The impact of serial cyclone clustering on extremely high sea levels in the Baltic Sea

Extremely high sea levels in the Baltic Sea and the resulting coastal flooding events are typically caused by the passage of extratropical cyclones (ETCs). ETCs raise the sea level by their strong winds and low atmospheric pressure. The role of individual ETCs for the extreme sea levels in the Baltic Sea has been studied in some case studies, but less attention has been paid to the serial clustering of ETCs (SCC, the passage of multiple cyclones within a short period of time). In particular, little research has been done on whether extremely high sea level events in the Baltic Sea are typically caused by just one extreme ETC or by the combined effect of several consecutive ETCs. In this study, the role of SCC for the extreme sea levels in the Baltic Sea is investigated. 

We use objectively determined cyclone tracks from ERA5 reanalysis and sea level observations from four tide gauges: Kemi (Finland), Helsinki (Finland), Pärnu (Estonia) and Riga (Latvia). All ETCs from the October-March period in 1980-2019 that pass within 700 km of the tide gauge are included in the analysis. Clustering and non-clustering days are defined as days when the 7-day running sum of ETCs is ≥ 3 and 1, respectively. Then, SCC and single cyclone periods are defined by adding ±1 day to the dates of the first and last ETC around the clustering and non-clustering days.

We find that SCC periods tend to produce on average 30 cm higher sea level than the climatology at the tide gauges, and about six days after the SCC onset dates. In addition, the daily maximum sea level peaks about 40 cm higher during SCC periods than during single cyclone periods. Thus, this result implies that the SCC periods are typically associated with higher sea level than the periods when only one intense ETC passes the tide gauge. However, when we look at the extreme sea level events at the tide gauges, we find that in Helsinki, Pärnu and Riga about 40 % of the events can be attributed to the SCC periods and about 15 % to the periods when only single ETC passes the tide gauge.

Our results demonstrate that serial cyclone clustering is an important phenomenon for the occurrence of extreme sea levels in the Baltic Sea, and in fact relatively few extreme sea levels and the associated coastal flooding events in the Baltic Sea are caused by only one single ETC.