The restless ocean

The waters of the ocean move because of three main mechanisms: cooling near the Polar Regions and warming near the equatorial region, the force exerted by the winds on the ocean surface, and the tides arising from the gravitational pull of the Moon and the Sun. The rotation of the Earth (360 degrees in 24 hours from west to east) about an axis passing through its poles, the shape of the oceans (long horizontal dimension in comparison to vertical), and the stable stratification (lighter waters overlying denser waters) in the oceans influence the movement of waters in the oceans.

Water is cooled near the poles (Fig. 12). As a result, its density increases and it sinks to great depths, setting up a global scale circulation known as the conveyor belt or thermohaline circulation (Fig. 15). Water sinks in the north Atlantic and upwells (rises) in the Pacific and Indian Oceans. The velocity associated with this circulation is typically a small fraction of a cm/s and the time scale associated with it is a few centuries. Since it is difficult to measure directly, its existence must be inferred from analysis of the distribution of temperature, salinity, and other fields.

Motion forced by winds is most noticeable within the upper 1 km from the surface, but the strongest motions are in the uppermost few hundred metres. The nature of these currents is determined by the nature of the winds. The most striking feature of the wind-forced surface circulation is the sub-tropical gyre, in which a strong poleward (towards the pole) current near the western boundary (east coast of a continent) is balanced by a gentler equatorward drift over the rest of the basin (Fig. 16). In the Atlantic and Pacific Oceans, the strong western boundary currents are called the Gulf Stream and Kuroshio respectively. The strongest ocean current is the Antarctic Circumpolar Current, which flows all round the globe in the Southern (or Antarctic) Ocean.

The circulation in the north Indian Ocean is unique because it experiences strong seasonal winds called the monsoons (Fig. 17). Hence, unlike in much of the Pacific and Atlantic Oceans, the currents in the north Indian Ocean change with season (Fig. 18). Recent research shows that the Arabian Sea, the Bay of Bengal, and the equatorial Indian Ocean function as a single dynamical entity, making the current at any location in the basin a response not just to the local winds, but also to winds elsewhere in the basin. For example, the strongest currents along the east coast of India occur not only during the summer monsoon, when the winds are strongest, but during March-April, a period of weak winds.

Motion due to tides is strongly periodic and leads to upward and downward movement of the water surface. The up and down motion of the surface is accompanied by horizontal currents that are also periodic (Fig. 19). Tidal motion is most conspicuous in shallow coastal areas. Its importance, in comparison to wind-driven circulation, reduces in deeper waters.

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