  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 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.
