Description of the worlds oceans

The main focus of this book is the study of large-scale circulation in the world's oceans. As a dynamical system, the circulation in the world's oceans is controlled by the combined effects of external forcing, including wind stress, heat flux through the sea surface and seafloor, surface freshwater flux, tidal force, and gravitational force. In addition, the Coriolis force should be included, because all our theories and models are formulated in a rotating framework. In this chapter, I first describe surface forcing and the distribution of physical properties. I then discuss the classification of different kinds of motion in the world's oceans, and briefly review the historical development of theories of oceanic general circulation.

1.1 Surface forcing for the world's oceans

The ocean is forced from the upper surface, including wind stress, and heat and freshwater fluxes. In addition, tidal forces affect the whole depth of the water column, and geothermal heat flux and bottom friction also contribute to the establishment and regulation of the motions in the ocean. However, the surface forces are the primary forces for the oceanic circulation, and these are the focus of this section.

1.1.1 Surface wind forcing

Wind stress is probably the most crucial force acting on the upper surface of the world's oceans. The common practice in physical oceanography is to treat the effect of wind as a surface stress imposed on the upper surface of the ocean. The sea surface wind stress is usually calculated from the geostrophic wind 10 m above the sea surface, using bulk formulae. However, the air-sea interface is actually a transition zone between the atmospheric boundary layer and the oceanic boundary layer. Most importantly, the oceanic boundary layer is a wave boundary layer in which surface waves and turbulence play vitally important roles in regulating the vertical fluxes of momentum, heat, freshwater, and gases. Strictly speaking, therefore, the so-called wind stress on the sea surface should be replaced by the radiation stress between the wave boundary layer in the upper surface and the water below. Wind stress acting on the water below should depend on many dynamical aspects of these two boundary layers, such as the stability of the atmospheric boundary layer and the age of surface waves in the upper ocean. However, the discussion in this book follows the traditional approach, and the term "wind stress" is used for simplicity.

Furthermore, the distribution of wind stress on the upper ocean should be a final product of the atmosphere-ocean coupled system, and such interaction involves very complicated dynamical processes that are the subject of air-sea interactions and are beyond the scope of this book. Thus, in this book we will treat the wind stress as an external forcing for the oceanic general circulation.

Wind stress at sea level is the surface expression of the turbulent motions in the atmosphere, which occupy rather broad spectra in both space and time. It is common knowledge that wind stress changes over different time scales, from seconds to interannual and centennial time scales. The most important cycles in wind stress include the diurnal cycle and the seasonal cycle, in addition to changes on longer time scales, from interannual to decadal. Similarly, wind stress varies on spatial scales over a very broad spectrum. However, for the theory of oceanic general circulation, wind stress is normally referred to the smoothed wind stress data for large spatial scales and long time scales.

The dominant player in setting up the global wind stress pattern is the equator-pole temperature difference. Owing to this surface differential heating, atmospheric circulation is organized in the form of "Hadley cells." The prime feature of the surface wind stress is the strong westerlies associated with the Jet Stream at mid latitudes of both hemispheres. The existence of a quasi-steady circulation requires a near balance of the surface frictional torque; therefore, easterlies should exist at low latitudes. In fact, both the equatorial Pacific and equatorial Atlantic Oceans are dominated by easterlies (also known as trades) (see Fig. 1.1).

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Fig. 1.1 Annual mean wind vector (in m/s) on the world's oceans, based on the European Centre for Medium-Range Weather Forecasts (ECMWF) dataset (Uppala et al, 2005).

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Fig. 1.1 Annual mean wind vector (in m/s) on the world's oceans, based on the European Centre for Medium-Range Weather Forecasts (ECMWF) dataset (Uppala et al, 2005).

Owing to the large-scale distribution of land and ocean, wind stress on the surface is far from being zonally symmetric. Another major player in setting up the global wind stress pattern is the Earth's rotation. For example, at low latitudes the near-surface branch of the equatorward return flow of the Hadley cell is turned westward and appears as the northeast trade wind in the Northern Hemisphere and the southeast trade wind in the Southern Hemisphere. Under many such dynamical constraints, the sea surface wind stress pattern takes complicated forms. In fact, the most outstanding feature in the North Pacific Basin is the huge cyclonic wind stress pattern in the subpolar basin and the anticyclonic wind stress pattern in the subtropical basin. Similar features also exist in the Atlantic Basin and in the Southern Hemisphere, including the South Pacific, South Atlantic and South Indian Oceans.

Strong circulation is induced by wind stress in the upper kilometer of the world's oceans. The most outstanding features of wind-driven circulation include gigantic anticyclonic gyres in subtropical basins, and cyclonic gyres in subpolar basins, as shown in Figure 1.2. In addition, there is a strong circumpolar current system in the Southern Ocean, which is one of the most crucial branches of circulation in the world's oceans.

Wind stress is one of the most crucial driving forces of the oceanic circulation. As explained in Chapter 4, the westerlies at mid latitudes are responsible for the equatorward surface drift, the so-called "Ekman drift," and the easterlies at low latitudes are responsible for the poleward surface drift. The anticyclonic wind stress in the subtropical basin drives the anticyclonic circulation in the subtropical basin, and the cyclonic wind stress in the subpolar basin drives the cyclonic circulation there. In the Southern Ocean the westerly wind appears as a continuously strong belt around the whole latitudinal band; this wind

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stress gives rise to the strong northward Ekman transport and upwelling, and is a direct driving force of the Antarctic Circumpolar Current (ACC).

Although a layman can observe the surface waves created by wind blowing on the sea surface, and a yachtsman can discern the surface wind drift from observation, the dynamical effects of wind stress include phenomena of huge spatial scales on the order of hundreds or thousands of kilometers - phenomena undetectable to the layman's eyes. A comprehensive understanding of the wind-driven motions in the oceans can only be achieved by systematic scientific research. In fact, the theory of the wind-driven circulation has to be developed side by side with the progress of in situ observations through the development of modern scientific instrumentation.

1.1.2 Surface thermohaline forcing

Heat and freshwater fluxes through the air-sea interface are the most critical forcing boundary conditions for the temperature and salinity distribution in the oceans. In addition, the oceans also receive geothermal heat from the seafloor; however, under present-day geological conditions, the amount of heat received from the seafloor is relatively small, approximately a thousand times smaller than that through the air-sea interface, so it is a rather minor contributor to the oceanic general circulation, except near the seafloor.

Surface heat flux

I first discuss the heat fluxes through the air-sea interface. The heat flux maps presented in this section are based on the NCEP-NCAR Reanalysis Project (Kistler et al, 2001). In the following figures, downward heat flux into the ocean is defined as positive, and upward heat flux, leaving the oceans, is defined as negative.

The most essential forcing for the climate system on Earth is solar radiation, and this energy is in the form of short waves. Most of the energy required for maintaining the climate system can ultimately be traced back to solar radiation. Since the atmosphere is nearly transparent for solar radiation, most of it can penetrate the atmosphere and reach the lower boundary of the atmosphere, over both the land and the oceans. The amount of short-wave radiation reaching the sea surface depends primarily on the latitudinal location. Furthermore, cloudiness may be another major player in regulating the amount of solar radiation which can reach the sea surface. On the sea surface, part of the incoming shortwave radiation is reflected; thus, what the ocean receives is the net short-wave radiation, as shown in Figure 1.3. The global maxima of net short-wave radiation are closely related to the cold tongues in the eastern part of the equatorial Pacific and Atlantic Oceans.

The net short-wave radiation at each station on the sea surface is balanced by the heat transport within the ocean through advection and diffusion, plus upward heat flux through the air-sea interface. The major term in the heat flux from the ocean to the atmosphere is the latent heat flux associated with evaporation. The latent heat content for water vapor is approximately 2,500 kJ/kg; thus a relatively small amount of evaporation can transfer a large amount of heat from the ocean to the atmosphere.

Net short-wave radiation (W/m2)

Net short-wave radiation (W/m2)

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