In the UK Initial Assessment 2012 (HM Government, 2012), long-term positive salinity trends were observed at the bottom waters of the northern North Sea. From 2011 a sharp decrease in salinity of the adjacent Atlantic waters was measured, which may reduce (or reverse) the long-term increases salinity reported in the previous assessment.

Background

Salinity affects marine ecosystems in many ways. Together salinity and temperature control water density affecting circulation patterns and the distribution and timing of stratification.

Changes in salinity in UK waters are largely influenced by changes in the global circulation and currents driving variability in flows between the North Atlantic and European shelf seas. Salinity is also influenced by changes in the balance between precipitation and evaporation. Salinity in coastal regions can be strongly influenced by seasonal and inter-annual variability in the freshwater input by rivers.

In the UK Initial Assessment (HM Government, 2012) an increase in the salinity of the north-western UK waters was related to a stronger inflow of subtropical waters in the area. Similarly, the deep waters of the northern North Sea were shown to be increasing salinity related to the inflow of these Atlantic waters. The coastal areas in the southern North Sea are more influenced by river inputs and they presented more variability and few clear trends except in the German Bight (beyond UK waters but included for context in the last assessment) where winter conditions were freshening in the presence of large year to year fluctuations. No clear trend in salinity was observed in the Western Channel or in the Irish Seas.

Monitoring Salinity

Here we make the assessment using long-term salinity time series records at the West Channel Observatory, Rockall Trough, Faroe Shetland Channel, Fair Isle, Felixstowe and Helgoland Roads (included for broader context in the southern North Sea) which are published in the ICES Report on Ocean Climate.

The winter bottom salinities are measured in the North Sea by the ICES International Trawl Survey Quarter 1 and shown for the period 1971-2012.

Climate Projections

The UK Climate Projections (2009) suggested that the salinity will slightly decrease in the future in the shelf seas and adjacent ocean (a decrease of approximately 0.2 by 2070-2098). Change is thought to be weaker in the Celtic and Irish Sea (approximately 0.1). The uncertainty of salinity in these projections is very high and will likely change as models develop.

Results

Conclusions

Salinity in UK seas is the result of a combination of adjacent oceanic water and freshwater inputs from rivers and net rainfall minus evaporation. Deep ocean stations around the UK (Rockall Trough and Faroe Shetland Channel) show that the adjacent Atlantic waters have been freshening since 2011. This is likely to alter the long-term trend in salinity observed at the bottom waters of the northern North Sea, however the current analysis is too short to make this a confident finding. For the northern North Sea and southern North Sea observations of salinity find very high year-to-year variability. While no clear long-term trend is evident, conditions in the northern North Sea station were notably fresher than the long-term average in 2013 to 2015.

Knowledge gaps

Salinity data are generally sparse in comparison to temperature data and so confidence in the assessments of long-term trends and spatial variability is lower. Data is particularly scarce in deeper waters both on and off the European shelf. Further under-sampling of seasonal variability adds difficulty for interpreting long-term change. The Argo float programme and installation of thermo-salinographs on ferries and voluntary observing ships will help to reduce this gap for the deep ocean and shelf seas waters, respectively. Emerging technologies, such as autonomous gliders and integrating observations with models will help fill the gaps for future assessments. Uncertainties in shelf sea salinity associated with the supply of freshwater from rivers, rainfall and climate change are areas where our understanding can be improved.

References

HM Government (2012) ‘Marine Strategy Part One: UK Initial Assessment and Good Environmental Status’ (viewed on 5 July 2018)

The information, figures and all the results presented in this assessment has been extracted from the following reports:

ICES Report on Ocean Climate 2015:

Larsen KMH, Gonzalez-Pola C, Fratantoni P, Beszczynska-Möller A, and Hughes SL (Editors) (2016) ‘ICES Report on Ocean Climate 2015’ ICES Cooperative Research Report Number 331, 79 pages (viewed on 14 January 2019)

Charting Progress 2 report for temperature and salinity:

UKMMAS (2010) ‘Temperature and Salinity’ Charting Progress 2 Feeder Report: Ocean Processes, Huthnance J (editor). Department for Environment Food and Rural Affairs on behalf of the UK Marine Science Co-ordination Committee (viewed on 14 January 2019)

UK Marine Climate Change Impacts Partnership Science Review 2013 for salinity:

Dye SR, Holliday NP, Hughes SL, Inall M, Kennington K, Smyth T, Tinker J, Andres O, Beszczynska-Möller A (2013) ‘Climate change impacts on the waters around the UK and Ireland: Salinity’ MCCIP Science Review 2013, 60-66. doi:10.14465/2013.arc07.060-066 (viewed on 14 January 2019)

 

 

Acknowledgements

Assessment Metadata

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