Systematic Drifts of Rossby Wave Breaking in Medium-Range Forecasts Over the Northern Hemisphere Cold Season

This study examines the dynamics and systematic forecast drifts of wintertime Rossby wave breaking (RWB) events over the Northern Hemisphere in 15-day forecasts. Using ECMWF and NCEP data from the THORPEX TIGGE archive, RWBs are classified into cyclonic (CWB) and anticyclonic (AWB) types and quantified using two metrics: breaking extent (BE) and breaking amplitude (BA). Both models consistently underestimate BE and BA, with especially large biases for CWBs over the Pacific and AWBs over western North America, indicating persistent errors along major storm tracks. These drifts are largely driven by the loss of small, frequent breakings, while larger events are better preserved. To understand the dynamical origin of these biases, we define a barotropic shear index based on meridional wind shear across the jet and assess its relationship to RWB. The impact of shear depends on wave evolution: when waves remain on one side of the jet, local shear there is most influential; when waves cross the jet, weak shear on the origin side facilitates transition into a more favorable shear zone. Conditional composites show that shear regimes influence regional jet structure and associated circulation. Moreover, large-scale climate modes such as ENSO, NAO, and PNA modulate shear and thus RWB predictability. ECMWF outperforms NCEP in capturing sharp meridional wind gradients and RWB evolution, likely due to its higher resolution. These findings offer a physically grounded explanation for systematic RWB forecast errors and underscore the importance of accurately representing jet dynamics in medium-range prediction.