Abstract
Simultaneous heavy precipitation with high sea level can lead to more severe flooding than if the hazards occur individually. In recent years, the significance of compound flooding has gained attention in many coastal areas, but studies on compound flooding in Finland are still lacking. In this paper, we investigate the co-occurrence of heavy precipitation and high sea level events (hereafter compound events) on the Finnish coast, because the simultaneous occurrence of these events can be considered as an indicator for compound flooding. We use daily sea level observations from tide gauges and gridded precipitation data to extract the days when both sea level and precipitation are extreme on the same day. Reanalysis data is used to derive composite maps of the compound events, and these maps are then qualitatively compared to the maps of non-compound events (i.e. when only one of the two variables is extreme alone). Finally, the dependence of the compound events on monthly indices of different atmospheric circulation patterns such as North Atlantic Oscillation is studied with correlation analysis. Our results show that the occurrence of the compound events is mostly controlled by sea level variability, with climatologically the most compound events taking place in autumn and winter. In addition to a large inter-annual variability of the compound events, statistically significant increasing trends were observed at some tide gauges, especially in the Bothnian Bay. The composite maps of meteorological variables indicate that the compound events are associated with strong extratropical cyclones which bring moisture and push the storm surge towards the coast. In average, the compound events are associated with stronger winds near the tide gauges than those events when only sea level or precipitation is extreme alone. Finally, we found that a negative phase of Scandinavian pattern (i.e. upper-level through over Scandinavia) is the most favorable atmospheric circulation pattern for the occurrence of compound events, in particular during autumn.
Highlights
The frequency of extreme weather events has traditionally been investigated for single hazard events
We investigate the co-occurrence of heavy precipitation and high sea level events on the Finnish coast, because the simultaneous occurrence of these events 5 can be considered as an indicator for compound flooding
When calculating the numerical values corresponding to the percentiles of precipitation, days with no precipitation were taken into account. Both elevated level (EL) and high level (HL) thresholds were defined separately for each tide gauge, and the actual numerical values corresponding to the thresholds range from 44 (55) to 66 (86) cm for EL (HL) sea level and 6.7 (10.3) to 9.0 (13.3 ) mm/day for EL (HL) 70 km x 70 km spatial average precipitation amounts (Table 1). 170 The joint occurrence of the events were obtained by counting the number of times when both precipitation and maximum sea level exceed the percentile thresholds on the same day
Summary
The frequency of extreme weather events has traditionally been investigated for single hazard events. This can po tentially underestimate the risk of a harmful weather event and its impacts, because climate drivers causing extreme weather events often interact and are dependent on each other, spatially or temporally. The co-occurrence of anomalous weather events, often called compound weather events, is a result of a combination of two or more variables which are not necessarily extreme themselves (Seneviratne et al, 2012; Leonard et al, 2014; Zscheischler et al, 2020).
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