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  • Surface waters to the north and west of the UK have become relatively more saline since the 1970s. There are no clear trends in the shallow coastal waters of the Irish Sea, southern North Sea and western Scotland.
  • Deep waters of the North Atlantic have freshened over the past 40 years.
  • Climate-driven changes in precipitation, evaporation, ocean circulation and ice-melt might influence salinity.

The salinity of the upper ocean (0-800m) to the west and north of the UK has been generally increasing since a fresh period in the 1970s.  A minimum occurred in the mid 1990s, and present day conditions are saline.  The decadal-scale pattern of change around the UK reflects the mean conditions of the North Atlantic and Nordic Seas, which has evolved from a maximum in the early 1960s and a minimum in the mid 1990s.

West of the UK the water of the deep oceanThe part of the ocean that does not cover the continental shelf margins (the shallower water adjacent to land masses). (>1000m) comes from the Labrador SeaA region of the North Atlantic Ocean located between southwest Greenland and northeast Canada. It is one of two main locations where cold, dense surface water sinks to produce south-flowing North Atlantic Deep Water, the other location being the Greenland Sea. and has freshened since 1975.  North of the UK, the deep water (800m) flows come from the Nordic Seas; they have freshened since 1950 but have been stable for the last decade.

In the northern North Sea the salinity is heavily influenced by inflowing North Atlantic water and has become more saline since the 1970s.  The salinity of the southern North Sea is dominated by river run-off and there is no clear trend since the 1970s.

Since the mid-1960s the salinity of the Irish Sea shows no significant long-term trend.  The decadal pattern is different to the deep offshore water; maxima occurred in the late 1970s and late 1990s; present conditions are close to the long-term mean.

There is no clear trend in the shelf waters off the west coast of Scotland; observed changes in salinity are due to an east-west migration of salinity gradients, with warm periods being associated with higher inshore salinities.

What is already happening - Medium
(consensus=high, evidence=moderate)

What could happen in the future - Low
(consensus=low, evidence=low).

Measurements of salinity at offshore sites are made 1-3 times per year, under-sampling the seasonal cycle which may alias the results . Shelf sea and coastal stations are sampled more frequently (up to daily), so the seasonal cycle is usually better resolved. Calibration is good (although data prior to 1970 are less reliable), so high confidence can be put on actual measurements.

The number of sites for which long-term records exist are limited, so it is difficult to make an overall assessment of changes in salinity around the UK.  However, the variability at the deep oceanThe part of the ocean that does not cover the continental shelf margins (the shallower water adjacent to land masses). sites on time scales of years to decades are consistent across the region and with the North Atlantic region, giving us overall moderate confidence in the results.

Sparse availability of data

The number of deep oceanThe part of the ocean that does not cover the continental shelf margins (the shallower water adjacent to land masses). sites for which long term measurements have been made are small.  This problem is being addressed in the  deep ocean by the Argo floatFree-drifting profiling floats that measure the temperature and salinity of the upper 2000m of the ocean. Argo floats are able to surface and relay observations directly to receiving stations. Globally, there are ~3000 total floats deployed across all of the world's major oceans. programme, which has greatly increased the amount of subsurface data since the early 2000s.  The  free-ranging instruments are programmed to float at a depth of 2000m, carried along by the ocean currents. Every 2 weeks they rise  to  the  surface  to  report  their  new  location and the temperature and salinity of the water they rose through. This  new  data  source  across  the  global  ocean,  combined  with  numerical models, will  in  time  reduce  some  of  the  difficulties  due  to  sparse  observations.  The  recent installation of thermosalinographsAn oceanographic instrument, typically attached to ships, designed to measure the temperature and salinity of a body of water. on a number of ferries and voluntary observing shipsThe World Meteorological Organisation's Voluntary Observing Ships programme is a scheme recruiting ocean-going vessels to collect and report meteorological observations. At present there are approximately 4000 ships involved in the programme. using UK waters should help to redress the shortfall of surface salinity observations in shelf waters.

Under-sampling of the seasonal cycle.

The surface and upper layers of the ocean exhibit a strong seasonal salinity cycle with an amplitude greater than longer-term changes.  Usually surface salinity is higher in the winter.  When looking at long term variability in time series that are sampled only 1-3 times per year, we need to take account of the season in which the measurements were made.  The under-sampling of the open ocean seasonal cycle and how it may be changing over time is a major uncertainty for interpreting long term changes.  The key to resolving the open ocean signal cycle lies in assimilating temperature and salinity from profiling floats and other devices into numerical models.  The models can fill the gaps between the data points, and the data can keep the model close to reality.  Progress is being made with developing this technique.

Not stated.

Penny Holliday
National Oceanography Centre, Southampton, European Way, Southampton, SO14 3ZH

Sarah Hughes
Fisheries Research Services, FRS Marine Laboratory, PO Box 101, 375 Victoria Road, Aberdeen, AB11 9DB.

Theresa Shammon
The Isle of Man Government Laboratory, Ballakermeen Road, Douglas, IM1 4BR, Isle of Man.

Toby Sherwin
The Scottish Association for Marine Science (SAMS), Dunstaffnage Marine Laboratory, Oban, Argyll, PA37 1QA