Contrasting characteristics of atmospheric rivers and their impacts on 2016 and 2020 wildfire seasons over the western United States

An atmospheric river (AR) is a strong filamentary water vapor transport that plays a critical role in regional hydroclimate systems. While climate conditions can affect wildfire activities, the process by which ARs are associated with wildfire patterns remains unclear. Here, we characterize ARs in 2016 and 2020, and associate them with fire spread and burned areas along with other climate conditions in the western U.S. We found the record-high wildfire activity in 2020 was associated with hotter, drier, and windier conditions, with its peak shifted from July to August, unlike the climatological fire seasonality in the western U.S. It was also linked to satellite-observed low soil moisture during pre- and on fire season but high vegetation greenness, a proxy of fuel load, during the pre-fire season. ARs were more frequent but weaker in the summer, while ARs were less frequent and short-lived in the fall of 2020 than those of 2016. The year 2016 experienced a 'coupled' precipitation-wind pattern (i.e. higher wind accompanying high precipitation). In contrast, precipitation was much lower in 2020 than in 2016, showing a 'decoupled' precipitation-wind pattern, particularly in the spring and fall. Under ARs, the contrasting precipitation-wind patterns in 2020 (dry-windy) and 2016 (wet-windy) were more evident. For example, the surface wind (precipitation) in the AR cases was higher by 9% (34%) than in the non-AR cases in 2020 (both years) (p < 0.01) over land. The daily fire activity records demonstrate that long-lived, successive, and coastal ocean originated (centered) ARs with high precipitation help suppress fire activity (e.g. September-November 2016), while short-lived or no ARs with strong wind and little precipitation rather yield fire activity (e.g. August and September 2020). This result highlights how ARs can be associated with wildfire activity patterns during the pre-fire and fire seasons in the western U.S.