Introduction to How the Ocean Affects Climate
Whether the forecast calls for a dry spell, a balmy day, heavy snow, or a hurricane, the ocean has played a major role in it. Although the sun is the engine that drives all weather on Earth, the ocean and atmosphere steer the sun's energy along certain paths to produce both regional climates and individual weather phenomena. For example, the climate of the West Coast of the United States is kept moderate by winds warmed by the Pacific Ocean. And Hurricane Mitch, which resulted in the deaths of more than 10,000 people in Central America in 1998, was spawned—like all hurricanes—by the ocean.
The ocean plays a crucial role in determining climate because of its ability to absorb, store, and transport heat from the sun. Ocean water also affects atmospheric temperature and circulation around the world. Furthermore, seawater is the source of most precipitation.
The top 3 meters (10 feet) of water in the ocean—most of this in the tropics and subtropics—hold as much heat as the entire atmosphere. The ocean's immense heat capacity means that it takes a huge amount of heat energy to change the temperature of a region of the sea. For instance, there must be many cold days to make an area of the ocean a little cooler and many hot days to make it warmer.
The upshot of this is that the ocean responds very slowly to changes in the seasons, causing it to have a moderating effect on climate. The sea makes winters in coastal regions a bit warmer and summers near the coast a bit cooler than they are farther inland.
Ocean-atmosphere Interactions
The ocean can warm or cool the air in a number of different ways. For example, when the air is at a lower temperature than seawater, the ocean transfers heat to the lower atmosphere, which becomes less dense as the heat causes molecules in the air to move farther apart. As a result, a low-pressure air mass forms over that part of the ocean. (Conversely, cool or cold waters lead to the formation of high-pressure air masses as air molecules move closer together.) Because air always flows from areas of higher pressure to those of lower pressure, winds are diverted toward the low-pressure area.
Among winds that are affected by such pressure changes are the jet streams, bands of fast-moving, high-altitude air currents. Jet streams supply energy to developing storms at lower altitudes and then influence their movement. In this way, the ocean alters the direction of storm tracks. Some storms even reverse direction as the result of ocean-influenced air-pressure changes.
The ocean's currents make it possible for these weather effects to be widely distributed. Some currents carry warm water from tropical and subtropical regions toward the poles, while other currents move cool water in the opposite direction. The Gulf Stream is a current that transports warm water across the North Atlantic Ocean from Florida toward Europe. Before reaching Europe, the Gulf Stream breaks up into several other currents, one of which flows to the British Isles and Norway. The heat carried in this current warms the winds that blow over these regions, helping to keep winters there from becoming bitterly cold.
In this way, the ocean's circulation compensates somewhat for the sun's unequal heating of the Earth, in which the tropics receive more energy from the sun than the poles. Were it not for the moderating effects of ocean currents on air temperatures, the tropics would be much hotter than they are and the polar regions even colder.
Besides transferring heat to the atmosphere, the ocean also adds water to the air through evaporation. When the sun's heat causes surface water to evaporate, warm water vapor rises into the atmosphere. As the water vapor rises higher, it cools into tiny water droplets and ice crystals, which collect together to form large clouds. The clouds soon return their moisture to the surface as rain, snow, sleet, or hail. Most evaporation occurs in the warm waters of the tropics and subtropics, providing moisture for tropical storms.
Virtually all rain comes from the evaporation of seawater. Though this may seem surprising, it makes sense when one considers that about 97 percent of all water on Earth is in the ocean. The Earth's water cycle, or hydrologic cycle, consists largely of the never-ending circulation of water from the ocean to the atmosphere and then back to the ocean.
The Role of Ocean Phenomena
The sea's influence on climate is periodically highlighted by two ocean phenomena that exert dramatic influences on weather patterns across the United States and many other countries. These phenomena are called El Nino and La Nina.
During an El Nino event, which usually lasts about a year and recurs every two to seven years, east-to-west trade winds in the tropical Pacific weaken or reverse direction. The change in winds causes ocean currents to flow eastward, transferring warm water from the western Pacific to the central and eastern Pacific. A low-pressure air mass—the type of air mass in which stormy weather develops—builds over the warm waters of the central and eastern Pacific. This air system carries heavy rainfall to the Pacific coast of South America. At the same time, a high-pressure system forms over the cool western Pacific and may lead to drought conditions in Southeast Asia.
Because the changes in air pressure associated with El Nino disrupt the normal circulation of the atmosphere, weather patterns in other parts of the world are also altered. In the United States, for example, El Nino events usually result in milder winters in the Midwest, heavy rains in the South, and dry conditions in the Pacific Northwest. Meteorologists said the El Nino of 1997-1998 led to severe flooding, landslides, and several deaths in North Carolina, Tennessee, and California.
A La Nina event often develops after an El Nino. La Nina is the climatic opposite of El Nino and occurs when strong trade winds push warm surface water westward, exposing lower cool waters in the east. As a result, a La Nina episode is characterized by cooler-than-normal water in the central and eastern Pacific and warmer-than-normal water in the western Pacific. This situation can lead to severe storms in Southeast Asia and drought in South America. Meteorologists said a La Nina that occurred in 1998-1999 also brought heavy rain and snow to the upper Midwest and the Pacific Northwest.
Oceanographers noted that conditions typical of La Nina continued into 2001, well beyond the one-to-two year length of a typical La Nina episode. They said this was probably due to the development of a long-term ocean condition called the Pacific Decadal Oscillation. Many scientists believe that this condition, which is characterized by cold waters off the Pacific coasts of North and South America, can last 20 to 30 years and may recur every few decades. They said this cold-water phase might cause harsh winter weather across the Midwest and Northeast for years to come.
Short-term and Long-term Influences
Patterns of warm and cold water in the ocean lead to the development of seasonal weather events as well. Two dramatic examples of such events are hurricanes and monsoons.
Hurricanes—called "typhoons" when they occur over the Northwest Pacific and "tropical cyclones" when they occur near Australia or over the Indian Ocean—are infamous for their powerful, destructive winds. They form where ocean water with a temperature of more than 26.5 degrees C (80 degrees F) evaporates. The water vapor is absorbed by the surrounding air. As the warm, moist air rises, the atmospheric pressure of the air below drops. Cooler air then swirls into this low-pressure area and begins to rotate around the low. As the air flow gains velocity, winds increase in intensity.
Hurricanes are warm-weather phenomena. In the Atlantic and Northeast Pacific, they are most common in August and September, when water temperatures are highest. Typhoons can form in the Northwest Pacific throughout the year, but usually do so in summer. Tropical cyclones often strike the Indian Ocean region in May and November. The only significant hurricane threat in the United States is on the East Coast and along the coast of the Gulf of Mexico. Hurricanes in the Atlantic Ocean can grow more intense during La Nina episodes.
Like hurricanes, monsoons are a type of seasonal weather event resulting from special ocean conditions. Unlike the destructive nature of hurricanes, however, the winds of a monsoon bring needed rains that irrigate rice crops for tens of millions of people in Asia. Disruptions in monsoon patterns can lead to famine.
The winds of a monsoon blow continuously between April and October from the southwest over the northern Indian Ocean and onto the land. These winds are caused by differences in temperature between the land and sea. During summer, the sun heats the land more than the sea, and the air over the land warms and rises. To replace this air, moisture-laden air from the sea flows inland. As the moist air rises over the land, it cools, and the water vapor condenses to form clouds. The water in the clouds then falls as rain. In winter, when the sea is warmer than the land, the process is reversed. Then, cold, dry air over the land sinks and spreads out to the sea from the northeast.
The ocean not only influences weather and climate on a short-term basis, it also exerts a long-term effect on global climate. One of the ways it does this is by absorbing carbon dioxide gas from the atmosphere. Carbon dioxide is a type of greenhouse gas, a chemical compound that acts much like a greenhouse to trap solar heat in the lower atmosphere. Many scientists believe that increasing amounts of carbon dioxide, released into the air by the burning of fossil fuels, are gradually raising the temperature of Earth's surface in a process called global warming. Researchers anticipate that one result of global warming may be the melting of glaciers and ice caps, with a subsequent rise in ocean levels.
But the ocean may be helping to slow global warming through its ability to remove vast quantities of carbon dioxide from the atmosphere. It is able to do this mainly because of the activity of tiny organisms called phytoplankton. These microbes carry out photosynthesis, the use of energy from the sun to make food from the chemical combination of carbon dioxide and water. Many researchers believe that were it not for the absorption of carbon dioxide by marine organisms, global warming would be occurring at a much faster rate that it actually is.
Much Remains Unknown
Whether harmful or beneficial to people, the climate depends on the ocean. The interactions between the ocean and atmosphere are so intricate and complex that weather forecasters in 2001 had a long way to go until they could make reliable long-term forecasts. Nonetheless, the interplay between water and air is a basic fact of everyday life. So the next time you walk in the rain, watch snow fall, bundle up against the cold, or bask in bright sunshine, remember the awesome power that the ocean has over the weather.