A novel index for regional stratospheric wave reflection and its impact on surface air temperature over North America

Accurately identifying stratospheric wave reflection is crucial for understanding wintertime extremes. Using ERA5 (fifth generation ECMWF atmospheric reanalysis) data from 1979 to 2023, this study establishes a novel objective index for regional planetary wave reflection based on the leading EOF (empirical orthogonal function) mode of 100-hPa eddy heat flux over North Pacific–North America. This mode reveals a significant “Aleutian–North America” dipole structure. Diagnostic analyses demonstrate that reflection events are triggered by the deceleration of background westerlies and the reversal of vertical wind shear in the upper stratosphere, which cause the refractive index to become negative. This creates a critical layer that forces planetary waves to refract downward. Upon entering the troposphere, the reflected waves amplify Alaskan blocking and the North American trough through linear interference, inducing a robust positive PNA (Pacific–North American) pattern. This circulation configuration steers polar cold air southward, resulting in widespread and persistent cold anomalies across North America. The proposed index effectively captures these dynamical processes, overcoming the limitations of previous fixed-region metrics.