The Most Effective Remote Forcing in Causing US-Wide Heat Extremes as Revealed by CESM Green's Function Experiments

We make use of the Community Atmosphere Model version 5 Green's function q-flux perturbation experiments to explore the most effective remote forcing in driving U.S.-wide summer heat extremes. We find that positive q-flux forcing over the western North Pacific Ocean is the most effective in causing an increased heat extreme frequency. This works by driving increased sea surface temperature and precipitation over western North Pacific and an eastward propagating Rossby wave train with an anomalous ridge over the contiguous U.S. In comparison, negative q-flux forcing over the eastern tropical Pacific and its resulting surface cooling also leads to an increased heat extreme frequency but is less effective. Furthermore, guided by the Green's function results, we separate the role of western North Pacific warming and eastern tropical Pacific cooling in U.S. heat extremes in prescribed sea surface temperature experiments and ERA5 reanalysis data and find overall consistent conclusions.

Automated summary

We use the Community Atmosphere Model version 5 Green's function q-flux perturbation experiments to investigate the impact of remote forcing on U.S.-wide summer heat extremes. Our findings reveal that positive q-flux forcing over the western North Pacific Ocean is the most effective in increasing the frequency of heat extremes. This is achieved through elevated sea surface temperature and precipitation over the western North Pacific, generating an eastward propagating Rossby wave train and an anomalous ridge over the contiguous U.S. Negative q-flux forcing over the eastern tropical Pacific also leads to increased heat extreme frequency but with less effectiveness.