Extreme Precipitation Events Over the Iberian Atlantic Margin: The Role of Atmospheric Rivers

Abstract:

Atmospheric Rivers (ARs), elongated structures of high values of water vapor flux carried along the warm conveyor belt of extratropical cyclones, are a prominent feature in the poleward transport of moisture, and are often associated to extreme precipitation events (EPEs). Here we examine the link between EPEs on the Atlantic Margin of the Iberian Peninsula and ARs in the period 1979-2007, and show that for about 75% of cases there is a direct relation, and for many others, a second order or indirect connection; that is, they originate from moisture transported by an AR to a neighboring region the previous days that feeds another cyclone causing the actual event. EPEs in western Iberia present a pronounced seasonal cycle, being much more likely to occur in the autumn and practically inexistent in the summer. The incidence of ARs is however much higher and doesn’t show the same temporal frequency as EPEs, with a maximum in winter and a minimum, very marked in the south but not so much in the north, in summer. Only about 30% of ARs result in EPEs in autumn-winter and less than 10% in summer, likely due to differences in forcing, duration and thermodynamic characteristics of the ARs themselves. The interannual variability of EPEs and ARs can be quite large and uncorrelated, with years of high AR activity not necessarily years with high number of EPEs; nevertheless, EPEs are consistently linked to ARs.

The interannual variability of AR arrivals to the Iberian Peninsula mirrors that of AR activity in the southeast North Atlantic and it’s enhanced by negative NAO conditions. There are uncorrelated and important yearly variations in other quadrants of the North Atlantic, some being favored over others depending on persistent circulation anomalies. For periods of several years the entire North Atlantic can also present a significant amount of interannual variability in AR activity.