On the Rivers in the Euro-Atlantic Sky

We study the spatiotemporal variability of Atmospheric Rivers (ARs) over Euro-Atlantic region using long-term reanalysis datasets. Winds, temperature and specific humidity at different pressure levels during 1979-2018 are used to study the water vapour transport integrated between 1000-300 hPa (IVT300) as a proxy to ARs. The intensity of ARs in the North Atlantic has been increasing in recent times (2009-2018) with large decadal variability and poleward shift (~5o towards the North) in landfall (1999-2018). Significant bias shown by different reanalysis products in IVT300 compared to ERA5 data is attributed to bias in specific humidity and winds. Different reanalysis datasets show similar spatial patterns of IVT300 in mapping ARs but has a bias of around 40-60 kgm-1s-1 compared to ERA5. The magnitude of winds and specific humidity in the lower atmosphere (below 750 hPa) dominates the total column water vapour and intensity of Ars in the north Atlantic. IVT300 in all reanalysis datasets in the North Atlantic show a standard deviation of 200 kgm-1s-1 which is around 60% of the IVT300 climatology (>300 kgm-1s-1). Though ARs have higher frequency of landfalling over Western Europe in winter half-year (WHY); the intensity of IVT300 in winter is 3% lower than the annual mean. On the other hand, lower frequency of ARs in summer half-year (SHY) shows 3% higher intensity than the annual mean. There is a significant impact of the North Atlantic Oscillation (NAO) and Scandinavian blocking on the location of landfall of ARs. Furthermore, there is a strong latitudinal dependence of the source of moisture flux in the open ocean, contributing to the formation and enhancing AR's strength.