7.11: Chapter Summary

Atmosphere and Water Vapor. 

Air moves vertically when a change in temperature or in its concentration of water vapor causes its density to change. In the lower atmosphere, vertical movements are generally limited to the troposphere (about 12 km altitude). The addition of water vapor to air decreases its density because lighter water vapor molecules displace heavier nitrogen and oxygen molecules. Atmospheric convection is caused by heating or evaporation at the sea or land surface. Water vapor continuously evaporates from the oceans into the atmosphere. As air rises, its pressure decreases, causing it to expand and cool. The saturation pressure of water vapor in air decreases with decreasing temperature. Hence, as air cools, it becomes supersaturated with water, which condenses to rain or ice crystals.

Water and Heat Budgets. 

Enough water is evaporated from land and oceans each year to cover the world about 1 m deep. About 93% of this water comes from the oceans, but nearly 30% of the resulting precipitation falls on land. The excess of precipitation over evaporation on land enters lakes, streams, and rivers and returns to the oceans as runoff.

Almost 25% of the solar radiation reaching the Earth is absorbed and converted to heat in the atmosphere. About 50% is absorbed by oceans and land, and the rest is reflected to space. Of the heat absorbed by the oceans, about half is lost by radiation, and half is transferred to the atmosphere as latent heat of vaporization. Solar energy per unit area received by the Earth is at a maximum at the equator and decreases toward the poles. However, the amount of heat radiated and reflected per unit area varies significantly with latitude and other factors. At the equator, more heat energy is received than is lost to space, whereas at the poles, more heat is lost than is received. Heat is transferred from the tropics to the polar regions by atmospheric and oceanic currents.

Climatic Winds. 

Ocean-to-atmosphere heat transfer, particularly through sensible and latent heat fluxes, plays a key role in fueling atmospheric convection, especially over tropical oceans. The atmospheric three-cell per hemisphere system consists of Hadley, Ferrel, and polar cells arranged between the equator and the pole in each hemisphere. This structure is modified, especially over the continents, as a result of land-ocean-atmosphere interactions driven by the far greater heat capacity of ocean waters compared to the land surface. Trade winds in the Hadley cell blow westward and toward the equator, and westerly winds in the Ferrel cell blow eastward and toward the pole. Between cells, winds are generally calm. Rainfall and clouds are heavy at upwelling regions, and winds are light and skies are clear at downwelling regions. The cells shift north and south seasonally as the Earth’s angle to the sun changes. Because solar heat is released to the atmosphere relatively slowly by evaporation, the location of the atmospheric cells lags behind the location of greatest solar heating as the cells migrate seasonally.

Climate and Ocean Surface Water Properties.

Ocean surface water temperatures generally decrease with latitude, but currents and upwelling distort the pattern. Surface water salinity is primarily determined by differences between evaporation and precipitation rates, except near continents, where freshwater runoff is high. Salinity is highest in the subtropics and polar regions, where rainfall is low. Salinity is low at mid latitudes, where rainfall is high and evaporation is less than at lower latitudes, and at the equator, where evaporation is reduced by persistent cloud cover and lack of winds. Extreme high salinity occurs in marginal seas where evaporation exceeds precipitation, and extreme low salinity occurs in marginal seas where precipitation exceeds evaporation.

Interannual Climate Variations. 

El Niño/Southern Oscillation is a complex, coupled ocean-atmosphere phenomenon that occurs across the equatorial region of the Pacific Ocean. Warm surface water is transported westward by the trade winds until it accumulates near Indonesia. When the winds slow, the water flows back to the east along the equator, where there is no Coriolis Effect. During El Niño, upwelling is stopped near Peru, with often disastrous effects on marine life. Droughts occur in locations as far away as Central Europe, and severe storms occur in California and other places. When an El Niño ends, the system may overshoot to produce La Niña. However, the strength and severity of La Niña events vary, and in some regions La Niña can have impacts that are equally, or even more, pronounced than those of El Niño.

In addition to ENSO, interannual oscillations of ocean-atmosphere characteristics have been identified in the North Pacific, North Atlantic, Arctic, and Indian Oceans. Each of these oscillations affects regional climates, but the effects are generally smaller and less widespread than those due to ENSO. Studies of the Pacific Decadal Oscillation suggest that it, and probably other such oscillations, and their ecosystem effects are chaotic and may cause unpredictable irreversible ecosystem change.

Global Climate Zones. 

Climate zones in the oceans are generally arranged in latitudinal bands. Land climate zones are more complex and depend on proximity to the ocean and the locations of mountain ranges. The ocean moderates coastal climates because surface waters do not change much in temperature either during the day or during the year. The moderating influence of the oceans can extend far into the continents in regions where the winds are generally onshore and no mountain chains block the passage of the coastal air inland.

Weather Systems. 

In the free atmosphere, above the influence of surface friction, winds around high- and low-pressure systems often flow approximately geostrophically, or nearly parallel to isobars or height contours due to a balance between the pressure gradient force and the Coriolis force. Near the surface, however, friction disrupts this balance, causing winds to cross isobars toward lower pressure. Winds flow counterclockwise around a low and clockwise around a high in the Northern Hemisphere, and vice versa in the Southern Hemisphere. Tropical cyclones form over warm water in latitudes high enough that the Coriolis effect is significant. Tropical cyclone winds blow in toward the eye in a rising helical pattern. Winds accelerate as heat energy is added by evaporation from the ocean surface. but the hurricane loses strength when it moves over land or cool water, where its energy supply is removed. Extratropical cyclones form at the polar front as warm air flows over cold polar air and is deflected by the Coriolis effect.

Local Weather Effects.

The Earth’s surface heats by day from solar radiation and cools at night as the heat is lost. Primarily because of water’s high latent heat, the ocean surface water temperature varies little in this daily cycle. Thus, land temperatures vary more. During the day, land next to an ocean heats, then loses some of its heat to the air. The warmed air rises and is replaced by cooler air from over the ocean, creating a sea breeze. At night, the land cools rapidly, which creates a land breeze that flows seaward to displace warmer, less dense air over the ocean. Coastal fogs form when warm, moist air from over the ocean passes over a cold coastal water mass as it enters the sea breeze system.

When moist air encounters mountains, it is forced to rise, expand, and cool. If it rises high enough, water vapor condenses or deposits onto nuclei and causes precipitation. After crossing the mountains, the air mass descends, is compressed, and warms. When it is no longer saturated with water vapor, precipitation ceases. Hence, the windward side of the mountains is wet, and the leeward side is arid.