Ocean Currents

The existence of different climates is due to the variation with latitude and season of the solar radiation arriving at the earth. This affects climate both directly, through the heating effect of the radiation, and indirectly, through the general circulation of the atmosphere that results. A secondary but nevertheless important element is the influence of the oceans.

The oceans affect climate in two ways. First, they act as reservoirs of heat which moderate the temperature extremes of the seasons. In spring and summer the oceans are cooler than the regions bordering them, since the heat they absorb is dissipated in a greater volume than in the case of solid, opaque land. The heat retained in the ocean depths means that in fall and winter the oceans are warmer than the regions bordering them. Heat flows readily between moving air and water; with a sufficient temperature difference, the rate of energy transfer from warm water to cold air (or from warm air to cold water) can exceed the rate at which solar energy arrives at the top of the atmosphere. With no such heat reservoir nearby, continental interiors experience lower winter temperatures and higher summer temperatures than those of coastal districts. In Canada, for instance, temperatures in the city of Victoria on the Pacific Coast range from an average January minimum of 36 °F to an average July maximum of 68 °F, whereas in Winnipeg, in the interior, the corresponding figures are — 8°F and 80°F.

Also influencing climate are surface drifts in the oceans produced by the friction of wind on water. Such drifts are much slower than movements in the atmosphere, with the fastest normal surface currents having speeds of about 7 mi/h.

The wind-impelled surface currents parallel to a large extent the major wind systems. The northeast and southeast trade winds drive water before them westward along the equator, forming the equatorial current. In the Atlantic Ocean this current runs head on into South America, in the Pacific into the East Indies. At each of these, points the current divides into two parts, one flowing south and the other north. Moving away from the equator along the continental margins, these currents at length come under the influence of the westerlies, which drive them eastward across the oceans. Thus gigantic whirlpools called gyres are set up in both Atlantic and Pacific Oceans on either side of the equator. Many complexities are produced in the four great gyres by islands, continental projections, and undersea mountains and valleys.

Currents also occur deep in the ocean, though their speeds are usually slower than those of surface currents. In the polar regions of both hemispheres cold water sinks because of its greater density and flows toward the equator several miles below the surface. These cold currents keep tropical waters cooler than they otherwise would be, and they also bring oxygen to the lower depths of the ocean which enables plant and animal life to occur there.

Thus the oceans, besides acting as water reservoirs for the earth's atmos­phere, play a direct part in temperature control—both by preventing abrupt temperature changes in lands along their borders and by aiding the winds, through the motion of ocean currents, in their distribution of heat and cold over the surface of the earth.

Check your understanding.

1. Why are there different climates?

2. How do oceans affect climate?

3. What is the speed of the surface drifts in the ocean?

4. What is a gyre?

5. What influence do cold currents have on tropical waters?