Scale-Dependent Dynamics of Alaskan Ridge Amplification: A Holistic View of Circulation Driving North American Cold Spells

North American cold spells are frequently associated with the amplification of a large-scale atmospheric circulation pattern known as the Alaskan Ridge. This pattern is characterised by a persistent high over Alaska and two low-pressure centres over the Pacific Ocean and Hudson Bay. While the Alaskan Ridge (particularly the high over Alaska) has been widely discussed in the literature, a comprehensive understanding of the multiple drivers behind its amplification remains elusive. Here, we consider the dynamical drivers of the intensified high over Alaska leading to cold spells in central North America. First, we separate the cold spells based on whether they are associated with stratospheric wave reflection. This separation reveals two distinct atmospheric states resulting in upper-tropospheric high formation. Second, we employ a wave decomposition technique based on normal-mode functions to understand the role of tropospheric dynamics on different scales in favouring the Alaskan Ridge amplification. This methodology's advantage is the ability to separate Rossby and inertia-gravity regimes as opposed to the widely utilised Fourier decomposition. The focus is on planetary (zonal wavenumbers 1-3) and synoptic (zonal wavenumbers 4-8) scales. The results show enhanced synoptic-scale Rossby wave activity prior to the Alaskan Ridge amplification. Based on the shape and location of the synoptic anomalies, we attribute the enhancement to an extratropical-midlatitude interaction, a driver previously proposed in the literature. Our approach enables a holistic picture of the atmospheric evolution leading to the central North American cold spells, supporting a dynamical understanding of their origin.