Whenever the U.K. is hit by major flooding, attention quickly turns to the performance of the nation’s flood defenses. Some defenses, such as London’s Thames Barrier, are regularly recognized for their vital role in protecting people and property. The value of other mitigation measures, however, has been frequently challenged, such as when defenses failed to prevent significant flooding in Cumbria during storm Desmond in 2015.
Stefano Zanardo, Principal Modeler, RMS
Ludovico Nicotina, Senior Director – Modeling, RMS
Arno Hilberts, Vice President, Model Development, RMS
Steve Jewson, Scientific Research Consultant, RMS
The North Atlantic Oscillation (NAO) describes the fluctuations in the difference of atmospheric pressure at sea level between two semi-permanent centers of low and high pressure in the North Atlantic: the Icelandic Low and the Azores High. Fluctuations between these centers control the strength and direction of westerly winds and location of storm tracks across the North Atlantic.
Why is this important? The NAO signal is Europe’s dominant mode of climate variability and correlates highly with European precipitation patterns. Typically, when the NAO is positive – characterized by a higher than average pressure difference between low and high latitudes of the Northern Hemisphere, Northern Europe experiences strong westerly winds. This causes stormier and wetter than usual conditions in Northern Europe, while Southern Europe is drier and colder than usual.
In contrast, when the NAO is negative, Southern Europe experiences westerly winds and the meteorological pattern is somewhat opposite, with Southern Europe being generally wetter than average. The NAO is significantly stronger in winter than in the other seasons, therefore, most studies on the NAO focus on winter months, when the influence of the NAO on surface temperature and precipitation is highest.
When climate patterns result in changing prevailing conditions, such as increased storm activity and rainfall, it is important to understand their effect in relation to the severity of flood events – responsible for significant property damage, business disruption and loss of life in Europe. And there is a need to understand its ongoing impact as the climate and the distribution of exposures change over time.