How Global Warming Is Drying Up the North American Monsoon

New insights into the droughts and wildfires of the southwestern US and northwestern Mexico.

Researchers have struggled to accurately model the changes to the abundant summer rains that sweep across the southwestern United States and northwestern Mexico, known to scientists as the "North American monsoon."

In a report published Oct. 9 in the journal Nature Climate Change, a team of Princeton and National Oceanic and Atmospheric Administration (NOAA) researchers have applied a key factor in improving climate models - correcting for sea surface temperatures - to the monsoon.
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When they corrected for persistent sea surface temperature (SST) biases and used higher-resolution data for the regional geography, the researchers created a model that accurately reflects current rainfall conditions and suggests that future changes could have significant consequences for regional water resources and hazards.

"This study represents fundamental science relating to the physics of the North American monsoon, but feeds back onto weather to climate predictions and building resiliency for our water supply and responses to hazards," said Kapnick.  "I am excited about this leap forward to improve our models and for the potential applications that they will provide in the future to society."

Their results highlight the possibility of a strong precipitation reduction in the northern edge of the monsoon in response to warming, with consequences for regional water resources, agriculture and ecosystems.

"Monsoon rains are critical for the southwest U.S. and northwest Mexico, yet the fate of the North American monsoon is quite uncertain," said Pascale, the lead author on the paper.   "The future of the monsoon will have direct impacts on agriculture, on livelihoods."
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The researchers discovered that the monsoon is not simply delayed, but that the total precipitation is facing a dramatic reduction.

That has significant implications for regional policymakers, explained Kapnick.  "Water infrastructure projects take years to a decade to plan and build and can last decades.  They require knowledge of future climate ... to ensure water supply in dry years.  We had known previously that other broadly used global models didn't have a proper North American monsoon.  This study addresses this need and highlights what we need to do to improve models for the North American monsoon and understanding water in the southwest."

The new model also suggests that the region's famous thunderstorms may become less common, as the decreased rain is associated with increased stability in the lower-to-middle troposphere and weakened atmospheric convection.

"The North American monsoon is also related to extreme precipitation events that can cause flash floods and loss of life," Kapnick said.  "Knowing when the monsoon will start and predicting when major events will happen can be used for early warnings and planning to avoid loss of life and property damage.  This paper represents the first major step towards building better systems for predicting the monsoon rains."

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