The El Ni¿o/Southern Oscillation (ENSO) and stratospheric sudden warmings (SSWs) each induce significant surface climate anomalies in Northern latitudes during winter. Nonetheless, possible connections between the impacts of the ENSO and SSWs remain relatively unexplored. Using both observational analysis and global climate model (GCM) experiments, we show that the impacts of El Ni¿o and SSWs interfere over North America. The interference includes constructive interference, or synergistic impacts of the El Ni¿o and SSWs over the southeastern US through northern Mexico, resulting in enhancement of colder and wetter climate when El Ni¿o and SSWs occur in the same winter. The interference is of practical importance such as in extended-range weather forecasts, since the impacts of the two phenomena affect seasonal averages and increase the probability of extreme weather conditions in the region.