Severe Local Coastal Storms: New Perspectives on Frequency, Intensity and Impacts

There are significant gaps in our knowledge with regard to the frequency, intensity and associated physical impacts of severe local coastal storms. This creates further uncertainties when associated with a changing climate. Severe local coastal storms affecting the southern county of Dorset, United Kingdom, have been investigated in detail with the aid of a new customized metadata repository designed to facilitate extensive secondary data collection and provide a medium to analyze fundamental relationships between these data.

A measure of intensity was developed in order to facilitate the identification of local coastal storm types. The Coastal Impacts Intensity Scale (CIIS) provides guidance and classification on storms producing physical impacts for a given range of forcing parameters and possible outcomes. Therefore, local coastal storms producing physical impacts can be classified using a numeric intensity value. The CIIS is a refined grading scale based on a number of factors including theoretical wind speeds, atmospheric pressure, storm duration, sea states and return periods. A scale from 1-9 represents, Light Coastal Storms (CIIS1); Mild Coastal Storms (CIIS2); Moderate Coastal Storms (CIIS3); Strong Coastal Storms (CIIS4); Severe Coastal Storms (CIIS5); Intense Coastal Storms (CIIS6); Extensive Coastal Storms (CIIS7); Extreme Coastal Storms (CIIS 8) and Super Coastal Storms (CIIS9).

In total, 251 severe local coastal storms were analyzed for a 140 year period between 1865 and 2005. There was no CIIS 8 or 9 local coastal storms affecting the study region, with a 1% occurrence of CIIS7 events. The most common were those of CIIS3 with a 29% occurrence level and CIIS4 at 24%. As storm intensity increases above CIIS4 the number of events decline, with the highest ranking event at CIIS7 indicating a wind speed range of 96-110 mph (155-177 km/h) and a local atmospheric pressure range of 975-965 mb.

Twelve of the fifteen most intense storms occurred post-1965 and results showed that the number of local coastal storms have increased in intensity in recent decades with the most intense occurring in 1987 (October 6th) and 1998 (January 3rd). A key turning point in intensity was identified between 1935 and 1942. Prior to 1940 there were a greater number of less intense storms. Whilst the number can be partly explained by reporting and increased vulnerability, the intensity suggests that more damaging severe local coastal storms are affecting the coast in these years. Increased local coastal storm intensity is closely linked to recent changes in local atmospheric pressure depth of these storms. The frequency of deeper pressure systems has increased over the last 140 years with local low pressure less than 965 mb occurring in the series for the first time post-1985. A swing in increased local coastal storm activity from October-December to that of January-March was identified.

Any reduction in storm frequency in the north of the United Kingdom and an increase in the south is consistent with a southerly movement of the North Atlantic storm track. Whilst severe local storms are more likely related to local gradients of pressure than to large-pressure differences over the Atlantic, it is also possible they are influenced and modified by the long-term changes on the large-scale. The progressive intensity result could be part of a natural multi-decadal oscillation of the storm climate or indeed, evidence towards anthropogenic influences on the climate.