The combination of two extreme events, like heavy local precipitation and high surge levels, can have a more devastating impact than their single-hazard equivalents. Researchers from R & D Weather and Climate Models at KNMI shed light on atmospheric rivers, one of the mechanisms causing these "compound events".
The study shows that atmospheric rivers are a useful tool for the early identification of possible harmful meteorological conditions over the Netherlands.
A solid understanding of the processes leading to a compound event, and their interactions, is crucial to understand the future development of flood risk under a changing climate. Researchers at KNMI (Nina Ridder, Hylke de Vries, and Sybren Drijfhout) have focused on the large-scale climatologic conditions leading to the simultaneous occurrence of heavy precipitation and high surge levels. In particular, they have focused on atmospheric rivers. An AR is a long, narrow regions of high water vapour content in the atmosphere that transport moisture to the high latitudes (Figure 1). When they reach land, ARs are often associated with heavy rainfall. Atmospheric rivers (AR) play a significant role in the simultaneous occurrence of high coastal water levels and heavy precipitation in the Netherlands.
"We find that the majority of compound event between 1979 and 2015 have been accompanied by the presence of an atmospheric river over the Netherlands," says lead author Dr Nina Ridder.
The study shows that compound events have a 3 to 4 times higher chance of occurring on days with an AR over the Netherlands compared to any random day. In contrast, the occurrence of a compound event on a day without AR is 3 times less likely than on any random day.
By isolating and assessing the prevailing sea level pressure (SLP) and sea surface temperature (SST) conditions with and without AR involvement up to 7 days before the events, the study shows that the presence of an atmospheric river constitutes a specific type of forcing conditions that (i) resemble the SLP anomaly patterns during the positive phase of the North Atlantic Oscillation (NAO+) with a north–south pressure dipole over the North Atlantic (Figure 2b) and (ii) cause a cooling of the North Atlantic subpolar gyre and eastern boundary upwelling zone while warming the western boundary of the North Atlantic. These conditions are clearly distinguishable from those during compound events without the influence of an atmospheric river which occur under SLP conditions resembling the East Atlantic pattern with a west–east pressure dipole over northern Europe and are accompanied by a cooling of the West Atlantic (Figure 2d).
Ridder, N., de Vries, H., and Drijfhout, S.: The role of atmospheric rivers in compound events consisting of heavy precipitation and high storm surges along the Dutch coast, Nat. Hazards Earth Syst. Sci., 18, 3311-3326, https://doi.org/10.5194/nhess-18-3311-2018, 2018.