North Atlantic storm driving of extreme wave heights in the North Sea

The relationship between storms and extreme ocean waves in the North Sea is assessed using a long-period wave data set and storms identified in the Interim ECMWF Re-Analysis (ERA-Interim). An ensemble sensitivity analysis is used to provide information on the spatial and temporal forcing from mean sea-level pressure and surface wind associated with extreme ocean wave height responses. Extreme ocean waves in the central North Sea arise due to intense extratropical cyclone winds from either the cold conveyor belt (northerly-wind events) or the warm conveyor belt (southerly-wind events). The largest wave heights are associated with northerly-wind events which tend to have stronger wind speeds and occur as the cold conveyor belt wraps rearward round the cyclone to the cold side of the warm front. The northerly-wind events provide a larger fetch to the central North Sea to aid wave growth. Southerly-wind events are associated with the warm conveyor belts of intense extratropical cyclones that develop in the left upper tropospheric jet exit region. Ensemble sensitivity analysis can provide early warning of extreme wave events by demonstrating a relationship between wave height and high pressure to the west of the British Isles for northerly-wind events 48 h prior. Southerly-wind extreme events demonstrate sensitivity to low pressure to the west of the British Isles 36 h prior. Plain Language Summary This study investigates how large-scale storms, known as extratropical cyclones, are related to extreme wave events in the North Sea. The North Sea houses oil and gas platforms as well as provides passage to cruise ships, thus, extreme wave events can impact the safety of ongoing operations in these industries. For this study, we examined 20 years of data to determine what storms led to extreme wave events and what characteristics those storms, or the environment that they are in, have in common. We found that in the two days prior to a storm which results in an extreme wave event, the strengthening of a high pressure system to the west of the British Isles was key. These new findings can improve warnings of extreme wave events in the North Sea and therefore enhance safety for industry operations.