7.5: Climate and Ocean Surface Water Properties

Ocean surface water salinity and temperature are controlled by solar radiation, transfer of heat and water between atmosphere and oceans, ocean currents, vertical mixing, and locally, river runoff.

Ocean Surface Temperatures 

Figure 7-8 shows the generalized latitudinal distribution of solar radiation that reaches the Earth’s surface. Ocean surface water temperatures generally decrease from the equator toward the poles (Figs. 7-14, 7-18), and thus, they reflect the distribution of solar radiation.

Figure 7-14. Average surface water temperatures in the world oceans in (a) February and (b) August.

On an Earth without ocean currents or winds, ocean surface water temperatures would decrease uniformly with latitude, and the isotherms in Figure 7-14 would be horizontal lines parallel to lines of latitude. Although such a basic pattern is evident in Figure 7-14, complications are also readily apparent.

First, a band of low-temperature water lies along the equator on the eastern side of the Pacific Ocean. The band is a surface water divergence where cold, deep water is continuously upwelled (Chap. 8). The relatively low surface temperature at the equator in the Atlantic Ocean may be a result of similar, but less well-developed, upwelling. In the Indian Ocean, upwelling at the equator is inhibited by the monsoon winds.

The second complication evident in Figure 7-14 is that isotherms in mid latitudes are inclined toward higher latitudes on the western sides of the oceans. Such a pattern is especially evident in the North Pacific and North Atlantic Oceans. It is a result of warm surface ocean currents that flow poleward from the subtropics on the western sides of the ocean basins and then across the ocean to the east. The currents are part of the subtropical gyres discussed in Chapter 8. In the North Atlantic Ocean the current is the Gulf Stream, and in the North Pacific Ocean it is the Kuroshio Current.

Ocean surface water temperature responds to seasonal changes in the sun’s angle. High temperatures typical of tropical and subtropical latitudes extend to higher latitudes in summer (Fig. 7-14b). However, because of the heat-buffering action of ice (Chap. 5), surface water temperatures do not vary significantly with the seasons in the Arctic Ocean or the ocean surrounding Antarctica.

Ocean Surface Salinity

Differences in the salinity of ocean surface waters are caused by variations in the rate of ocean surface water evaporation, by variations in the rate of freshwater input from rainfall and land runoff, and, in limited areas, by upwelling and downwelling. Because geographic variations in evaporation and precipitation rates are somewhat complex, the distribution of ocean surface water salinity (Fig. 7-15) is somewhat more complicated than that of surface temperatures. Salinity is generally highest in surface waters in subtropical regions that are remote from land. It is generally lower at the equator, in polar and subpolar regions, and near continents. Surface salinity is higher in the subtropical Atlantic Ocean than in the subtropical regions of the other oceans.

Figure 7-15. Surface water salinity in the world oceans.

The distribution of salinity is related to variations in evaporation and precipitation with latitude (Fig. 7-16). Precipitation is high near the equator and at mid latitudes around 40° to 60°N and S, which are the atmospheric convergence regions between the Northern and Southern Hemisphere Hadley cells and between the polar and Ferrel cells (Figs. 7-10, 7-11). In these regions, warm, moist air converges and rises through the atmosphere. As the air rises, its water condenses to form clouds and precipitation. Precipitation is lower in subtropical latitudes (the horse latitudes) and near the poles because these regions are under the centers of the atmospheric convection cells or at divergences.

Evaporation varies with latitude in a different way than precipitation does (Fig. 7-16). The evaporation rate is higher at warmer ocean surface water temperatures. Hence, evaporation generally decreases progressively from equator to pole. However, the evaporation rate is lower in equatorial latitudes than in subequatorial regions for two main reasons. First, persistent cloud cover reduces the solar intensity in the equatorial region. Second, the equatorial region is persistently calm, whereas the trade wind regions have higher winds and hence greater evaporation. The higher rate of evaporation is due to both increased airflow over the water and the increased surface area of the water caused by waves, particularly breaking waves, which create water droplets and bubbles with relatively large surface areas.

Figure 7-16. Precipitation and evaporation as a function of latitude. (a) Precipitation is high at the atmospheric upwelling zones at the equator and at 50° to 60°N and S, somewhat lower in mid latitudes, and very low at high latitudes. Evaporation generally increases from the poles toward the equator, but it is slightly lower at the equator because of the extensive cloud cover. Thus, there is a net excess of evaporation over precipitation in mid latitudes, whereas there is a net excess of precipitation within a few degrees north and south of the equator and at latitudes above approximately 40° in both hemispheres. (b) The salinity of surface ocean waters varies according to the difference between evaporation and precipitation. It is high where evaporation exceeds precipitation, and low where precipitation exceeds evaporation.

Figure 7-16b shows that precipitation exceeds evaporation across the equatorial oceans and at latitudes above about 40°N and 40°S. Therefore, surface salinity tends to be lower in these regions. In contrast, evaporation exceeds precipitation in subtropical regions. Consequently, surface salinity tends to be higher in these regions (Fig. 7-16b). The pattern of higher salinity in subtropical regions and lower salinity in equatorial regions and at high latitudes is observed in surface waters of the central parts of each ocean (Fig. 7-15). The distribution is more complex in some regions near continents because of large quantities of freshwater runoff.

The distribution of precipitation with latitude is substantially altered by the presence of landmasses (Fig. 7-17). The higher surface salinity in the central North and South Atlantic oceans (Fig. 7-15) is the result of interactions of the mountain chains of North and South America with the prevailing winds, and the outflow of high-salinity water from the Mediterranean (Chap. 10). In the westerly wind zones, moist air masses that move from the Pacific Ocean onto the American continents lose their moisture as rainfall on the west side of the Andes in South America and on the west side of the Rocky Mountains, the Sierra Nevada, and the coastal mountain ranges in North America. Consequently, the precipitation runs off into the Pacific Ocean. In contrast, the Northern Hemisphere trade winds in the Atlantic Ocean blow across the relatively narrow, low-altitude neck of Central America and therefore carry their moisture all the way to the Pacific. The tongue of low-salinity water that extends westward into the Pacific Ocean from Central America (Fig. 7-15) is evidence of this transport.

Figure 7-17. Distribution of mean annual atmospheric precipitation. Precipitation is high in the tropical atmospheric upwelling zone at the equator, generally low at mid latitudes in the atmospheric downwelling zones between the Hadley and Ferrel cells, higher in the atmospheric upwelling zones between the Ferrel and polar cells, and low again in the polar atmospheric downwelling zones. This pattern is more evident in the Southern Hemisphere because it is less modified by the presence of landmasses than it is in the Northern Hemisphere.

The Southern Hemisphere trade winds in the Atlantic Ocean carry their moisture predominantly into the Amazon and Orinoco river basins. Both of these major rivers discharge into the Atlantic Ocean equatorial region, where salinity is generally low.

Extremely high and extremely low salinities are most common in marginal seas, which have limited water exchange with the open oceans. Hence, if evaporation exceeds precipitation, salinity increases in such seas, and the high-salinity water is not mixed effectively with the lower-salinity water of the open ocean. Surface water salinity is high in the Mediterranean and the Arabian Gulf, and it is especially high in the Red Sea. All of these regional seas are in arid regions (Fig. 7-17) with low river runoff and high evaporation rates. Similarly, if precipitation and river runoff exceed evaporation, the salinity of a marginal sea is low. The best examples are the Baltic Sea and the marginal seas of Southeast Asia. In the Baltic, evaporation is low, and rainfall and runoff are moderately high (Fig. 7-17). In the marginal seas of Southeast Asia, rivers fed by monsoon rains discharge huge volumes of freshwater, and high rainfall rates overcome a relatively high rate of evaporation.