Significant increase of global anomalous moisture uptake feeding landfalling Atmospheric Rivers

Atmospheric Rivers (ARs) are narrow, elongated corridors of strong water vapor transport, typically in the warm sector of extratropical cyclones, that deliver a large fraction of poleward moisture from subtropics to mid- and high latitudes. They are strongly associated with extreme precipitation and flooding when they make landfall, especially over mountainous coasts, and their absence is often linked to droughts. Global warming increases atmospheric moisture (~7% per K) and is expected to intensify moisture transport and AR activity, though dynamic changes (winds, anticyclones) may modulate this response. Climate projections generally indicate more frequent and intense ARs, but it remains unclear which oceanic regions provide anomalous moisture to landfalling ARs (LARs) and whether these source anomalies exhibit warming-linked trends. The study aims to identify, globally, the regions of anomalous moisture uptake (AMU) feeding LARs and to quantify their interannual variability and trends, testing whether increases align with Clausius–Clapeyron scaling.

Prior research established ARs as primary conduits for horizontal water vapor transport and key drivers of extreme precipitation and floods in the western U.S. and western Europe, with substantial socio-economic impacts. ARs bridge oceanic evaporation and continental precipitation and affect cryosphere mass balance. Thermodynamic arguments and modeling suggest ~7%/K increases in atmospheric water vapor, and studies project increases in AR intensity, frequency, and integrated water vapor transport (IVT) under warming, though dynamic changes may also play roles. Observational trends in some regions (e.g., western U.S.) are mixed for IVT and precipitation. The relative importance of local moisture convergence vs. long-range (sub)tropical advection in ARs is debated, with evidence supporting contributions from both. A key gap highlighted is the lack of a global assessment of where ARs obtain anomalous moisture and how these source regions are changing.

• Identification of landfalling AR (LAR) regions: Used the global AR landfall database (Guan & Waliser 2015) based on ERA-Interim (ECMWF) with 6-hourly time steps (00, 06, 12, 18 UTC), 1.5° resolution, and IVT threshold plus geometric coherence criteria. LAR events were aggregated along global coastlines at 12° spatial resolution for 1980–2017. Regions with LAR frequency exceeding 10% of total days (≥1388 days) were flagged as maximum occurrence areas. To ensure extratropical cyclone association, regions lacking a negative winter mean sea level pressure (MSLP) anomaly (from ERA-Interim) were excluded. From 24 initial regions, 20 were retained for AMU analysis; some tropical/monsoon-influenced zones were excluded. The U.S. West Coast was subdivided into four subregions to reflect differing AR regimes and orientations; one southern subregion was excluded from AMU analysis due to lacking negative MSLP seasonality.
• Lagrangian AMU diagnostics: Applied FLEXPART v9.0 to track approximately 2 million equal-mass air parcels advected by ERA-Interim 3-D winds (1° horizontal grid, 61 vertical levels from 1000 to 0.1 hPa; 6-hourly data) over 1980–2017. For each LAR event, parcels arriving in the AR were traced backward in time over an optimal, region-specific mean monthly integration time (reflecting atmospheric water vapor residence times ~8–10 days, adjusted per Nieto & Gimeno 2019). Moisture budget along parcel trajectories was computed via e−p = m dq/dt, and surface freshwater flux (E−P) was diagnosed by vertically integrating over parcels resident over each area. Moisture source regions are defined where (E−P) > 0.
• Anomalous Moisture Uptake (AMU) definition: For each LAR event, only positive (E−P) contributions were summed to yield moisture uptake fields. Anomalies were obtained by subtracting the climatological moisture uptake for the same Julian date and 6-hour time step (1980–2017 period) from the event-specific uptake, retaining only positive anomalies. This yields AMU fields showing where LARs gain anomalous moisture along their pathways. Thousands of events per region were processed (e.g., 6201 events in one region).
• Statistical analyses and mapping: For each of the 20 regions, AMU fields were summarized (e.g., 90th percentile maps) and tested for trends using the non-parametric Mann–Kendall test (95% significance). Global AMU fields were also aggregated across all LARs to assess spatial patterns, zonal means, interannual variability, and linear trend (1980–2017). Latitudinal distribution and hemispheric asymmetries were quantified. IVT trends were also evaluated regionally and globally for comparison with AMU trends.

• Global increase in anomalous moisture uptake: AMU associated with landfalling ARs increased significantly by about 0.9% per decade over 1980–2017, consistent with thermodynamic expectations.
• Clausius–Clapeyron consistency: Given ~0.5 °C global surface warming from 1980 to 2016, the AMU increase is ~3% over that period, implying ~6% per K—close to the Clausius–Clapeyron scaling (~7–7.3%/K) for atmospheric moisture and extreme precipitation.
• Spatial AMU patterns: Highest AMU values are concentrated over subtropical oceans (25°–40° latitude) in both hemispheres, with a dominant maximum over the Western Hemisphere Warm Pool (WHWP: Gulf of Mexico and Caribbean Sea). This region acts as a major moisture reservoir for North Atlantic ARs making landfall along the U.S. East Coast, Greenland/Iceland, and Western Europe.
• Hemispheric asymmetry: Zonal AMU exhibits a bimodal distribution with peaks near 40° latitude; values are approximately twice as high in the Northern Hemisphere, aligning with higher LAR occurrence there. Interannual variability in the latitudinal position of AMU is small.
• Regional trends: Most of the 20 LAR regions show significant positive AMU trends (95% level), indicating generalized enhancement of anomalous moisture supply. A notable exception is the U.S. Western region, where no significant trend was found, consistent with prior studies reporting limited long-term IVT/precipitation trends.
• Basin contributions: The North Atlantic, particularly via the WHWP, accounts for nearly half of global AMU associated with LARs. Even when WHWP is isolated or excluded, significant increasing AMU trends persist.
• IVT consistency: Trends in IVT are similar to AMU regionally in areas of strong AR activity and show a significant global increase, reinforcing the intensification signal.

The observed significant increase in AMU feeding landfalling ARs indicates that a warmer, moister atmosphere is already enhancing the moisture available for AR-driven precipitation, aligning with Clausius–Clapeyron scaling and projections of increased AR intensity and frequency. The dominance of AMU over subtropical oceans and the strong role of the WHWP highlight key source regions whose warming and variability (e.g., linked to North Atlantic SST changes and AMV, with aerosol forcing influences) can modulate AR moisture supply and downstream hydroclimate impacts in Europe and North America. Although not all ARs produce extreme precipitation, their enhanced moisture content raises the potential for heavier rainfall and flooding, especially with orographic or warm-conveyor-belt uplift. The results also reconcile the debate on moisture sources, showing that both long-range (sub)tropical advection and local oceanic convergence near landfall contribute substantially to AR moisture. The findings underscore the need to assess the relative contributions of SST, near-surface wind speed, and near-surface humidity to AMU changes, and to better understand changes in moisture residence time and AR dynamics under continued warming.

This study provides a first global identification of anomalous moisture uptake (AMU) regions that feed landfalling atmospheric rivers (LARs) and demonstrates a significant increase in AMU from 1980 to 2017 (~0.9% per decade), consistent with Clausius–Clapeyron scaling. The Western Hemisphere Warm Pool emerges as the dominant global AMU maximum, supplying moisture to North Atlantic ARs impacting the Americas and Europe. Most coastal LAR regions exhibit significant AMU increases, with a few exceptions (e.g., western U.S.). The alignment between AMU and IVT trends suggests strengthening AR-related moisture transport in the present climate, implying heightened risks of heavy precipitation and flooding. Future work should quantify the relative roles of SST, winds, and humidity in driving AMU anomalies, assess potential dynamic changes in ARs, evaluate moisture residence time changes, and extend analyses to different reanalyses and climate model projections to refine regional impact assessments.

• Regional selection relied on excluding areas without negative winter MSLP anomalies to ensure extratropical cyclone association; some tropical/monsoon-influenced coastal regions were omitted.
• AMU estimates depend on ERA-Interim reanalysis and Lagrangian modeling assumptions (parcel representation, residence times, and only positive E−P contributions), which may introduce uncertainties.
• The study emphasizes thermodynamic changes; potential dynamic changes (e.g., storm track shifts, wind trends, anticyclonic activity) are not comprehensively attributed.
• The relative impacts of SST, near-surface winds, and humidity on AMU trends are not disentangled; quantifying these drivers is identified as future work.
• Not all regions show significant trends (e.g., western U.S.), and causality with regional precipitation extremes is not directly established for each LAR event.