Mid-Pliocene North American Monsoon in Weather Resolving Coupled Simulations

The North American Southwest (SW NA) has recently experienced periods of extreme drought, largely due to an increased intensity in evaporation. Yet, there remains large uncertainty in the predicted future changes of precipitation over this region. As a result, the future of SW NA hydroclimate remains uncertain.  The North American Monsoon (NAM) is an atmospheric circulation feature of SW NA hydroclimate that is generated by interactions between topography and moisture surge from the Gulf of California and the Gulf of Mexico.  Previous research has shown a weakened NAM in response to elevated levels of atmospheric CO₂. Consistent with this result, PlioMIP2 simulations also show reductions in precipitation throughout this region. However, when analyzing proxy paleoclimate reconstructions during the Pliocene, various records suggest wetter conditions during that time.  We use the mid-Pliocene (3.3 – 3.0 Millions of years ago) as an analog for ongoing climate change because this interval featured topography, geography, and biome assemblages similar to today, but a warmer global mean temperature by 2 - 4 °C compared to pre-industrial, and a sustained 400 ppm CO₂.  Here we are testing whether a high resolution simulation (25 km) can better capture the NAM and provide different sensitivity to boundary conditions compared to low resolution (100 km) simulations, using the same Community Earth System Model.  Our pre-industrial simulations with high elevation in the Sierra Madre Occidental mountain range display a more realistic distribution of the NAM rainbelt and its dependency on topography. In the Pliocene simulations, despite that the lowered mountain range reduces precipitation in the NAM region, an increase in summer precipitation is seen in the high resolution run on the eastern side of the mountains and along the gulf of Mexico. Ongoing work will explore the sources for this resolution dependency, and will quantify contributions of mesoscale systems, such as tropical and extratropical cyclones, to precipitation in the monsoon region.