Ocean temperature, salinity and circulation affect marine ecosystems in many ways. Some species are sensitive to temperature and/or salinity; circulation and currents distribute salt, deep-ocean heat and pollutants; currents affect habitats; many species are carried by the flow during their life cycle. Temperature and salinity control water density, which drives its motion in tandem with tides and winds. In return, circulation patterns and currents influence the temperature and salinity of the UK seas. From above, the atmosphere provides warming and cooling and changes the amount of freshwater arriving into the sea, through the balance of precipitation and evaporation as well as via rivers. For the shelf seas, the physical properties of the water column are controlled by a balance between mixing by tides and winds and buoyancy changes through warming, cooling and changes in salinity.

We have assessed variability and trends in these factors using time-series that span several decades. Temperature and salinity data come from: volunteer observing ships, drifting and moored buoys, repeated cross-sections measured from ships, bottom trawl surveys, coastal stations, and satellite radiometers. There are also some important recent developments. Over the past ten years the international Argo programme has established a global array of 3000 free-drifting profiling floats, measuring temperature and salinity between the surface and 2000 m depth; these now provide essential monitoring data in deeper waters west of the British Isles. The Natural Environment Research Council (NERC) and The Department for Environment, Food and Rural Affairs (Defra) have supported FerryBoxes on some ferries (see photo), and long-term series in the western English Channel,the Isle of Man and Liverpool Bay.

Figure 2.1 Annual average sea surface temperature (SST) for the UK seas, between 1870 and 2007. The 1961–1990 average was 11.09 °C. The red line smoothes out fluctuations shorter than 5 to 10 years to illustrate the longer-term trend.

Temperature

Globally, sea surface temperatures rose by about 0.3 °C from around 1910 to 1940, remained steady until the 1970s and have then risen again by about another 0.4 °C. Since the mid-1980s, Atlantic surface waters adjacent to the UK have warmed by between 0.5 and 1 °C. Superimposed on this background trend are seasonal cycles of many degrees, variations between regions, and interannual-to-decadal variability (for any season) of 0.5 to 2 °C.

In shallower UK shelf seas, mixing of water masses and especially local weather largely control the temperature, on timescales of a day (for 1 m water depth) to a few months (for 100 m water depth). There is also some influence from adjacent Atlantic water where it moves onto the shelf. The annual sea surface temperature, averaged around the UK coastline, has increased by about 0.5 to 1 °C for the period 1870 to 2007 (Figure 2.1). Much of the warming took place in the 1920s and 1930s and again since the mid-1980s; this later warming was especially pronounced in the Southern North Sea, Irish Sea and the Minches and Western Scotland. Spatial and interannual temperature variability in UK waters is of the order of 0.5 °C; but can be up to 2 to 3 °C in shallow areas for an extreme month.

Figure 2.2 Schematic of the North Atlantic ocean circulation showing the northward transport of relatively warm surface water (red/orange) and the southward return of cooler, subsurface water (blue). The Gulf Stream is a component to the overall circulation. Note the partial recirculation of northward-flowing water by the subtropical and sub-polar gyres before water enters the far North Atlantic.

Salinity

Salinity is influenced primarily by Atlantic water, slightly by rainfall and evaporation, and locally by the influx of fresher water from rivers via estuaries; values are usually between 34 and 35.6 in salinity units. Atlantic waters adjacent to the UK have experienced an increase in salinity of 0.05 to 0.1 units since the late 1970s and this in turn has caused a salinity rise in the nearby UK shelf waters. The picture is rendered more complex by spatial and interannual-to-decadal variability, of up to 0.1 in salinity. Irish Sea salinities are especially variable; they are typically between 34 and 35 in the west but sometimes as low as 31 approaching the English coast where freshwater inputs are relatively important. Typically salinity is most variable, with potential impacts on biota, near the head of an estuary where the fresh-salty water transition may move according to river flow and stage in tidal cycles.

Circulation

North-East Atlantic temperature and salinity are controlled by the large-scale circulation (Figure 2.2) and history of these waters. The Atlantic Meridional Overturning Circulation (AMOC) brings warm surface water past the west of the UK, strongly influencing our climate by warming the prevailing westerly airflow. Instantaneous currents in UK shelf seas comprise tidal flows, wind-driven flows and flows driven by differences in density that arise from seasonal heating and salinity differences. ‘Residual’ flow, after averaging out oscillatory tidal flow, is mainly driven by winds and by density differences in many areas. Tides, winds and density all change on a range of timescales, so that observed and residual flows can be very variable. On the shelf, transport of water in a single storm can be significant relative to a year’s total.

We have assessed the long-term circulation in the adjacent North Atlantic using tracks of drifters and Argo floats, and in shallower UK waters using distributions of tracers, drifter tracks or numerical hydrodynamic models. We have also used data from current-meter measurements in a few long-term mooring arrays and from submarine cables. For components with timescales longer than a day, we inferred circulation from ship-based temperature and salinity measurements. High Frequency radar gives spatial coverage for surface currents, although the range is limited to the order of 50 to 100 km. A recent development is the NERC-funded RAPID programme which maintains an array of moored sensors to study the sub-surface temperature and salinity distribution, and hence monitor transport of the AMOC, across a section of the Atlantic Ocean at 26° N where it is strongest.

Five ship-based cross-sections of the Atlantic near 24° N suggest that the AMOC declined in strength from 1957 to 2004. However, continuous measurements starting in 2004 show this to be within the range of variability on timescales of weeks to months, so that we cannot be sure of an overall trend. Deep outflows of cold water from Arctic seas are likewise too variable to infer any trend.

Future monitoring of the Atlantic circulation in RAPID extends to 2014; this will help to clarify variability and the statistical confidence in any trend. However, changes in circulation at 26° N have proved hard to relate to patterns of sea surface temperature (Figure 2.3), or to circulation at higher latitudes, where AMOC correlation with surface heat fluxes is suggested by models. Other measurements, especially at higher latitudes, may help us to understand how changes in the AMOC are relayed from place to place and possibly to establish proxies for easier monitoring.