Dynamics and model representation of the boreal spring subseasonal variability over North America

Abstract The dynamics and model representation of boreal spring subseasonal variability over North America are examined, focusing on four leading modes identified through empirical orthogonal functions. Mode 1 is active over the high latitudes of the North Pacific, exhibiting a meridional dipole structure. Mode 2 features a localized circulation anomaly over northeastern North America. Modes 3 and 4 are wave-train-like patterns extending from the eastern North Pacific to North America from lower- and mid-latitudes, respectively. Despite a bias in Mode 2, the Community Atmosphere Model version 5 (CAM5) reproduces the overall spatiotemporal features and associated temperature and precipitation responses to the subseasonal modes. A scale-resolving quasi-geostrophic geopotential tendency analysis reveals that Modes 1 and 2 develop quasi-stationarily followed by westward movements, primarily driven by vorticity fluxes. Heat fluxes act to maintain the barotropic structure of the related circulation anomalies. Modes 3 and 4 exhibit eastward propagation throughout their lifecycles, explained by both vorticity and heat fluxes. Scale separation indicates that both high- and low-frequency eddy vorticity fluxes explain the formation of Modes 1 and 2, whereas the latter dominates the propagating nature of Modes 3 and 4. These dynamical characteristics are well replicated in CAM5. The bias in Mode 2 is likely attributed to weaker contributions of high-frequency components in CAM5. Further analyses suggest that current models in the Coupled Model Intercomparison Project Phase 6 also reasonably capture the basic statistics of the subseasonal modes, lending confidence in using them for the study and prediction of long-lasting weather and climate extremes.