The salinity of the upper ocean (0-800 m) to the west and north of the UK (Region 8) 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.

West of the UK the water of the deep ocean (>1000 m) comes from the Labrador Sea and has freshened since 1975. North of the UK, the deep water (800 m) flows from the Nordic Seas; they have freshened since 1950 but have been stable for the last decade.

In the northern North Sea (Region 1) the salinity is heavily influenced by inflowing North Atlantic water and has become more saline since the 1970s, though the trend is not as clear. The salinity of the southern North Sea (Region 2) is dominated by river run-off balanced with flow through the Dover Strait and there is no clear trend since the 1970s.

The western English Channel (Region 4) is influenced by North Atlantic Water, tidal currents and local weather conditions. There is no discernible long-term trend in over a century of observations, and recent years have been higher than average in salinity.

Since the mid-1960s the salinity of the Irish Sea (Region 5) 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 (Region 6); observed changes in salinity are due to an east-west migration of salinity gradients, with warm periods being associated with higher inshore salinities.

In the future the shelf seas and adjacent ocean may be slightly fresher (less saline) than the present. On the shelf the oceanic influence will dominate the mean long-term salinity.

There remains uncertainty in the causes of large-scale, long-term changes in salinity as there are considerable uncertainties on the effects of climate-driven changes in precipitation, evaporation, ocean circulation and ice-melt.

What is already happening: Medium

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 can be difficult to make an overall assessment of changes in salinity around the UK. However, the variability at the deep ocean 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.

What could happen: Low

There is less confidence in regional climate-change prediction than in global predictions, and less confidence for the hydrological cycle (for salinity) than for temperature. Note that the UKCP projections for salinity present a single run under one climate scenario (medium - Lowe et al., 2009) and they note a lower confidence in salinity in the model.

The top priority knowledge gaps that need to be addressed in the short term to provide better advice to be given to policy makers are:

1. Sparse availability of data The number of deep ocean sites for which long term measurements have been made are small. This problem is being addressed in the deep ocean by the Argo float programme which has been greatly increasing the amount of global subsurface data since the early 2000s. This new data source is helping to reduce some of the difficulties due to sparse observations, but it is important that new floats continue to be deployed in order to maintain the level of sampling required as older floats cease to function. The installation of thermo-salinographs on a number of ferries and voluntary observing ships using UK waters will help to redress the shortfall of surface salinity observations in shelf waters.
2. 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 in the open ocean. 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.
3. The Hydrological Cycle We know that the North Atlantic ocean undergoes large-scale, long-term changes in salinity, and so freshwater content, at all depths. But at present there remains uncertainty in the causes of those changes and their relationship with anthropogenic climate change. Climate models are not able to predict future evolution of the salinity fields with confidence. In particular it is unclear how changes in the hydrological cycle relate to changes in ocean circulation and what the feedback mechanisms might be. Modelling of salinity trends in the UK shelf seas is still being developed, and the accuracy of such models is very much dependent on boundary inputs either from the open ocean or from the river catchments that discharge onto the northwest European shelf.

There has been little research into the direct socio-economic impacts of variations in salinity around the UK; rather, the main focus has been on temperature and sea-level rise. However a recent study has shown that phytoplankton, zooplankton and some commercial fisheries are affected by changes in the circulation of the subpolar gyre (Hatun et al., 2009). Periods of high salinity in the northern North East Atlantic are associated with higher temperatures, while periods of low salinity tend to have cooler temperatures. These opposing scenarios are linked to the strength of the subpolar gyre circulation which determines the balance between subtropical and subarctic water flowing into the region. It has been shown that when the Rockall Trough is more strongly influenced by warm saline subtropical water due to a weak subpolar gyre (as it is presently), phytoplankton abundance is high, the abundance of warm-water zooplankton is high, and the density of blue whiting and pilot whales in the Iceland-Faroe region is high.

As a key component of the ocean climate and dynamics (through its control of density) salinity is partly responsible for driving ocean circulation. The potential socio-economic impacts of large-scale circulation change are considered in the MCCIP ARC Science Review 2010-11 Atlantic Heat Conveyor (Atlantic Meridional Overturning Circulation) (Cunningham et al. 2010).

Holliday, N.P., S.L. Hughes, S. Dye, M. Inall, J. Read, T. Shammon, T. Sherwin & T. Smyth (2010) Salinity in MCCIP Annual Report Card 2010-11, MCCIP Science Review, 10pp. www.mccip.org.uk/arc