Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Temperature extremes not only directly affect the marine environment and ecosystems but also indirectly influence hydrodynamics and marine life. In this study, the role of heat wave events in the occurrence and persistence of thermal stratification was analysed by simulating the water temperature of the North Sea from 2011 to 2018 using a fully coupled hydrodynamic and wave model within the framework of the Geesthacht Coupled cOAstal model SysTem (GCOAST). The model results were assessed against reprocessed satellite data and in situ observations from field campaigns and fixed Marine Environmental Monitoring Network (MARNET) stations. To quantify the degree of stratification, the potential energy anomaly throughout the water column was calculated. The air temperatures and potential energy anomalies in the North Sea (excluding the Norwegian Trench and the area south of 54<span class="inline-formula"><sup>∘</sup></span> N) were linearly correlated. Different from the northern North Sea, where the water column is stratified in the warm season each year, the southern North Sea is seasonally stratified in years when a heat wave occurs. The influences of heat waves on the occurrence of summer stratification in the southern North Sea are mainly in the form of two aspects, i.e. a rapid rise in sea surface temperature at the early stage of the heat wave period and a higher water temperature during summer than the multiyear mean. Another factor that enhances the thermal stratification in summer is the memory of the water column to cold spells earlier in the year. Differences between the seasonally stratified northern North Sea and the heat wave-induced stratified southern North Sea were ultimately attributed to changes in water depth.

Highlights

  • The increased number of extreme events with the global climate change has attracted more attention from research in regional and global earth systems (IPCC, 2012; Herring et al, 2015)

  • At MARNET station NSB III, which is located in the centre of the eastern part of the southern North Sea, the model data followed the seasonal cycles of the temporal variations in the observed air temperature from 2011 to 2018

  • This study investigated the influence of extreme climate events, i.e., European heatwaves, on the occurrence of summer stratification in the North Sea

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Summary

Introduction

The increased number of extreme events with the global climate change has attracted more attention from research in regional and global earth systems (IPCC, 2012; Herring et al, 2015). Summer heatwaves are extreme meteorological events that frequently occur (Perkins and Alexander, 2013). They cause anomalously warm seawater in discrete periods via local air-sea heat flux exchanges, which are known as marine heatwaves (MHWs) (Pearce et al, 2011; Hobday et al, 20 2016). MHWs have been identified with a trend of increasing intensification (Oliver et al, 2020). Over the last two decades, record-breaking MHW events have been reported in the Mediterranean Sea (Bensoussan et al, 2010), Tasman Sea (Oliver et al, 2017; Perkins-Kirkpatrick et al, 2019), west Australia (Feng et al, 2013), northeast Pacific (Hu et al, 2017), western South Atlantic (Manta et al, 2018) and East China Sea (Tan and Cai, 2018).

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