Occurrence of meteorological summer dry spells and dry days in northern Europe during the 20th century

Abstract

<div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> Abstract</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> In spite of the relatively humid climate of Northern Europe, prolonged meteorological dry</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> spells do occasionally cause problems for the water supply in different sectors of society.</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> During recent decades, total annual precipitation has increased in the region, especially</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> during winter. A linear change in total precipitation does not necessarily indicate a change in</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> the occurrence of meteorological drought across different time scales. In this study,</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> temporal changes of meteorological summer (May-August) dry spells (MDS) and dry days</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> (MDD) are analysed using measured precipitation observations from 12 weather stations</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> located around Northern Europe. The statistics studied are the number of MDDs (&lt;1.0 and</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> &lt;0.1 mm) per selected periods, plus the lengths of the longest MDSs during which the total</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> accumulated precipitation remains under certain thresholds, namely 10 and 100 mm. The</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> results suggest that, in general, the lengths of the longest MDSs and the numbers of MDDs</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> do not differ remarkably between the stations, median value being 26/80 days (&lt;10/&lt;100</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> mm rain) and 87/70 days (&lt;1.0/&lt;0.1 mm/day), respectively. A distinct exception is Bergen,</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> in Norway, where the lengths of the longest MDSs are shorter (19 and 41 days, on average)</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> and the numbers of MDDs lower (ca. 64 and 50 days) than at the other stations. During the</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> period of homogeneous instrumental precipitation observations, the occurrence of summer</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> MDSs and MDD have remained the same at most of the stations. Only a few statistically</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> significant increasing temporal trends appear in the time series of MDDs in the southern</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> parts of the region. In the north, one statistically significant decreasing trend has been</div> <div id="_mcePaste" style="position: absolute; left: -10000px; top: 0px; width: 1px; height: 1px; overflow: hidden;"> detected.</div> <div> In spite of the relatively humid climate of Northern Europe, prolonged meteorological dry spells do occasionally cause problems for the water supply in different sectors of society. During recent decades, total annual precipitation has increased in the region, especially during winter. A linear change in total precipitation does not necessarily indicate a change in the occurrence of meteorological drought across different time scales. In this study, temporal changes of meteorological summer (May-August) dry spells (MDS) and dry days (MDD) are analysed using measured precipitation observations from 12 weather stations located around Northern Europe. The statistics studied are the number of MDDs (&lt;1.0 and &lt;0.1 mm) per selected periods, plus the lengths of the longest MDSs during which the total accumulated precipitation remains under certain thresholds, namely 10 and 100 mm. The results suggest that, in general, the lengths of the longest MDSs and the numbers of MDDs do not differ remarkably between the stations, median value being 26/80 days (&lt;10/&lt;100 mm rain) and 87/70 days (&lt;1.0/&lt;0.1 mm/day), respectively. A distinct exception is Bergen, in Norway, where the lengths of the longest MDSs are shorter (19 and 41 days, on average) and the numbers of MDDs lower (ca. 64 and 50 days) than at the other stations. During the period of homogeneous instrumental precipitation observations, the occurrence of summer MDSs and MDD have remained the same at most of the stations. Only a few statistically significant increasing temporal trends appear in the time series of MDDs in the southern parts of the region. In the north, one statistically significant decreasing trend has been detected.</div>