During atmospheric river (AR) landfalls on the Antarctic ice sheet, the high waviness of the circumpolar polar jet stream allows for sub-tropical air masses to be advected towards the Antarctic coastline. These rare but high-impact AR events are highly consequential for the Antarctic mass balance; yet little is known about the various atmospheric dynamical components determining their life cycle. By using an AR detection algorithm to retrieve AR landfalls at Dumont d’Urville and non-AR analogues based on 700 hPa geopotential height, we examined what makes AR landfalls unique and studied the complete life cycle of ARs to affect Dumont d’Urville. ARs form in the mid-latitudes/sub-tropics in areas of high surface evaporation, likely in response to tropical deep convection anomalies. These convection anomalies likely lead to Rossby wave trains that help amplify the upper-tropospheric flow pattern. As the AR approaches Antarctica, condensation of isentropically lifted moisture causes latent heat release that – in conjunction with poleward warm air advection – induces geopotential height rises and anticyclonic upper-level potential vorticity tendencies downstream. As evidenced by a blocking index, these tendencies lead to enhanced ridging/blocking that persist beyond the AR landfall time, sustaining warm air advection onto the ice sheet. Finally, we demonstrate a connection between tropopause polar vortices and mid-latitude cyclogenesis in an AR case study. Overall, the non-AR analogues reveal that the amplified jet pattern observed during AR landfalls is a result of enhanced poleward moisture transport and associated diabatic heating which is likely impossible to replicate without strong moisture transport.

The state and evolution of the North Pacific jet (NPJ) stream strongly influences the character of the downstream synoptic-scale flow pattern over North America. This study employs data from nine models within the Subseasonal-to-Seasonal Reforecast Database hosted by the European Centre for Medium-Range Weather Forecasts to examine the subseasonal (2 weeks–1 month) predictability of the NPJ through the lens of an NPJ phase diagram. The NPJ phase diagram provides a visual representation of the state and evolution of the NPJ with respect to the two leading modes of NPJ variability. The first mode of NPJ variability corresponds to a zonal extension or retraction of the climatological jet-exit region, whereas the second mode corresponds to a poleward or equatorward shift of the climatological jet-exit region. The analysis reveals that ensemble forecasts of the prevailing NPJ regime, as determined from the NPJ phase diagram, are skillful into week 3 of the forecast period. Forecasts initialized during a jet retraction, or verifying during a jet retraction and equatorward shift, generally feature the largest errors during the forecast period. Examination of the worst-performing 21-day forecasts from each model demonstrates that the worst-performing forecasts are uniformly associated with development, maintenance, and decay of upper-tropospheric ridges over the high-latitude North Pacific. These results demonstrate that bias-corrected NPJ phase diagram forecasts have the potential to identify periods that may exhibit enhanced forecast skill at subseasonal lead times based on the anticipated NPJ evolution.