The Influence of Atmospheric River Seasonality and Orientation on Pacific Northwest Surface Temperatures

Abstract Atmospheric rivers (ARs) influence weather and climate along the North American west coast, yet their effects on Pacific Northwest (PNW) surface temperatures remain unclear across seasons and synoptic configurations. Motivated by the 2021 PNW heatwave—which was preceded by a poleward‐tracking AR—we analyze how ARs modulate daily maximum temperatures ( T max ) during summer (JJA) and winter (DJF) from 1989 to 2024. Using reanalysis data and an AR detection algorithm, we composite integrated vapor transport (IVT), Z500, total precipitation, cloud cover, and T max anomalies for AR detection days and the subsequent 10 days, and classify summer ARs according to the tilt of their IVT plumes as meridionally oriented (MARs) or zonally oriented (ZARs). Winter ARs exhibit a coherent southwest to northeast orientation and generate widespread, statistically significant PNW warming that persists for over a week. In contrast, composite summer ARs produce no consistent T max response due to the cancellation of ARs that track north versus south of the region. Separating JJA ARs by orientation reveals a strong dynamical divide: MARs induce substantial warming (>3°C in the median T max ), associated with a northeast‐tilted ridge that enhances subsidence and poleward heat transport, whereas ZARs generate significant cooling tied to zonal flow and inland advection of cool, moist marine air. These results show that AR seasonality and orientation jointly influence the sign and magnitude of PNW temperature anomalies. Importantly, AR orientation at a fixed offshore detection region provides predictive value for downstream thermal impacts beyond AR intensity and ridge strength, improving heat‐extreme forecasting in a warming climate.