地区再次成为网络的超级节点，进一步证实了它们的远程连接结构。几乎所有北美和亚洲的节点都消失了，这表明极端事件可以增强高降水梯度，导致系统地缺乏远程模式。
