External Influences on Precipitation Mean State and Variability

Monday, May 12, 2014 - 07:00
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Changes in global (ocean and land) precipitation are among the most important and least well-understood consequences of climate change. In a simple case, future changes in regional precipitation would result from a gradual change in mean precipitation superimposed upon precipitation changes arising from internal climate variability modes with largely unaltered patterns and amplitude. Several questions, however, arise from this simple scenario. Can we already detect the gradual changes in mean precipitation in observations? Can we attribute those changes to specific external forcings? Will ENSO-driven precipitation teleconnections evolve in response to anthropogenic radiative forcing, invalidating this scenario? During this presentation, we will show some of our progresses to address those questions. In particular, we will focus on two basic mechanisms affecting the zonal-mean distribution of precipitation in a warming world: the intensification of the hydrological cycle (thermodynamic changes), and the poleward displacement of current zonal wet-dry patterns (dynamic changes). We will demonstrate that both these changes are occurring simultaneously in global precipitation, that this behavior cannot be explained by internal variability alone, and that external influences are responsible for the observed precipitation changes. We will also present some evidence that the observed changes in precipitation likely result from human activities. In the second part of the presentation, we will briefly describe a novel approach developed to examine the evolution of ENSO-driven precipitation teleconnections in the 21st century. Acknowledgments: This work was funded by the U.S. Department of Energy Office of Science and was performed at the Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

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