The influence of the North Sea on coastal precipitation in the Netherlands in the present-day and future climate

Coastal precipitation in the Netherlands for present-day and future climate using observations and simulations with the KNMI regional climate model RACMO2 is studied. Overall a good correspondence between RACMO2 and the observations is found for the present-day climate. Yet, the model generally simulates too sharp gradients in precipitation amounts near the coast line, and the maximum in precipitation is shifted towards the sea compared to the observations. The seasonal cycle of coastal precipitation (here the difference between precipitation near the coast and inland) is well linked to the land sea temperature contrast. Yet, this is not a one-to-one relation and other effects (like atmospheric stability) are likely to be important as well. Variations from year to year in monthly coastal precipitation correlate well with variations in strength of the cyclonic westerly circulation in the periods August to November. A moderate sensitivity to sea surface temperatures (a parts per thousand 6 mm month(-1) K-1) has also been found. In simulations of climate change a small increase in coastal precipitation is generally found, except for the spring period. Using a more realistic prescription of North Sea temperatures (from a slab ocean model) only marginally changes these results. The increase in coastal precipitation is tempered by a decrease in land sea temperature contrast, and considerably larger increases of coastal precipitation could be expected if the temperature rise over sea equalled the temperature rise over land. Despite the fact that the overall changes in coastal effect appear relatively moderate, impact on more extreme events could be considerable, increasing the probability of exceeding extreme thresholds by a factor two or more.