Effects of Air-Sea Interchange
The atmosphere and the oceans have tre-mendous effects on each other. These effects are principally in the realms of temperature and water vapor. The heat balance of the oceans is main-tained by the processes of radiation, the exchange of sensible heat, and the evaporation and conden-sation of water vapor on the sea surface.
The amount of radiant energy absorbed by the sea depends upon the amount of energy reaching the surface and the amount of reflection by the surface. When the Sun is directly overhead, the amount of its energy reflected amounts to only about 3 percent. Even when the Sun is 30° above the horizon, the amount of reflection is just 6 percent. However, there is a reflection of about 25 percent of the energy when the Sun is 10° above the horizon. (See fig. 6-7-1.) Reflection loss is especially great in the presence of waves when the Sun is low.
Much of the insolation is absorbed in the first meter of seawater. This is true of the clearest water as well as of quite turbid (opaque) water. In water that is extremely turbid, the absorption is in the very uppermost layers. Foam and air bubbles are two major causes of a proportionately greater amount of absorption in the uppermost meter of the sea. However, due to vertical mixing, the heat absorbed in the upper layer is carried
to great depths of the ocean, which acts as a great
to great depths of the ocean, which acts as a greatheat storage reservoir.
There is an exchange of energy between the oceans and the atmosphere. The surface of the oceans emits long-wave heat radiation. The sea surface at the same time receives long-wave radia-tion from the atmosphere. Although some of this incoming radiation from the atmosphere is reflected from the surface of the oceans, most of it is absorbed in a very thin layer of the water sur-face. The difference between the incoming long-wave atmospheric radiation and the outgoing long-wave radiation from the sea surface is known as the EFFECTIVE BACK RADIATION. The effective back radiation depends primarily on the temperature of the sea surface and on the water vapor content of the atmosphere. The time of day and the season have little effect on effective back radiation, since the diurnal and annual variation of the sea-surface temperature and of the relative humidity of the air above the oceans is slight.
For conduction to take place between the oceans and the atmosphere, there must be a temperature difference between the ocean surface and the air immediately overlying it. On the average, the temperature of the surface of the oceans is higher than that of the overlaying air.
It might be expected that all of the ocean’s surplus of heat is either radiated or conducted to the atmosphere. This is not the case. Only a small percentage of the ocean’s surplus heat is actually conducted to the atmosphere. About 90 percent of the surplus is used for evaporation of ocean water.
Due to the processes of radiation and mixing, the oceans act as a thermostat relative to the atmosphere. The energy stored at one place dur-ing one season may be given off at another locality and during a later season. Hence, there seems to be a constant effort by the atmosphere and the oceans to keep their temperatures in balance by an interchange of heat.
STABILITY.—The deciding factor of most weather phenomena is the stability of the at-mosphere. Air masses may become more stable or less stable as they move over ocean surfaces. The temperature contrast between the ocean sur-face and the lowest layers of the overlying air determines whether the ocean will promote sta-bility or instability.
When the air moving over the ocean has ahigher temperature than that of the ocean surface, the lower layers of the air become stable in time. On the other hand, when the air mass is colder than the ocean surface over which it is moving, instability results. As the colder air is warmed by the ocean, convective activity eventually develops. If the warming is sufficiently intense, thunderstorms develop.
MOISTURE CONTENT.—The interchange of moisture between the atmosphere and the oceans is one of the most important features of the whole meteorological picture. Without this in-terchange, weather, as we know it, could not exist; there would be no clouds and no precipita-tion. The oceans are by far the greatest source of moisture for the atmosphere. Other moisture sources are negligible in comparison. Whether the atmosphere gives up some of its moisture to the ocean or vice versa depends greatly upon vapor pressure. Vapor pressure is the pressure exerted by the molecules of water vapor in the atmosphere or over the surface of liquid water. When the vapor pressure of a liquid is equal to that of the atmosphere above the li-quid, there is little or no apparent interchange of moisture. In other words, at equal vapor pressure, just as many molecules escape from the liquid to the atmosphere and vice versa. This is the case when air becomes saturated. The saturation vapor pressure increases with increasing temperature.
If the temperature of the surface water is warmer than that of the air, the vapor pressure of the water at its surface is greater than that of the air. When this condition exists, there can beabundant evaporation from the ocean surface. This evaporation is aided by the turbulence of the air brought on by the unstable condition of the lower layers. It follows, then, that the greatest evaporation takes place when cold air flows over warm ocean waters.
Let us consider the opposite condition-warmair flowing over a relatively cold body of water. When this happens, there is stable stratification in the lower layers of the atmosphere. The vapor pressure of the air soon reaches a state of equilibrium with that of the water surface. Evaporation stops. However, if the warm air is quite moist, it is possible for the moisture in the air to condense on the water surface. Contact of the warm air with the cold water may result in the formation of fog by lowering the air temperature to the dew point.
The direct interchange of moisture from theatmosphere to the oceans occurs through precipitation and, to a lesser extent, condensation. The direct interchange, however, is not as impor-tant meteorologically as the indirect interchange. The indirect interchange is a sequence of events beginning with the evaporation of water from the ocean surfaces and ending with the subsequent condensation and precipitation over land areas. Generally, precipitation occurs more fre-quently over land than over the oceans. Though the oceans are a source of abundant moisture, they normally lack the required precipitation mechanisms, such as vertical mixing, strong temperature contrasts, and orographic lifting.