NWC REU 2018
May 21 - July 31



Photo of author

Global Annual Precipitation Cycles and Variability

Erica Bower


What is already known:

  • The wet season for any given region is defined by the magnitude of rainfall received, the frequency of precipitating days, and the timing of the enhanced precipitation.
  • Different regions of the world are experiencing changes in the annual precipitation cycle due to natural variability and anthropogenic forcing.
  • There are many different method to objectively identify the onset and cessation of the wet season for a region.

What this study adds:

  • Analysis of daily data to high spatial resolution to identify the wet season for all locations on earth to at least 1° resolution
  • Historical models and observational data show onset dates became later throughout most of Europe and Antarctica, while cessations became earlier in Europe and later near the poles.
  • Future CMIP5 models project shorter wet seasons in the Amazon, the Caribbean, the CONUS, and northeastern Europe. Longer wet seasons could occur in Japan and throughout the western Pacific.


Changes in the timing of precipitation within a year can impact ecosystems, agriculture, infrastructure, and many other aspects of life. A shift in the wet season can lead to droughts or flooding, causing financial losses as well as risking human lives. This study uses high resolution daily precipitation data to objectively identify the timing and duration of the wet season across the globe in historical and future periods. Datasets include the Global Precipitation Climate Project (GPCP), the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks (PERSIANN) project, and 10 historical and 8 future model simulations from the Coupled Model Intercomparison Project Phase 5 (CMIP5). In general, shorter wet seasons have been occurring worldwide according to the historical simulations and the observational data. Exceptions can be found in eastern Asia and in many southern hemisphere ocean areas, where longer wet seasons have been observed. The future model simulations, using representative concentration pathway 8.5, project different changes in timing and duration of wet season for locations worldwide. Across the majority of the globe, the wet season is projected to become shorter in the 21st century. Supporting prior regional work, shorter wet seasons are expected in the coming years in the Amazon, southern Africa, and much of the contiguous United States. The Arctic could see a reduction or extension of the wet season by as much as 15 days per decade due to a later onset and earlier cessation. These results have important consequences for planning and managing water resources in the future.

Full Paper [PDF]