Twentieth-century atmospheric river activity along the west coasts of Europe and North America: algorithm formulation, reanalysis uncertainty and links to atmospheric circulation patterns

Abstract

A new atmospheric-river detection and tracking scheme based on the magnitude and direction of integrated water vapour transport is presented and applied separately over 13 regions located along the west coasts of Europe (including North Africa) and North America. Four distinct reanalyses are considered, two of which cover the entire 20th-century: NOAA-CIRES Twentieth Century Reanalysis v2 (NOAA-20C) and ECMWF ERA-20C. Calculations are done separately for the OND and JFM-season and, for comparison with previous studies, for the ONDJFM-season as a whole. Comparing the AR-counts from NOAA-20C and ERA-20C with a running 31year window looping through 1900-2010 reveals differences in the climatological mean and inter-annual variability which, at the start of the 20th-century, are much more pronounced in western North America than in Europe. Correlating European AR-counts with the North Atlantic Oscillation (NAO) reveals a pattern reminiscent of the well-know precipitation dipole which is stable throughout the entire century. A similar analysis linking western North American AR-counts to the North Pacific index (NPI) is hampered by the aforementioned poor reanalysis agreement at the start of the century. During the second half of the 20th-century, the strength of the NPI-link considerably varies with time in British Columbia and the Gulf of Alaska. Considering the period 1950-2010, AR-counts are then associated with other relevant large-scale circulation indices such as the East Atlantic, Scandinavian, Pacific-North American and West Pacific patterns (EA, SCAND, PNA and WP). Along the Atlantic coastline of the Iberian Peninsula and France, the EA-link is stronger than the NAO-link if the OND season is considered and the SCAND-link found in northern Europe is significant during both seasons. Along the west coast of North America, teleconnections are generally stronger during JFM in which case the NPI-link is significant in any of the five considered subregions, the PNA-link S. Brands,

Atmospheric rivers in 20 year weather and climate simulations: A multimodel, global evaluation

“…A broad definition is adopted in the current study, which, as in Zhu and Newell [], does not impose any predetermined geographical requirements to the existence of ARs. For example, the three flood‐producing “tropical plume” events in subtropical northwestern and tropical West Africa reported in Knippertz and Martin [] were all detected as ARs by Brands et al [] (http://www.meteo.unican.es/es/atmospheric-rivers), as also in the current study. While two of the three events are outside of active monsoon season, the third event occurred when the African monsoon was active and therefore could include contribution from the monsoon flow.…”

Evaluation of the moisture sources in two extreme landfalling atmospheric river events using an Eulerian WRF tracers tool

“…Both events were very intense in terms of IVT and IWV and are well detected by different methods (Guan and Waliser, 2015;Eiras-Barca et al, 2016;Brands et al, 2016). The first AR occurred on December 2007, affecting mostly the Pacific Northwest region of the United States (Fig.…”

“…The relationship between ARs and flood events has been extensively analyzed for the US West Coast region (e.g., Higgins et al, 2000;Ralph et al, 2005Ralph et al, , 2006Ralph et al, , 2004Ralph et al, , 2013Bao et al, 2006;Neiman et al, 2008a, b;Leung and Qian, 2009;Dettinger, 2011;Dettinger et al, 2011;Warner et al, 2012Warner et al, , 2014Rutz et al, 2013;Kim et al, 2013), Europe (Lavers et al, 2011(Lavers et al, , 2012Villarini, 2013, 2014;Ramos et al, 2015Ramos et al, , 2016Eiras-Barca et al, 2016;Brands et al, 2016) and other regions world-wide (e.g., Mahoney et al, 2016;Mundhenk et al, 2016). It is important to better understand the physical mechanisms leading to extreme flooding associated with ARs, considering their impacts on human and natural systems and the mounting evidence that ARs are projected to become more frequent and intense in the future (e.g., Dettinger, 2011;Payne and Magnusdottir, 2015).…”

“…There are several proposed methods of AR detection, most of which are based on thresholds of integrated water vapor (IWV) and/or IVT, shape criteria, and from satellite or reanalysis data (e.g., Ralph et al, 2004;Bao et al, 2006;Lavers et al, 2011Lavers et al, , 2012Dettinger, 2011;Nayak et al, 2014;Ramos et al, 2015;Eiras-Barca et al, 2016;Brands et al, 2016). Guan and Waliser (2015) developed a global detection method using filters of intensity, direction, geometry and coherence of the structures.…”

On the relationship between atmospheric rivers, weather types and floods in Galicia (NW Spain)

“…There has been a rise in the development of detection algorithms for ARs over the past few years, with significant contributions made by numerous authors (e.g., Lavers et al, 2012;Guan and Waliser, 2015;Eiras-Barca et al, 2016;Brands et al, 2016). Despite the fact that discrepancies in the finer details of their detection remain, all algorithms in the literature rely on an analysis of the integrated water vapor column (IWV) and IVT fields.…”