Both the atmosphere and oceans play significant roles in the determination of climate on seasonal and longer time scales. Due to latitudinal differences in solar heating, the Tropics have a net energy surplus whereas the polar regions have a net energy deficit. The redistribution of this energy is accomplished in large part by large-scale atmospheric and oceanic circulations (see Ahrens Fig. 2, page 60). The equator-to-pole energy and mass transport by the ocean, which occurs over great distances, is important in reducing the pole-to-equator temperature gradient. The surface ocean currents, for example, account for ~40% of the total heat transport in the Northern Hemisphere.

The oceanic role in global climate can be expressed in terms of the ocean's physical properties: albedo, heat capacity, and fluid character.

Albedo: The ocean has a low albedo, typically ~7%. Thus water is relatively transparent to solar radiation, meaning it is an excellent absorber of solar radiation. This is because the absorption of solar radiation is not limited to the surface but can occur through a volume of water down to a depth of about 100 m. Oceans receive more than half of the energy entering the climate system.

Heat capacity: The ocean has a high heat capacity. The tropical oceans are particularly large reservoirs of heat. Oceans provide the bulk of the thermal inertia of the climate system on the time scale of weeks to centuries. The ability of the oceans to store heat reduces the magnitude of the seasonal cycle in surface temperature by storing heat in summer and releasing it in winter. The moderating effect of oceans on air temperature occurs because of the thermal storage in the upper layers of the ocean. The upper 3 m of water has the same heat capacity as the whole of the atmosphere. The energy required to raise the temperature of the atmosphere by 1° C can be obtained by cooling the upper 3 m of water by 1° C. An additional factor of climatic significance is that the warmest ocean water is found in a relatively thin surface layer, which is separated from the deep water by what is called the thermocline. The warm water (main thermocline) is confined to the upper 1000 m and about 75% of the ocean water is cooler than 4° C. This temperature structure exists because the ocean is heated from above and warmer water, being less dense, lies above the cooler, denser water. The resulting density structure inhibits vertical motion in the ocean and has the effect of isolating the deep water from surface effects. The stable stratification over most of the ocean acts as a barrier to the sinking of near-surface water.

Fluid character: The ocean surface is a constant supply of water vapor for the atmosphere, i.e., the large amounts of net radiation on the ocean provides the energy for evaporation. In this way, the oceans act as a boiler that drives the global hydrologic cycle, which affects regional and global climate through the exchange of energy and mass with the overlying atmosphere. Cooling by evaporation and sensible heat transfer to the atmosphere occur at the ocean surface.

The ocean behaves as a heat engine in a fashion similar to the atmosphere, but the ocean is far less efficient. This is because the oceanic heat sources and sinks are all near the surface at approximately the same pressure. A great deal of the driving of the ocean, particularly in the upper layers, is the result of wind stress on the surface. As air moves across the ocean surface faster than the strongest oceanic surface flows, the drag or resistance of the slower moving water slows the air flow in the layers of air nearest the surface. This imposes a surface wind stress (momentum transfer from the atmosphere to the ocean) on the water surface (this occurs in the form of turbulent motions in the atmosphere). Wind stress is strongest in the winter when winds are strongest, and it displays a distinct annual cycle. Wind stress is a very important part of the atmosphere-ocean interaction, and is a key link in the whole climate system. It is important to recognize that wind related flows are confined to a layer of water which has vertically uniform temperature, the so-called mixed layer, which has an average depth of about 70 m.

Although the atmospheric circulation drives the ocean circulation at the surface, the circulation of the ocean is quite different than that of the atmosphere. Nevertheless, the oceans’ surface circulation pattern bears a resemblance in many ways to global atmospheric circulation (see Fig. 11.14 in Ahrens) in that the oceans respond to the average of the wind stress over months or longer. An important difference is that the continents confine the oceans to specific basins. Recall, the average atmospheric motions in middle latitudes are zonal (east-west). Except for a rather small "leak" between South America and Antarctica, zonal (east-west) motion in the ocean is blocked. Characteristically, there are intense, narrow flows on the western sides of the basins and much broader slower flows on the eastern sides.

The oceanic influence on climate is most evident in the warming and cooling of coastal areas. Commonly, many of the world's major ocean currents are classified as warm or cold. Rather than in reference to the actual water temperature, these terms refer to the environment into which the water is going. That is, warm (cold) currents are warm (cold) relative to the latitudes into which they are flowing. Below is a brief discussion of some of the world's major ocean currents (see Fig. 11.14 in Ahrens).

The Gulf Stream of the North Atlantic is the most climatically significant surface ocean current. It is a large, warm current that flows northeastward between Cuba and the southern tip of Florida, parallels the eastern coast of North America almost to Newfoundland, then continues as the North Atlantic Drift. The North Atlantic Drift then moves northeastward past Britain and Norway around the northern tip of Scandinavia and into the Barents Sea of the Arctic north of Russia. The Gulf Stream has a profound effect on much of Europe since there are few terrain barriers at these latitudes to stop the westerly winds that carry the marine air eastward. In the Norwegian Sea in winter, the Gulf Stream causes surface air temperatures to be as much as 26° C above the normal for the latitude. Much of Britain, which lies north of the latitude of the Canadian border, experiences winters nearly as mild as Georgia which is 15° -20° latitude farther south.

The North Pacific Drift, which is an extension of the Kurishio Current, does not have as strong a climatic effect on the eastern side of the Pacific as the Gulf Stream does on the eastern side of the Atlantic because its influences are not permitted to extend as far poleward. High mountains paralleling the West Coast of North America do not allow much penetration of Pacific marine air across the continent. Also, the North Pacific is separated from the Arctic during the winter by the narrow and frozen Bering Straits, which block any movement of surface water from the North Pacific into the Arctic during the middle and late winter.

Warm currents are poorly developed in the Southern Hemisphere. For example, the Brazil Current, which moves along the southeast coast of Brazil, has a small flow because the triangular shape of eastern South America directs most of the south Equatorial Current into the Northern Hemisphere. The Indian Ocean currents are greatly affected by the seasonal reversal of the monsoons. A weak, diffuse warm current moves southwestward along the eastern coast of Australia

The two currents with the most pronounced climatic influences in the Southern Hemisphere are large cold currents. These are the largest cold currents in the oceans. The Peru (Humboldt) Current, has the greatest volume and is climatically the most significant, flows northward along the west coast of South America from about 50° S to almost the equator. This current has a profound effect on the climate along much of the west coast of South America, although its influences are limited to a narrow coastal strip because of the High Andes. Cold water of this current and others is due to upwelling in response to offshore pushing of surface water by winds (see the Ekman Spiral in Ahrens, Figure 11.17). The cold water and the atmospheric subsidence of the subtropical high produce a strong temperature inversion. Thus little precipitation occurs. Consistently cool, moist surface air with frequent fog and overcast of low stratus clouds characterizes the coast. From about 7° S to 30° S, the coast of Peru and Chile receives less than 50 mm (2 inches) of annual rainfall. At Arica, Chile, during the period 1911-1949 average annual rainfall was 0.7 mm (0.03 inches), which is probably the driest point on earth. About 1/3rd of the rain accounting for that annual average occurred on a single day in January 1918. The high Andes paralleling the west coast of South America abruptly terminates the eastern end of the South Pacific subtropical high and produces maximum subsidence in the coastal area.

The Benguela Current flows northward along the southwest coast of Africa in the eastern South Atlantic. Cold currents in the Northern Hemisphere are not as large or as strong. The Canary Current flows southwestward along the coasts of the Iberian Peninsula and northwestern Africa past the Canary Islands.

The California Current flows southward along the West Coast of North America. During the summer, the water off the coast near Cape Mendocino north of San Francisco is often the coldest along the entire North American west coast (see Figs. 11.16 and 11.18 in Ahrens). Local upwelling is more significant than the southward horizontal movement of water from Alaska. Comparison of ocean surface water temperature in late August illustrates the nature of the cold current compared to the warm current (Gulf Stream) along the East Coast.