These notes are about regional oceanography of shelf seas. Regional oceanography can be described as the successor to physical oceanography during the first half of this century, a branch of science that grew from physical geography. It studies the consequences of topographic detail of the world ocean for the oceanic circulation - the depth of ocean sills, the influence of land barriers, the importance of rivers - and investigates the effect of atmospheric forces such as the wind field and the distribution of rainfall and their seasonal variation on the hydrological structure of the sea.
Regional oceanography of the deep seas is a relatively straightforward matter; it deals with three oceans and addresses them by finding what they have in common and where and why they differ. The last fifty years have seen rapid advances in this field. More importantly, significant progress occurred in our understanding of ocean dynamics. As a result, oceanography is now closer to theoretical fluid dynamics than to its geographical roots, and regional description of the deep ocean has become impossible without at least some elementary knowledge of physical principles.
Regional oceanography of shelf seas, ie a description of the circulation and water masses in shallow ocean areas, relies even more on an understanding of the underlying physical principles. The world ocean has many shelf regions with many different characteristics. Without some organizational framework, a regional study of shelf seas develops into an encyclopaedia, a useful undertaking but not suitable for an undergraduate topic. The physical principles that govern fluid flow can provide us with a framework. They enable us to group shallow seas according to acting forces and to the balances between them, thus allowing us to discuss selected shallow seas as prototypes for others.
These notes attempt to introduce the principles of geophysical fluid dynamics at a level sufficient to open up an understanding of shelf dynamics but elementary enough to allow all students interested in regional oceanography of shelf seas access to the topic.
Most people who come into contact with the sea and want to learn more about it never get much further from solid land than as far as a day trip can take them. The deep ocean is out of reach of most of them; an ocean-going research vessel may not be available, or their interest is focussed on the coastal zone, or both. It is true that much oceanographic research can be done without ever venturing onto the deep sea. But it is not true, despite a widespread belief, that the shallow regions of the world ocean can be understood without recourse to an understanding of the deep ocean circulation. Oceanography has seen many attempts to present an introduction into the coastal ocean without a discussion of open ocean dynamics. The result is invariably an eclectic collection of various aspects of oceanography without a firm base. A systematic approach has to begin with an overview of deep ocean dynamics and proceed to applications of those elements that govern the dynamcis of the shallow ocean.
The main reason why a review of deep ocean dynamics has to precede a discussion of shallow seas is the unfortunate fact that the dynamics of shallow water near coastlines are much more difficult than the dynamics of the deep ocean. To give just a few examples, the circulation in the ocean interior can be treated as frictionless motion, while on the shelf friction can rarely be neglected. Tides in the open ocean are generally small, and a discussion of the open ocean circulation can disregard tidal effects. On the shelf, tides have important consequences for the stratification and water movement; in estuaries they become a dominant force. Wind-driven currents in the deep ocean can be understood without knowledge of the details of turbulent mixing; in shallow water the type of mixing in the surface layer is a determinant for the strength and direction of wind-driven water movement. These and other complications over the simple force balances found in the deep ocean make it more difficult to present regional oceanography of shelf seas on an introductory level. Nevertheless, the task is possible, as we intend to prove.
Figure 1.1
How and in which way, then, do shelf seas differ from the deep ocean? To answer this question requires a few definitions. We begin with a definition of the shelf.
The coast is the boundary between water and land; but it does not delineate the boundary between continents and oceans. The continents "float" on the earth's crust. Their mass distribution may be compared with that of icebergs floating on water: Most of the mass is submerged, pushing the crust down to make the combined layer of material sitting on the earth's mantle some 50 km thick (In comparison, the mantle is found only 5 km below the ocean floor). The flanks of the continents rise steeply from depth but do not reach the sea surface; they end more or less abruptly at about 200 m below present sea level (Figure 1.1). The region between the 200 m depth contour and the coast is known as the continental shelf. It is that part of the continents that is covered by ocean water, or that part of the ocean where its waters lap over the continents. The location where the continental shelf joins the flanks of the continents near the 200 m depth mark is termed the shelf break.
Figure 1.2
The width of the continental shelf varies considerably. Some regions which experience pressure from the movement of the continental plates, resulting in the subduction of the ocean floor under the continents, do not show much of a continental shelf at all; examples are the coastlines of Chile and Peru and the coastline of the Philippines. Other regions contain a number of apparently disconnected areas of dry land which in reality constitute the higher portions of a continuous continental land mass covered with extensive shelf seas; Ireland and the British Isles, for example, are the westernmost areas of dry land belonging to the Eurasian continent, on which the Irish and North Seas are shallow ponds. Figure 1.2 shows the extent of the continental shelf of the world ocean. The largest shelf regions are found in the Arctic Ocean, from Siberia to the Bering Sea and in the Canadian Archipelago south to Hudson Bay. Other large shelf regions include the European shelf seas, the shallow seas along eastern Asia (Sea of Okchotsk, East China and Yellow Sea), the shelf of south-east Asia and Australia (Sunda Shelf and Arafura and Timor Seas), the Grand Banks of Newfoundland, the Patagonian Shelf and the Persian Gulf.
The extent of the continental shelf leads to a definition of shelf seas as that part of the world ocean adjacent to a coast and within the 200 m isobath. Shelf seas encompass a variety of regions of different oceanographic character that requires further differentiation. The first and fundamental distinction is between the ocean proper and estuaries. We refer to those regions of the shelf seas with close affinity to the deep ocean as the coastal ocean and define the coastal ocean as those shelf sea regions that display the dynamics of the deep ocean, modified by shallow water depth and the presence of the coast. In most situations this means that the driving force for water movement is the wind stress and that thermohaline forces (the influence of temperature and salinity changes on the stratification) are of secondary importance. In contrast, estuaries are defined as those parts of the shelf seas where water movement is controlled primarily by thermohaline forcing.
A more refined definition of estuaries will be introduced with the detailed description of estuarine dynamics in the second part of these notes. In the present context it is important to clarify our definition of the coastal ocean and avoid misunderstanding. The term coastal ocean is sometimes used to describe processes in the immediate vicinity of the coastline where water (and sediment) movement is strongly affected by wave movement and wave breaking. This region, also often - and more appropriately - called the coastal zone, is the shallowest part of the shelf sea. Its dynamics are not included here, and a description of the coastal zone is not part of these notes. This does not imply that it is an irrelevant part of the shelf sea. But proper treatment of the dynamics of the coastal zone has to address processes of wave breaking, boundary layer dynamics associated with friction processes on scales of centimeters, processes of sediment movement and sorting according to grain size etc. This leads inevitably to considerations more appropriate for a course on ocean engineering than regional oceanography, a task we shall not attempt. We therefore stress again that we use the term coastal ocean as a synonym for all shelf seas except estuaries. Thus defined, the coastal ocean includes the coastal zone, but our description of coastal ocean dynamics does not include the dynamics of the coastal zone.
A distinction generally applicable to sub-regions of the oceans is that between adjacent or marginal seas and mediterranean seas. An adjacent or marginal sea is defined as a part of the ocean that is separated from the major ocean basins by topographic features such as islands or bay-like coastline configurations. In many instances identification of an ocean region as a marginal sea is merely geographic and does not relate to different dynamics, since the marginal sea is of oceanic depth and thus displays the same dynamic behaviour as the deep ocean. Consider for example the Tasman Sea: It is an adjacent sea of the Pacific Ocean, but its currents and water masses follow the same dynamics as those of the South Pacific Ocean. It is thus clear that all coastal ocean regions (with the exception of mediterranean seas) are adjacent seas but not all adjacent seas are coastal ocean regions. To be more specific, the coastal ocean comprises those adjacent seas where the dynamics are modified by shallow depth and the presence of the coast.
Figure 1.3
A mediterranean sea is defined (Tomczak and Godfrey, 1994) as a part of the world ocean that has only limited communication with the major ocean basins and where as a result the circulation is dominated by thermohaline forcing. The requirement of limited communication sets mediterranean seas apart from the major ocean basins geographically. Domination of thermohaline forcing sets them apart from the point of view of ocean dynamics, since water movement in the upper kilometer of the open ocean is usually dominated by wind forcing, as we shall see in more detail in the next chapter. All mediterranean seas found on the shelf belong to the coastal ocean; those with basins deeper than 200 m are not part of it. To put it in another way, the categories adjacent seas and mediterranean seas go beyond the coastal ocean. Figure 1.3 displays the relationships between the various definitions.
Without going into more detail, it is useful at this stage to point out the similarity of the definition of mediterranean seas with the definition given for estuaries. The dynamics of estuaries and mediterranean seas have indeed many parallels, and it is impossible to give a clear distinction between the two without a more detailed understanding of deep ocean dynamics. As a preliminary though dynamically superficial distinction we note that even the smallest mediterranean sea is an order of magnitude larger in size than the largest estuary. We will be able to assess the dynamic consequences of this size difference after the review of deep ocean dynamics in the next chapter.
Having spent some effort on definitions we return to our question how shelf seas differ from the deep ocean. Chapters 3 - 10 will be devoted to the answer to this question in some detail. On an introductory level it is useful to list a few observations. The first observation relates to the effects of shallow water depth. The presence of the sea floor in the depth range of moderate or strong water movement is responsible for frictional effects that cannot be ignored in the coastal ocean but are usually of no importance in the deep ocean basins. Water movement over the sea floor establishes a frictional boundary layer near the bottom, in the same way as wind blowing over the sea surface produces the Ekman layer, a frictional boundary layer at the sea surface through which momentum is transferred from the wind to the current. Bottom currents in the deep ocean are usually so sluggish that the associated frictional boundary layer can be disregarded in the dynamics. Chapter 3 investigates the role of frictional boundary layers in shallow water
The second effect of shallow water depth is an enhancement of tidal motion. Tides are oceanic waves of very long wavelength and generally small amplitudes in deep water. As the tidal wave approaches the continental shelf its amplitude increases (just as the amplitude of wind waves increases as they approach the shore and the water becomes increasingly shallower) and propagation of the wave crest is retarded. This is associated with a significant increase of the tidal current which in many shelf seas becomes the dominant water movement. These and related aspects are the topic of Chapter 5.
The third difference between shelf seas and the deep ocean relates to the presence of coasts. Coastlines are an obstacle to the free flow of water. A current approaching a coast will develop flow convergence, which will result in acceleration of the flow. Alternatively, convergence of surface currents can force surface water to sink to greater depth. Where a wind produces water movement away from the coast, the resulting surface divergence causes a rise of subsurface water to the surface, a process known as upwelling. Convergences and divergences of currents are, of course, observed in deep ocean regions as well; but they are generally much weaker, and the vertical velocities associated with them are orders of magnitude smaller than those produced in the vicinity of coastlines. Chapter 6 addresses these processes in detail.
A fourth, important aspect is again a consequence of the presence of coasts and related to variations in sea level produced by flow convergence and divergence. In the deep ocean any accumulation of water in a region of flow convergence is usually short-lived, resulting in sinking of water from the upper ocean to greater depth (a process known as Ekman pumping and described in Chapter 2). The reason is that the open ocean cannot support the pressure gradients associated with a sloping sea surface without the assistance of strong currents (the geostrophic currents discussed in Chapter 2). Shelf seas can support extreme pressure gradients without recourse to currents, since the water can "lean" against the coast. The associated slope of the sea surface can be supported, for example, by an onshore-blowing wind. This is known as a surge or - in extreme situations - a storm surge. Some shelf seas are more prone to storm surges than others, and in a few regions storm surges are among the most dangerous natural hazards. This aspect of shelf sea dynamics is discussed in Chapter 4.
The fifth aspect is related to stratification. Shallow water depth limits the volume of water available for mixing. This enhances the seasonal cycle of temperature, since the water warms and cools faster than in the open ocean. In some shelf regions salinity undergoes strong seasonal variation, too, as a result of highly seasonal freshwater input from rain or river runoff. These effects can lead to the establishment of a particularly strong seasonal thermocline during summer, which acts as an impediment to the exchange of properties. In other words, mixing across the water column is reduced, currents are restricted to the layer above the thermocline, and water quality may be adversely affected by the lack of turbulent exchange of properties. Chapter 3 includes these aspects in the discussion of boundary layers in shallow water; it looks particularly at the effect of stratification on boundary layer structure and extent.
Finally, the sixth aspect that sets shelf seas apart from the deep ocean is their exposure to terrestrial influences. Two of the three major elements of the hydrological cycle, evaporation and precipitation, act as sources or sinks distributed over the entire surface of the ocean. The third element, river runoff, constitutes an ensemble of point sources that impact exclusively on the coastal ocean. The introduction of substantial amounts of freshwater into a shallow sea can produce horizontal density gradients large enough to support buoyancy-driven flow (ie flow supported by thermohaline forces). The coexistence of wind-driven and buoyancy-driven flow in the upper ocean is a situation not normally encountered in the deep sea. Some shelf seas show quite convincing evidence that thermohaline forcing has to be included in any description of their dynamics. These questions are considered in more detail in Chapter 4.
Most likely, the six points listed above do not cover all processes found to be important for any particular shelf region. But they should capture the most important ones, and they represent the processes that have to be considered in all shallow water situations, regardless of local peculiarities. As stated earlier, a treatment of all possible shelf situations can only be offered by an encyclopaedia. The organizational framework developed here can help us discuss most aspects of shelf sea dynamics and thus produce something of relevance to most situations.