Understanding Midwinter Suppression of Baroclinic Wave Activity in the Pacific from Linear Perspective of Baroclinic Wave Growth

Abstract Midwinter suppression, characterized by reduced baroclinic wave activity over the North Pacific storm track in January despite elevated background baroclinicity, challenges the framework of linear baroclinic instability theory. A central feature of this phenomenon is the southward shift of baroclinic waves, guided by the jet. While this shift has been identified as a key factor in midwinter suppression, the precise dynamics remain unclear, partly due to the lack of explicit terms in eddy energetics that address the impact of meridional wave shifts. In this study, we apply linear regression analysis and a simplified perturbation vorticity equation to examine vorticity amplification in baroclinic waves during their linear growth phase. Vorticity amplification is dominated by vortex stretching, a mechanism conceptualized as the concentration or dilution of planetary vorticity driven by the divergent winds associated with the wave, linking wave growth to meridional migration. Our results show that the suppression relative to November is marked by a weakening of the wave growth rate in the central-eastern Pacific and a southward shift of the growth region, with no substantial changes in wave structure. This suppression is primarily driven by the weakening of planetary vorticity due to the wave’s southward migration, which inhibits vortex stretching intensity. In contrast, the suppression relative to March is mainly attributed to the stronger westward tilt of the wave in the western Pacific in March, with no significant change in vortex stretching intensity during this period.