Intense coastal rainfall in the Netherlands in response to high sea surface temperatures: analysis of the event of August 2006 from the perspective of a changing climate

August 2006 was an exceptionally wet month in the Netherlands, in particular near the coast where rainfall amounts exceeded 300% of the climatological mean. August 2006 was preceded by an extremely warm July with a monthly mean temperature of almost 1C higher than recorded in any other summer month in the period 1901-2006. This had resulted in very high sea surface temperatures (SSTs) in the North Sea at the end of July. In this paper the contribution of high SSTs to the high rainfall amounts is investigated. In the first part of this study, this is done by analyzing short-term integrations with a regional climate model (RACMO2) operated at 6 km resolution, which are different in the prescribed values of the SSTs. In the second part of the paper the influence of SSTs on rainfall is analyzed statistically on the basis of daily observations in the Netherlands during the period 1958-2006. The results from both the statistical analysis as well as the model integrations show a significant influence of SSTs on precipitation. This influence is particularly strong in the coastal area, that is, less than 30-50 km from the coastline. With favorable atmospheric flow conditions, the analyzed dependency is about +15% increase per degree temperature rise, thereby exceeding the Clausius-Clapeyron relation-which is often used as a temperature related constraint on changes in extreme precipitation-by approximately a factor of two. It is shown that the coastal area has consistently become wetter compared to the inland area since the 1950s. This finding is in agreement with the rather strong observed trend in SST over the same period and the dependencies of rainfall on SST reported in this study.