Physical oceanography studies all forms of motion in the ocean. It relates observations of motion to physical laws (such as Newton's Law that if a force F acts on a body of mass m, it undergoes an acceleration a such that F = m a ).
(Note: For the mathematically inclined, bold characters in this and all following lectures indicate vectors, characters in italics stand for scalars. If you do not know what that means, don't despair; it is more a matter of using the correct notation than a matter of practical importance for this topic.)
The prevailing wind system is the major driving force for ocean currents. Figure 2.1 shows that in the open ocean winds are nearly zonal (blowing east-west). The Trade Winds are easterly winds in the tropics and subtropics (between 30ºN and 30ºS). They are regions of extremely uniform wind conditions, where the wind blows steadily from the same direction with moderate strength throughout the year. Their strength increases slightly in winter. The Trade Winds of the two emispheres are separated by the Doldrums, a region of weak and variable winds near 5ºN.
Figure 2.1
Between 30º and 65º are the Westerlies. They are stronger in winter than in summer and are regions of frequent storms. Poleward of 65º the wind direction reverses again, and the Polar Easterlies blow from east to west.
Deviations from zonal wind direction occur near continents. This is particularly noticeable along the east coast of the oceans in the tropics and subtropics where the winds blow parallel to the coast towards the equator.
Figure 2.2
The present configuration of the distribution between land and water determines the ocean's response to the winds. It defines the major subdivisions of the world ocean, the Pacific, Indian and Atlantic Oceans. Their southern region around Antarctica is also known as the Southern Ocean.
Figure 2.2 is a map of surface currents. The combined action of the Trade Winds and the Westerlies produces large gyres, with clockwise rotation in the northern hemisphere, anti-clockwise rotation in the southern hemisphere, known as the subtropical gyres. A subpolar gyre is produced in the north Pacific Ocean by the combined action of the Westerlies and the Polar Easterlies; it consists of the Oyashio, North Pacific Current and Alaskan Current. An indication of a subpolar gyre is also seen in the north Atlantic Ocean (anti-clockwise rotation in the current system that includes the North Atlantic, East Greenland and Labrador Currents). The subpolar region of the southern hemisphere does not have land barriers and therefore is dominated by the Antarctic Circumpolar Current.
Note: The convention for indicating the direction of ocean currents differs from the convention used for wind directions. A "westerly" wind is a wind which blows from the west and goes to the east; a "westward" current is a current which comes from the east and flows towards west. This can cause confusion to people who rarely, if ever, go to sea; but it is easily understood and remembered when related to practical experience with winds and ocean currents. On land, it is important to know from where the wind blows: any windbreak must be erected in this direction. Where the wind goes is of no consequence. At sea, the important information is where the current goes: a ship exposed to current drift has to stay well clear from obstacles downstream. Where the water comes from is irrelevant.
(M. Tomczak and J. S. Godfrey: Regional Oceanography: an Introduction. Pergamon, New York (1994), 422 pp.)
So remember: westerly means from west, westward means towards west.
A feature to note is that as a general rule currents along the western coasts of ocean basins are much narrower and stronger than currents in the remainder of the ocean. Typical current velocities at the surface in the open ocean are 0.2 - 0.5 m s-1 (about 1 km h-1). In western boundary currents velocities are closer to 2 m s-1 (about 7 km h-1). These differences in current strength do not come out in most surface current maps.
The Indian Ocean is dominated by seasonal wind reversal (the Monsoons) and a corresponding reversal of surface currents. Figure 2.2 shows the situation during the Southwest Monsoon season when the Equatorial Countercurrent is suppressed and the circulation in the northern Indian Ocean differs significantly from that of the other ocean basins.
Just as in the atmosphere, where wind systems are linked to atmospheric pressure patterns, ocean currents are linked to pressure patterns in the ocean. Pressure at any depth in the ocean is determined by the weight of the water above, which is determined by the density of the water, which in turn depends on the water's temperature and salinity. It follows that ocean currents can be determined by measuring temperature and salinity, a task infinitely easier than direct current measurement. It is therefore appropriate to turn to a discussion of the basic properties of seawater before proceeding with a discussion of the oceanic circulation and the physical laws that govern it. This is the topic of the next lecture.