A classification of ambient climatic conditions during extreme surge events off Western Europe

Abstract

Principal factor analysis (PFA) of gridded daily mean sea level pressure and 500 hPa height data is used to create a classification of the large-scale ambient climatic conditions associated with severe storm surges in the Irish Sea and the North Sea. The original 100 pressure and 500 hPa height nodes are reduced to seven and four rotated factors, respectively. Each factor represents a single pressure or 500 hPa height system that can readily be combined with others to create complex climate scenarios. Using this property, the factor scores are tested against the dates of 23 known severe surges in the North and Irish Seas and important associated combinations of pressure system identified. A 100 years of data is searched for similar combinations and the results analysed. For the Irish Sea and the North Sea, the 1960s and 1970s indicate levels of surge activity unprecedented since the 1900s. This is followed by a sharp decline in the 1980s, taking the number of surges back to the levels of decades before the 1960s. The ambient pressure conditions for surges in the Irish Sea became more complex during the 1950s, and for the North Sea during the 1940s. This tendency persists to the present and could be a manifestation of shifts in storm tracks. Evidence from independent studies is provided to show that these changes are part of natural variability on decadal time scales rather than a long-term climatic change due to anthropogenic influences. Irish Sea and North Sea surges are associated with ambient conditions dominated by different pressure factors for each sea and with different steering mechanisms aloft. Copyright © 1999 Royal Meteorological Society