On the mean precipitation characteristics of North American heatwaves

Precipitation—or a lack thereof—can be both cause and consequence of heatwaves. Whereas most heatwave-precipitation research has emphasized summer heatwaves and extreme precipitation, heatwaves outside of summer are relatively more important for driving streamflow. Further, there is increasing recognition of how heatwaves impact river basin hydrology via their influence on the cryosphere, but fewer studies have considered the hydrological impacts of heatwaves via precipitation. Here, we aim to link and address these knowledge gaps by offering an analysis of a set of seasonally- and spatially-varying precipitation characteristics of heatwaves. We consider: (1) how the frequency of heatwave precipitation varies by precipitation intensity; (2) how the frequency and magnitude of precipitation differs between heatwave and non-heatwave periods; (3) how the frequency and magnitude of precipitation varies throughout a heatwave; and (4) how precipitation varies between periods before, during, and after heatwaves. We assess these characteristics for 14 425 basins across North America for winter, spring, summer, and autumn heatwaves. We find that there is a high degree of spatial and seasonal variability in all characteristics assessed. Overall, heatwaves are wetter in autumn and winter but are drier in spring and summer. We find that there are drier conditions overall in the continental interior and wetter conditions overall along the northwest and east coasts. Precipitation is generally greater and more frequent on the final day of heatwaves relative to other days, except for the west and east coast regions in winter. For those cases, heatwaves are generally wetter than periods that occur immediately before and after, indicating strong links between precipitation and relative warmth. These findings have implications for the processes that drive streamflow during heatwaves, and offer insights into the role that extreme temperature events play in modifying river basin hydrology.