Natural and anthropogenic influences of heat waves and droughts over the United States: variability, predictability, and future projections

Heat waves are responsible for the most weather-related cause of the death in the United States
(U.S.). Their number and severity have been increasing and are projected to continue increasing
throughout the 21st Century under anthropogenic climate change (ACC). Recently, the PIs of this
proposal published results on how internal variability and ACC modulate U.S. heat waves. The
study stressed that ACC will dominate heat wave occurrence over the Western and Great Lakes
regions, with Time of Emergence (the time at which ACC signal emerge against background
natural variability, or ToE) occurring as early as in 2020s and 2030s, respectively. The early ToE
was found to be caused by a reduction in atmospheric transient eddies (storminess), thus calling
for the need for greater mitigation and adaptation efforts in these regions. In contrast, internal
variability will govern heat wave occurrence in the Northern and Southern Great Plains, where
ToE occurs in the 2050s and 2070s; this later ToE is found to be a result of a projected increase in
circulation variability, namely the Great Plain low-level jet (GPLLJ) and associated moisture
transport. The PIs suggested that the enhanced GPLLJ and moisture transport due to ACC serves
to attenuate soil moisture depletion and thus heat wave occurence. This result calls for the need to
identify potential remote linkages of these extreme events over the Great Plains, which should aid
in their prediction and understanding of future projections.

With respect to naturally-occurring processes that modulates heat waves, preliminary analysis
suggests that convective latent heat release from the East Asian Monsoon enhances the likelihood
of droughts and heat waves over the U.S. through atmospheric teleconnection, promoting an
anticyclonic circulation over the Great Plains. Also, tropical Atlantic and eastern Pacific sea
surface temperature (SST), namely Western Hemisphere Warm Pool (WHWP) is found to be a
modulator of the GPLLJ during the summer, which in turn influence moisture transport into the
U.S., soil moisture, and eventually heat waves. Our goal is to advance our understanding of the
physical mechanisms that control the variability, predictability, and future projections of
extreme heat waves events and associated droughts over the U.S. To achieve our goal, we
propose the following research objectives: (1) diagnose the relative role of natural variability
versus anthropogenic forcing on heat waves and droughts over the U.S. within the context of the
CMIP6 21st Century projections. (2) Identify potential modulators of heat waves and droughts
(e.g., monsoonal teleconnections, Arctic amplification, tropical SST anomalies) under present
conditions and future climate projections. (3) Develop an integrated prediction system of heat
waves and droughts at national scale. To achieve these three objectives, CMIP6 21st Century
projections, atmospheric forced models, and fully coupled models experiments along with
observational estimates will be analyzed.

Relevance to the Competition: The proposed work contributes directly to NOAA CPO FY2019
MAPP funding Competition: 21st Century Integrated U.S. Climate Predictions and Projections
Priority Area A: “Develop integrated predictions/projections of long-term changes affecting the
U.S. within the global context at national or large regional scale, and/or for specific applications”,
as well as Priority Area B: “Develop integrated process-level understanding of
predicted/projected climate changes for the purpose of characterizing associated confidence and
uncertainties.” The proposed research will be carried out as part of the Cooperative Institute for
Marine and Atmospheric Sciences (CIMAS) under the research theme: Climate Research and
Impact. The proposed work is aligned with the following two NOAA’s goals: (1) Climate
Adaptation and Mitigation. (2) Weather-Ready Nation.

Climate Risk Area: Extreme Heat