OSPAR / QSR 2000 Chapter 2 - Geography, hydrography and climate

2.1 Introduction

This chapter defines the principal physical characteristics of the OSPAR maritime area. It forms the basis for more detailed descriptions of the chemical and biological properties of the area (in Chapters 4 and 5) and the impact of human activities (in Chapter 3).

The North Atlantic circulation forms part of the global ocean circulation system that has been called the ‘Ocean Conveyor’ by oceanographers. Warm, salty, nutrient-rich surface waters flow north through the Atlantic at a rate more than a hundred times that of the Amazon River. They then sink to the depths of the Greenland and Labrador Seas, and return to the Southern Ocean at two to three kilometres below the surface as ‘North Atlantic Deep Water’. The warm surface waters release heat into the cold northern atmosphere at a rate equivalent to a hundred times the world’s energy consumption sufficient to warm the air over Europe by about 5 °C.

2.2 Definition of the OSPAR Convention area

Together, the five regions of the OSPAR Convention area cover most of the North-East Atlantic Ocean (Figure 2.1, having a surface area of about 13.5 x 10⁶ km² and a volume of approximately 30 x 10⁶ km³. However, because the northern North Atlantic is relatively narrow and shallow, these figures represent only ~ 4% of the surface area and ~ 2% of the volume of the world’s oceans. The southern and northern limits of the area are the 36º N parallel and the North Pole respectively. The 42º W meridian, the Atlantic coast of Europe and the 51º E meridian in the Arctic Ocean form the other borders of the area.

Figure 2.1 The OSPAR maritime area covers an overall oceanic area of about 13.5 x 10⁶ km² and has a volume of 30 x 10⁶ km³ .

2.3 Bottom topography

The major topographical features in the OSPAR maritime area are the Mid-Atlantic Ridge (with the Azores and Iceland as its highest points), and the Greenland-Scotland Ridge (which separates the Atlantic Basin from the Nordic Seas) (Figure 2.2). Water depths range from around 5000 m on either side of the Mid-Atlantic Ridge, to less than 200 m on the continental shelf along the European coast. In some places seamounts occur as submerged single mountains or chains of mountains along the ocean floor.

The most extensive continental shelf areas are found in the North Sea and Celtic Seas. Other shelf seas are found around Iceland, Greenland and in the Barents Sea. In contrast, along the Iberian coast and to the west of Norway, the shelf break is quite close to the coastline (Figure 2.2).

Figure 2.2 Bottom topography. Source of data: ETOPO5 database, National Geophysical Data Centre, US.

2.4 Geology and sediments

The North Atlantic began to form approximately 200 million years ago as the European and North American plates separated either side of the active mid-ocean ridge. The current rate of spreading due to formation of basaltic oceanic crust at the mid-ocean ridge is ~ 2 cm/yr. The OSPAR area can be divided into three distinct geological regimes: the oceanic basin and the continental shelf, separated at the shelf break by the passive continental margin.

In the deep ocean basin an abyssal plain extends either side of the Mid-Atlantic Ridge to the continental margins consisting of a 4 – 6 km thick basaltic basement overlain by 0.1 – 2 km thick accumulations of sediment. The sediment (pelagic ooze) consists largely of the remains of microscopic organisms (mostly foraminifera and diatoms) from the overlying waters, as well as minor amounts of windblown atmospheric dust and turbidity currents.

At the continental margins huge wedges of sandy to muddy sediments extend down into the deep-sea basin. These are the result of submarine landslides (turbidity flows). Owing to the intermittent nature of these events, the deposits (which can be up to 10 km thick and 700 km in length) consist of interbedded terrigenous muds and pelagic sediments. These deposits are sometimes rich in hydrocarbons.

The continental crust is generally 30 – 40 km thick, thinning below the sedimentary basins. It has a varied composition of igneous, metamorphic and sedimentary rocks as a result of a succession of separate tectonic periods. Underlying the whole NW European Shelf is the Pre-Cambrian basement (> 600 million yr). Oil is found in the Jurassic sediments of the central North Sea graben and in fractured Tertiary strata of the Faroe-Shetland Basin. Shelf sea sediments are of mainly terrestrial origin. In northern latitudes, including the North Sea and Irish Sea, much of the sea floor sediment is of relict glacial or periglacial origin.

2.5 Description of the coastal margin

A diverse mixture of coastal landscapes is found in the OSPAR area, although generally the western margins are deeply indented with fjords, estuaries and rias and, in northern latitudes, the coast is dominated by high mountains. Around the North Sea and Celtic Sea, the coast exhibits a range of features, including cliffs of varying heights and rock types, bays and estuaries, sandy and shingle beaches, dunes and island archipelagos. Further south, the French coast of the Bay of Biscay is low-lying with lagoons. The Iberian coast comprises alternating cliffs and beaches, while cliffs predominate on the oceanic islands such as the Azores, Iceland and the Faroe Islands.

2.6 Estuaries, fjords, rias and wetlands

Many rivers do not discharge directly to the sea, but flow out through an estuary. In Greenland and Norway in particular glaciers formed deep fjords during the Quaternary period. Further south in the OSPAR area, for example in south-west Ireland and on the north-western Spanish coast, drowned river valleys, or rias, were formed by rising sea levels. Wetlands, areas of land either seasonally or permanently waterlogged, occur predominantly around the North Sea coast and on the west coast of France. Wetlands are highly productive areas. An estuarine circulation pattern, with lighter brackish water flowing seawards at the surface and intruding salt water below, is a common feature of all these areas.

2.7 Catchment area and freshwater run-off

The catchment areas and run-off of rivers discharging into Regions I to V (Figure 2.3) are given in Table 2.1. The total catchment of the Convention area covers approximately 5 140 000 km², made up of:

Region I: the Norwegian coast north of 62° N, the Fennoscandian and Kola peninsulas and Arctic islands (including Svalbard), Iceland, Greenland and the Russian rivers Pechora and Dvina;

Region II: catchments draining directly into the North Sea and, indirectly, through the Baltic outflow from a large Baltic catchment;

Region III: the western part of the United Kingdom (including Northern Ireland) and Ireland;

Region IV: the Franco-Iberian area; and

Region V: the Azores.

Mean total river run-off into each Region ranges from a negligible input to Region V to ~ 25 000 m³/s to Region III, but may vary considerably between years. Much of the Arctic region is covered by permafrost and ice and run-off to these areas is relatively low. However, huge amounts of freshwater enter the sea by calving from glaciers from the Greenland ice sheet. Data are also included in Table 2.1 for run-off from the major Siberian rivers – Lena, Ob and Yenisey – which drain into the Arctic and can have a considerable influence on the hydrography and ecosystems of Region I.

Figure 2.3 Catchment areas of the OSPAR Regions and the Baltic Sea. The inset shows the major rivers discharging to the Russian Arctic (see Table 2.1).

Table 2.1 Catchment areas and river run-off.

Box 2.1
A water mass is defined as a large body of water with a distinctive set of properties, typically identified by its temperature and salinity. A newly formed water mass will mix and sink to a given equilibrium depth, depending on its density relative to that of the surrounding waters.
Water density is determined by water temperature and salinity. Atmospheric cooling or heating may change the temperature of the water mass, while precipitation, evaporation, run-off and ice melt can modify its salinity. If the density of a water mass is increased, it sinks, often along a front. As it sinks, it mixes with surrounding water until it reaches a depth where its density is equal to that of the surrounding water.
Vertical mixing can occur in a variety of ways. For example, winds play a key role and, if they are directed along the shore, may generate coastal upwelling. On the shelf, tidal movements are important in mixing bottom waters and, depending on the depth and strength of the currents, can influence the development of thermally stratified waters.

2.8 Water masses

Sea water properties can vary significantly and play a major role in the hydrography of the oceans (Box 2.1).

In Region I, different water masses represent surface and upper layer waters. These include two warm, high-salinity water masses, which enter from the Atlantic. On entering the Nordic Seas, these waters have temperatures ranging from 7 °C to 10 °C and salinities mainly between 35.1 and 35.4. Within the region, the Atlantic water is cooled and diluted and may change considerably in character. ‘Norwegian Coastal Water’ enters the region from the south-east; its salinity increases to a maximum around the Lofoten Islands. While there is a general northward temperature decrease, the seasonal temperature fluctuations are large. ‘Polar Water’ is formed in the Arctic Ocean where it occupies the 30 – 50 m thick surface mixed layer. ‘Arctic Surface Water’ is observed in the upper layers of the central Greenland and Iceland Seas. The Polar Front separates the warm Atlantic water from the cold northern water masses.

In Region II, water originates from the North Atlantic and from freshwater run-off in different admixtures. The salinity and temperature characteristics of different parts of the region are strongly influenced by heat exchange with the atmosphere and by local freshwater supply. The deeper waters of the North Sea consist of relatively pure water of Atlantic origin, also partly influenced by the surface heat exchange (especially winter cooling) and, in certain areas, slightly modified with admixture of less saline surface water.

Region III waters vary from being oceanic at the shelf break to the west, through the relatively shallow semi-enclosed Irish Sea, to estuarine and fjordic inlets on its eastern boundary. In very general terms, the overall movement of water masses is from south to north, with oceanic water from the North Atlantic entering from the south and west of the region. This moves northwards through the area, to exit either into Region I to the north or, after flowing around the north of Scotland, into Region II. The general pattern of salinity distribution indicates that the water is mainly of Atlantic origin.

The major part of Region IV corresponds to the continental margin of the southernmost part of the Convention area. Most of the water masses found in the region either have a North Atlantic source, or result from interaction between waters formed in the Atlantic with water of Mediterranean origin. Winter vertical convection is also likely to give rise to water mass formation in the upper ocean levels (0 – 500 m) beyond the continental slope north of 40° N, particularly at the western Bay of Biscay, in a process subject to significant interannual variability.

Region V is the region where cold, low salinity water masses coming from the polar seas and the warm, salty waters originating from the south are transformed by mixing and cooling. Many of the water masses found in the Atlantic contain high concentrations of dissolved oxygen and are rich in nutrients. Throughout the deep waters of the North-East Atlantic, concentrations of dissolved oxygen never fall low enough to limit aerobic biological activities.

Most areas of the OSPAR region are vertically well mixed in the winter months of the year, down to a depth of more than 600 m in the eastern Atlantic. In spring, as solar heat input increases, a thermocline (a pronounced vertical temperature gradient) is established over much of the region, separating a heated and less dense surface layer from the rest of the water column. These waters are said to be stratified. In shallow shelf areas, with strong tidal movements, the waters remain mixed throughout the year.

The distinction into stratified and permanently tidally mixed areas is of considerable importance to the structuring of both pelagic and benthic ecosystems. The stability induced by the establishment of the spring thermocline allows phytoplankton to remain near the surface where both high light and nutrient levels are found. After the spring bloom, nutrients become limiting above the thermocline. As a result, phytoplankton production is reduced in the summer. Where the thermocline outcrops at the surface, the boundary between the different water masses is known as a tidal front and is a region of intense biological activity. In oceanic waters to the south of the OSPAR area, there is a deep, permanent thermocline.

2.9 Circulation and volume transport

Within the OSPAR area, warm Atlantic surface water flows in a north-westerly direction towards the Norwegian Sea as the North Atlantic Current (NAC). An eastward-directed flow in the Azores Current (AzC) roughly coincides with the southern boundary of the OSPAR maritime area. As extensions of the Gulf Stream, these two currents form the southern edge of the subpolar gyre and the north-eastern edge of the subtropical gyre respectively. On the margins of Europe a warm northward-flowing Eastern Boundary Current (EBC) is found intermittently. A western boundary current flows south from the Fram Strait as the East Greenland Current (EGC) and, its extension, the Labrador Current (LC). The northward transport of warm surface waters towards the Arctic Ocean is balanced by a southward return flow of intermediate and deep water from the Nordic Seas via the Denmark Strait and from both the Faroe–Shetland Channel and the Labrador Sea. Mean flows for these currents derived from modelling and observational studies are given in Figure 2.4.

The NAC and AzC, together with the dominant mid-latitude westerly winds and a mean meridional density gradient, combine to push oceanic water against the European coast. This effect, influenced by the Coriolis force, generates the northward-flowing EBC. Although the EBC does not appear to be continuous, it is evident from southern Portugal to northern Norway. It may also reverse its surface mean flow to the south in the summer upwelling period, especially off the coast of the Iberian Peninsula.

The water circulation of the European shelf seas is dominated by tidal and wind generated currents. In the North Sea the residual circulation is anticyclonic (anti-clockwise), passing out along the Norwegian coast after mixing with the outflow from the Baltic in the Skagerrak (Figure 2.5). This low salinity outflow continues to the north towards the Arctic and into the Barents Sea. Elsewhere on the narrow shelves of the eastern margin of Europe and in Region III, the shelf currents are predominantly from south to north. Coastal upwelling that occurs typically between April and October off the Iberian Peninsula complicates the coastal currents in Region IV. In Region I, currents exhibit complex patterns, particularly around the Region’s islands. Off Iceland, the coastal current circles in a clockwise direction.

	Figure 2.4 Mean surface currents.
	Figure 2.5 A schematic diagram of the general circulation in the North Sea. The width of the arrows is indicative of the magnitude of volume transport. Source: after Turrell et al. (1992).

2.10 Waves, tides and storm surges

2.10.1 Waves

In the open ocean, the wave climate is conditioned by changing wind regimes. Ocean currents and, in shallow waters, strong tidal currents may modify wave fields. In shallow waters, wave activity may also contribute to the mixing of water masses by weakening or destroying stratification. Statistical studies have shown that North Atlantic storms in both open ocean and coastal areas are not increasing in intensity. Although there has been a noticeable increase in mean significant wave height in the North Atlantic, this appears to be correlated with the increasing intensity of the North Atlantic Oscillation (NAO) seen in recent decades. Therefore, the increase in mean significant wave height appears to be positively correlated with the atmospheric zonal mean flow strength, rather than with storm intensification.

2.10.2 Tides

Tides are semi-diurnal throughout the area. In the Atlantic, their amplitude is small relative to those occurring in many of the continental shelf regions. In addition to regular sea level changes, tides also induce oscillating currents over the same period, and these too are strongest on the shelf regions.

Tidal ranges are greatly amplified near the coasts of semi-enclosed seas. The best examples are in the North Sea and the Irish Sea, where heights of up to 8 m or more can be observed. Strong oscillatory currents are often associated with these high tidal ranges and vigorous mixing and sediment resuspension is common in such areas. Tides can generate a net residual flow, and may dominate the circulation of certain coastal regions. Where they are strong enough, the tidal currents can keep the water column mixed in zones marked by tidal fronts. Tidal circulation and fronts affect the distribution of biota and the transport, dispersion and aggregation of pollutants.

2.10.3 Storm surges

A storm surge is an unusually high stand of sea level produced when strong winds blow water shoreward and when the ocean surface rises in response to low atmospheric pressure. Partially enclosed shallow water areas are particularly vulnerable to storms surges, which may increase sea level by several metres. Operational numerical models are used in the North Sea to produce reliable forecasts of storm surges.

2.11 Transport of solids

Input of suspended particulate matter (SPM) to the marine environment occurs mainly from rivers and, to a lesser extent, from the atmosphere and from sea-ice. Particle sizes of SPM range from sand (millimetres) via silt to clay (micrometres). In general, the finest particles travel the greatest distances, depending on the dynamic intensity in the conveying medium. Consequently, coastal and shallow water sediments are normally coarser than those found far away from the coast. There may be exceptions to this rule in semi-enclosed narrow bays, like fjords, or on tidal flats, where the dynamic activity of the water is low.

The supply of SPM from land depends both on the existence of soils subject to erosion in the hinterland and on the climatic situation. Therefore, under present climatic conditions, it would appear that SPM enters the OSPAR area mostly in its mid-latitudes, rather than in the drier south.

The mineral nature of the SPM is an important factor for the transport and fate of contaminants in the marine environment. Certain minerals, such as the clays, have a high adsorption capacity for a number of both organic and inorganic contaminants, whereas more silty and sandy fractions consist of inert minerals, which have a negligible adsorption capacity. This adsorption capacity/affinity strongly influences contaminant transport.

2.12 Meteorology

The atmospheric circulation is characterised by a westerly airflow associated with a meandering upper troposphere jet stream. Embedded within this belt of westerly winds are numerous cyclones, which develop along the zones of strongest temperature gradients, the Polar Front, and generally traverse the area from south-west to north-east. The cyclonic activity in the atmosphere is much stronger in winter than in summer.

The NAO index is defined as the difference in atmospheric pressure at sea level between the Azores and Iceland and describes the strength and position of westerly airflows across the North Atlantic. Fluctuations in the strength of the westerly winds over the North Atlantic are believed to play a major role in controlling oceanic ecosystems and ultimately North Atlantic fish stocks.

When the NAO index is high, the westerly winds over the North-East Atlantic are strong and numerous cyclones bring wet weather (particularly over the western parts of the British Isles and Scandinavia). In winter, high NAO values result in very mild weather over the eastern OSPAR area and north-western Europe, while summers often become unsettled and chilly. When this pressure difference is unusually small, anticyclones dominate large parts of this area and winter becomes colder. Further to the west, closer to southern Greenland, there is a tendency towards opposite reactions to the variations in the NAO. Thus high values on the NAO index are most often characterised by cold weather with outflow of very cold air from the Arctic region. Periods of low NAO values can be very mild in south-western Greenland, when warm air masses penetrate northwards into the Davis Strait.

2.13 Climate variability and climate change

The NAO index undergoes long-term cycles with varying periodicity. These oscillations have been linked to fluctuations in wind speed, sea temperature, heat fluxes, wave heights, storm tracks, and patterns of evaporation and precipitation. The relatively high NAO index over the past fifteen years (Figure 2.6) has been associated with milder than normal winters in Europe, and high sea surface temperatures, especially in winter. When considering the NAO index for the present decade, particularly within the context of this century, the 1960s were generally low-index years while the 1990s were high-index years.

The energy release by the North Atlantic warms the air over Europe. North-western Europe and the northern North Atlantic region, in particular, have a climate that is some 5 – 10 °C warmer than the zonal mean. This gives a climate, which is very benign relative to the same latitudes elsewhere. Yet this has not always been the case. Past climatic records reveal that there have been several occasions when, for reasons not yet firmly established, the climate of Europe suffered major cooling events that occurred very rapidly (10 to 100 years). The global thermohaline circulation (or 'Ocean Conveyor'), and its switching on and off in the North Atlantic, has been suggested as the source of these rapid and potentially disastrous fluctuations. A variety of historic records suggest that such a rapid shift could happen again, particularly if atmospheric levels of carbon dioxide increase steeply.

The UN Intergovernmental Panel on Climate Change (IPCC) has drawn a number of conclusions concerning the impact of climate change over Europe and the North Atlantic (IPCC, 1997). They noted that most of Europe experienced temperature increases this century larger than the global average together with enhanced precipitation in the northern half and decreased precipitation in the southern half of the region. Projections of future climate, not taking into account the effect of aerosols, indicate that precipitation in high latitudes of Europe may increase, with mixed results for other parts of Europe. The effects of aerosols mainly exacerbate the current uncertainties about future precipitation.

The IPCC further noted that water supply might be affected by possible increases in floods in northern and north-western Europe and by droughts in southern parts of the continent. A warmer climate could lead to reduced water quality, particularly if accompanied by reduced run-off. Warmer summers would probably also lead to increased water demand. Expected changes in snow and ice will impact upon European rivers, affecting, for example, summer water supply, shipping and hydropower.

The IPCC report also notes the ecological importance of coastal zones. Some coastal areas are already beneath mean sea level, and many others are vulnerable to storm surges. Areas most at risk in Europe include the Dutch and German coastal zones. Storm surges, changes in precipitation, and changes in wind speed and direction add to the concern of coastal planners. In general, major economic and social impacts can be contained with relatively low investment. This is not true, however, for a number of low-lying urban areas vulnerable to storm surges, nor for ecosystems, particularly coastal wetlands, which may be even further damaged by protective measures.

There is some evidence of a change in the climate of the OSPAR area, or, at least, of some changes in the ocean circulation and water mass characteristics. The amount of Atlantic water in the Arctic Ocean has increased during recent years, the temperature in the deep water of the Norwegian Sea has increased and there are indications of changes in the Iceland–Scotland Ridge overflow. The Annual ICES Ocean Climate Status Summary shows relatively high temperatures in the North Atlantic during the 1990s. Most areas show a warming trend, although the temperature has been going down in the subpolar North Atlantic, between Greenland and Iceland (Read and Gould, 1992).

There is a poor understanding of observed ocean climate variability, because of the complex interaction of forcing parameters. A better understanding of the cause of ocean climate variability is of major importance in predicting future climate impacts. Also the impacts of the climate change are rather uncertain. Some climate change models are predicting a global sea level rise. A mean sea level rise of 50 cm during the next 100 years has been forecast, putting low-lying coastal areas and wetlands particularly at risk from flooding.

Figure 2.6 Comparison between the observed NAO Index and the observed North European land/sea surface temperatures averaged over the box 5 - 50° E and 50 - 70° N, for the period 1900 to 1999. Source: after Rodwell et al. (1999).

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