Unraveling Salinity Extreme Events in Coastal Environments: A Winter Focus on the Bay of Brest

Coline Poppeschi,Loïc Quéméner,Guillaume Charria,Maximilian Unterberger,Paul Tréguer,Emilie Grossteffan,Michel Repecaud,Eric Goberville,Sébastien Theetten,Jean-François Le Roux,Peggy Rimmelin-Maury,Sébastien Petton,Nicolas Barrier

doi:10.3389/fmars.2021.705403

Abstract

Extreme weather events affect coastal marine ecosystems. The increase in intensity and occurrence of such events drive modifications in coastal hydrology and hydrodynamics. Here, focusing on the winter period (from December to March), we investigated multi-decade (2000–2018) changes in the hydrological properties of the Bay of Brest (French Atlantic coast) as an example of the response of a semi-enclosed bay to extreme weather episodes and large-scale atmospheric circulation patterns. The relationships between extreme weather events and severe low salinity conditions (as a proxy for changes in water density) were investigated using high-frequencyin situobservations and high-resolution numerical simulations. The identification of intense episodes was based on the timing, duration, and annual occurrence of extreme events. By examining the interannual variability of extreme low salinity events, we detect a patent influence of local and regional weather conditions on atmospheric and oceanic circulation patterns, precipitation, and river runoff. We revealed that low salinity events in Brittany were controlled by large-scale forcings: they prevailed during the positive phase of the North Atlantic Oscillation and periods of low occurrences of the Atlantic Ridge weather regime. The increase in severe storms observed in western France since 2010 has led to a doubling of the occurrence and duration of extreme low salinity events in Brittany.

Highlights

Coastal zones–including continental shelves–cover only a small fraction of the global ocean, they are key economical (e.g., 90% of the fish catch) and ecological (e.g., 30% of the global primary productivity; Wollast, 1998) regions
To evaluate the link between Bay of Brest hydrodynamics and atmospheric circulation, we investigated four weather regimes associated with surface pressure, wind, air-temperature and precipitation anomalies (Barrier, 2013) and which represent the main largescale atmospheric forcings that act in western Europe (Cassou et al, 2011; Barrier et al, 2014; Tréguer et al, 2014)
When investigating the duration of events, we found that MARS3D–BACH overpredicted the number of extreme low salinity events duration and that MARS3D–MARC underpredicted those durations with 2.4 days on average compared with 9.2 days observed (Figure 5B)

Summary

Introduction

Coastal zones–including continental shelves–cover only a small fraction of the global ocean (about 8%; Yool and Fasham, 2001), they are key economical (e.g., 90% of the fish catch) and ecological (e.g., 30% of the global primary productivity; Wollast, 1998) regions. Changes in large-scale atmospheric variability over the North Atlantic modulate precipitation patterns and hydrological processes in western Europe (Bojarriu and Reverdin, 2002; Msadek and Frankignoul, 2009), which in turn can modify both the properties and dynamics of the water column in coastal ecosystems (Goberville et al, 2010). Previous studies have demonstrated that air masses, that circulate over the North Atlantic Ocean, significantly influence the exchanges between the ocean and the atmosphere, with putative consequences on long-term trends in sea surface waters (such as alternating between warming and cooling episodes) and changes in salinity in coastal and estuarine ecosystems (Chaalali et al, 2013). In western Europe, Tréguer et al (2014) and Charria et al (2020) demonstrated the connection between changes in coastal waters of western Europe in winter and large-scale hydro-climate processes, as well as the influence of local factors, such as river discharge, during the 1998–2013 period

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