Weakening of the North American monsoon with global warming
Future changes in the North American monsoon, a circulation system that brings abundant summer rains to vast areas of the North American Southwest, could have significant consequences for regional water resources. How this monsoon will change with increasing greenhouse gases, however, remains unclear, not least because coarse horizontal resolution and systematic sea-surface temperature biases limit the reliability of its numerical model simulations. Here we investigate the monsoon response to increased atmospheric carbon dioxide (CO_2) concentrations using a 50-km-resolution global climate model which features a realistic representation of the monsoon climatology and its synoptic-scale variability. It is found that the monsoon response to CO_2 doubling is sensitive to sea-surface temperature biases. When minimizing these biases, the model projects a robust reduction in monsoonal precipitation over the southwestern United States, contrasting with previous multi-model assessments. Most of this precipitation decline can be attributed to increased atmospheric stability, and hence weakened convection, caused by uniform sea-surface warming. These results suggest improved adaptation measures, particularly water resource planning, will be required to cope with projected reductions in monsoon rainfall in the American Southwest.
Additional Information© 2017 Macmillan Publishers Limited, part of Springer Nature. Received 08 May 2017; Accepted 12 September 2017; Published online 09 October 2017. S.P. was supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by the University Corporation for Atmospheric Research, Boulder, Colorado and by the NOAA CICS grant - NA14OAR4320106. S.B. acknowledges support from the Caltech Davidow Discovery Fund. The authors thank N. Johnson and H. Zhang for comments on the manuscript. Author Contributions: S.P. designed the research and performed the analysis of the data. S.P. led the writing with the assistance of S.B., S.B.K. and W.R.B. S.P., W.R.B., S.B. and T.L.D. contributed in defining the methods and interpreting the results. All authors took part in the discussion of the results and refined and improved the manuscript. H.M. and G.A.V. designed the model experiments. H.M. and W.Z. performed the simulations. The authors declare no competing financial interests. Data availability: The data that support the findings of this study are available from the corresponding author upon request.
Supplemental Material - nclimate3412-s1.pdf