Surface waters freeze as they reach the arctic waters of the North Atlantic. The freezing process removes water molecules, but not salt, from the ocean. The result is an increase in the salinity of ocean waters. This process sets up a large, slow, deep-water "conveyor belt" that transports water along the ocean floor to Antarctica then through the Indian, Pacific, and eventually Atlantic oceans.
The combination of oceanic and atmospheric circulation drives global climate by redistributing heat and moisture. Areas located near the tropics remain warm and relatively wet throughout the year. In temperate regions, variation in solar input drives seasonal changes.
In the Northern Hemisphere where land masses are more concentrated, these seasons can involve pronounced changes in temperature. In the Southern Hemisphere where large land masses are located nearer to the equator and the majority of Earth's surface is covered with water, seasonal cycles revolve around the presence and absence of precipitation rather than major swings in temperature.
Global climate patterns are dynamic: They are continually changing in response to solar radiation, atmospheric greenhouse gas concentrations, and other climate forcing factors.
Among the more predictable of these changes are cyclical changes in solar radiation reaching the poles. These cycles, first described by Milutin Milankovitch , involve Earth's orbit, tilt, and the precession of the equinoxes.
Earth's elliptical orbit around the sun shifts under the gravitational pull of other planets in our solar system. In a ,year cycle, the orbit shifts from one that is nearly circular to one that is elongated, pulling the planet further from its energy source Figure 5A.
Earth's tilt relative to its orbit changes in a 41,year cycle from Finally, the axis north-south orientation of the Earth wobbles over time. This 23,year precession of the equinoxes changes the orientation of the planet relative to its location in orbit Figure 5C.
When all three Milankovitch cycles reinforce each other, they alter solar input and influence oceanic and atmospheric circulation patterns. This can lead to regular periods of cooling and glaciation. Figure 5: Milankovitch cycles A High eccentricity in Earth's orbit takes it further away from the sun.
B The degree of Earth's tilt relative to its plane of orbit changes the degree of warming in the polar regions. C Precession of the equinoxes occurs as Earth wobbles on its axis. All three cycles can influence warming and cooling periods by altering the amount of solar radiation that reaches Earth. Periods of cooling can be intensified through albedo; the presence of snow and ice reflects incident sunlight and heat, which serves to further cool the planet.
In this way, glaciers and polar ice caps continue to grow during periods when incident sunlight is low. As more water becomes locked up as ice, the surface level of oceans drops, which can alter oceanic circulation patterns. In addition, movement of continental land masses through the processes of plate tectonics can shift the flow of water, altering ocean currents and circulation patterns.
As Earth's precession and tilt increase polar exposure to sunlight, rapid melting events can occur. Freed from the grip of ice, soils thaw and previously frozen vegetation decays, releasing both carbon dioxide and methane gas — two noted greenhouse gases — into the atmosphere.
Increases in carbon dioxide and methane in the atmosphere help to further warm the earth, and these gases are thought to have contributed to historical rapid warming events. The current distribution of plants and animals reflects historical changes in both global climatic conditions and the location of land masses.
During cold periods, when much of the land was covered in snow and ice, the amount of land available for terrestrial organisms to inhabit decreased, increasing competition for resources. As the ice retreated during warming events, organisms migrated to fill newly-available areas, and many species flourished under the new environmental conditions. Over time, organisms evolved adaptations that better enabled them to exploit their new surroundings. Some of those adaptations persist in their modern-day descendants.
While climatic conditions were changing, so were the locations of large land masses as they shifted under the influence of magma currents beneath the crust. Continental collisions built mountain ranges and widening rifts became seas, both of which served to create barriers to organismal dispersal, restricting the ability of organisms to migrate. Restricted to smaller areas, organisms evolved traits that best suited them to the environmental conditions of their continent and region.
Today we recognize six biogeographic realms — Nearctic, Palearctic, Neotropical, Ethiopian, Oriental, and Australian — in which animals exhibit features distinctive to that region Figure 6. Realms that have experienced barriers to dispersal for longer periods of time contain animals with more distinctive traits. One of the best examples of this can be seen in the marsupial mammals of the Australian Region, which has a long history of isolation from other continents. Figure 6: Biogeographic realms Variation in climate and presence of barriers to dispersal has led to six major realms of organisms.
The realms are not clearly delineated, rather species mix along boundaries. Brown, J. Biogeography , 2nd ed. Sunderland, MA: Sinauer Associates, Ekman, V. On the influence of the Earth's rotation on ocean currents.
Gross, M. Oceanography: A View of Earth. Holton, J. An Introduction to Dynamical Meteorology , 4th ed. Lutgens, F. The Atmosphere: An Introduction to Meterorology , 8th ed. Milankovitch, Milutin. Canon of Insolation and the Ice Age Problem. National Oceanic and Atmospheric Administration.
Pidwirny, M. Ruddiman, W. Earth's Climate: Past and Future , 2nd ed. New York, NY: W. Freeman and Company, Trefil, J. These same tropical storms are known as hurricanes in the Atlantic Ocean, cyclone s in the northern Indian Ocean, and typhoon s in the western Pacific Ocean.
These tropical storms have a spiral shape. The spiral swirling counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere develops as a high-pressure area twists around a low-pressure area. Wind conditions that can lead to hurricanes are called tropical disturbances. They begin in warm ocean waters when the surface temperatures are at least If the disturbance lasts for more than 24 hours and gets to speeds of 61 kph 38 mph , it becomes known as a tropical depression.
When a tropical depression speeds up to kph mph , it is known as a tropical storm, and is given a name. Meteorologist s name the storms in alphabetical order, and alternate with female and male names.
When a storm reaches kph 74 mph , it becomes a hurricane and is rated from 1 to 5 in severity on the Saffir Simpson scale. A Category 5 hurricane is the strongest storm possible on the Saffir-Simpson scale. Winds of a Category 5 blow at kph mph. Hurricane Ethel, the strongest hurricane in recorded history, roared across the Gulf of Mexico in September Winds were sustained at kph mph.
However, Hurricane Ethel quickly dissipate d. Although its winds ultimately blew as far north as the U. Hurricanes bring destruction to coastal ecosystems and communities. When a hurricane reaches land, it often produces waves that can reach 6 meters 20 feet high and be pushed by high winds kilometers miles inland.
These storm surge s are extremely dangerous and cause 90 percent of all hurricane deaths. The deadliest hurricane on record is the Great Hurricane of Although sophisticated meteorological equipment was not available at that time, winds may have reached kph mph as the hurricane hit Barbados and other islands in the Caribbean Sea.
This may have been enough to strip the bark from trees. More than 20, people died as a result of the hurricane as it made its way across Barbados, St. Although it decreased in intensity, the hurricane was tracked through the U. Hurricanes can be destructive in other ways. High winds can create tornadoes. Heavy rains contribute to floods and landslides, which may occur many kilometers inland. Damage to homes, businesses, schools, hospitals, roads, and transportation systems can devastate communities and entire regions.
Hurricane Katrina, which blew through the Gulf of Mexico and into the southern U. New Orleans, Louisiana, was almost completely devastated by Hurricane Katrina. New Orleans, as well as Mobile, Alabama, and Gulfport, Mississippi, took years to recover from the damage done to their structures and infrastructure. The best defense against a hurricane is an accurate forecast that gives people time to get out of its way.
The National Hurricane Center issues hurricane watches for storms that may endanger communities, and hurricane warnings for storms that will reach land within 24 hours. Cyclones Cyclones blow through the Indian Ocean in the same way hurricanes blow across the Atlantic.
Cyclones blow in with air masses from the east, often the South China Sea, or the south. The most powerful and devastating cyclone in recorded history was the Bhola Cyclone. Its winds were about kph mph as it made landfall along the coast of the Bay of Bengal, in what is today Bangladesh. More than , people died, and more than a million were made homeless. Cyclone winds devastated fishing villages, and storm surges drowned crops.
Typhoon Typhoons are tropical storms that develop over the northwest Pacific Ocean. Their formation is identical to hurricanes and cyclones. Typhoons form as equatorial winds and blow westward before turning north and merging with westerlies around the mid-latitudes. Typhoons can impact a wide area of the eastern Pacific.
The islands of the Philippines, China, Vietnam, and Japan are the most affected. However, typhoons have also been recorded as far as the U. Typhoons are often associated with extremely heavy rainfall. The wettest typhoon ever recorded was Typhoon Morakot in Morakot devastated the entire island of Taiwan, with winds of about kph 85 mph. Storm surges and floods caused by those winds, however, caused the most damage. It blows from the northeast along the East Coast of the U.
The U. Weather Service calls a storm a blizzard when the storm has wind speeds of more than 56 kph 35 mph and low visibility. Visibility is the distance that a person can see—blizzards, like fog, make visibility difficult and a task like driving dangerous.
The storm must go on for a prolonged period of time to be classified as a blizzard, usually a few hours. Blizzards can isolate and paralyze areas for days, especially if the area rarely has snowfall and does not have the equipment to clear it from the streets.
The Great Blizzard of was perhaps the worst in U. More than centimeters 58 inches of snow fell across the region, causing freezing temperatures and massive flooding as the snow melted.
Monsoon A monsoon is a seasonal change in the prevailing wind system of an area. They always blow from cold, high-pressure regions. Monsoons are part of a yearlong cycle of uneven heating and cooling of tropical and mid-latitude coastal regions. Monsoons are part of the climate of Australia, Southeast Asia, and in the southwestern region of North America. The air over land is heated and cooled more quickly than the air over the ocean. During summer, this means warm land-air rises, creating a space for the cool and moist air from the ocean.
As the land heats the moist air, it rises, cools, condenses, and falls back to Earth as rain. During the winter, land cools more quickly than the ocean.
The warm air over the ocean rises, allowing cool land-air to flow in. Most winter monsoons are cool and dry, while summer monsoons are warm and moist. The famous summer monsoon, on the other hand, develops over the Indian Ocean, absorbing tremendous amounts of moisture. The summer monsoon is essential for the health and economies of the Indian subcontinent.
Aquifers are filled, allowing water for drinking, hygiene , industry , and irrigation. Tornado A tornado , also called a twister, is a violently rotating funnel of air. Tornadoes can occur individually or in multiples, as two spinning vortex es of air rotating around each other. Tornadoes can occur as waterspout s or landspouts, spinning from hundreds of meters in the air to connect the land or water with clouds above.
Although destructive tornadoes can occur at any time of day, most of them occur between 4 and 9 p. Tornadoes often occur during intense thunderstorms called supercells. A supercell is a thunderstorm with a powerful, rotating updraft. A draft is simply a vertical movement of air. This powerful updraft is called a mesocyclone.
A mesocyclone contains rotating drafts of air 1 to 10 kilometers 1 to 6 miles in the atmosphere. When rainfall increases in the supercell, rain can drag the mesocyclones down with it to the ground. This downdraft is a tornado. Depending on the temperature and moisture of the air, a tornado can last a few minutes or over an hour. However, cool winds called rear flank downdrafts eventually wrap around the tornado and cut off the supply of warm air that feeds it.
Most tornadoes have wind speeds of less than kph mph , and are about 76 meters feet across. They can travel for several kilometers before dissipating. However, the most powerful tornadoes can have wind speeds of more than kph mph and be more than 3 kilometers 2 miles across. These tornadoes can travel across the ground for dozens of kilometers and through several states. These violent storms occur around the world, but the United States is a major hotspot with about a thousand tornadoes every year.
The most extreme tornado ever recorded occurred on March 18, The tornado destroyed local communications, making warnings for the next town nearly impossible.
The Tri-State Tornado killed people in 3. The best protection against a tornado is early warning. In areas where tornadoes are common, many communities have tornado warning systems.
In Minnesota, for example, tall towers throughout neighborhoods sound an alarm if a tornado is near. Measuring Winds Wind is often measured in terms of wind shear. Wind shear is a difference in wind speed and direction over a set distance in the atmosphere. Wind shear is measured both horizontally and vertically. Wind shear is measured in meters per second times kilometers of height. Under normal conditions, the winds move much faster higher in the atmosphere, creating high wind shear in high altitudes.
Wind shear is higher near the coast, for example. The amount of force that wind is generating is measured according to the Beaufort scale. The scale is named for Sir Francis Beaufort, who established a system for describing wind force in for the British Royal Navy. The Beaufort scale has 17 levels of wind force. An anemometer is a device for measuring wind speed. Anemometers are used with tornado data collectors, which measure the velocity , precipitation, and pressure of tornadoes.
The scale has six categories that designate increasing damage. In , the Enhanced Fujita Scale was established in the U. The Enhanced Fujita Scale has 28 categories, with the strongest cataloging damage to hardwood and softwood trees.
Hurricanes are measured using the Saffir-Simpson scale. In addition to tropical depressions and tropical storms, there are five categories of hurricanes. The most powerful, Category 5, is measured by winds whipping at kph mph.
Impact on Climate Wind is a major factor in determining weather and climate. Wind carries heat, moisture, pollutants, and pollen to new areas. Many daily weather patterns depend on wind. A coastal region, for instance, undergoes changes in wind direction daily. The sun heats the land more quickly than the water. Warm air above the land rises, and cooler air above the water moves in over the land, creating an inland breeze.
Coastal communities are usually much cooler than their inland neighbors. Rain shadow s are created as wind interacts with a mountain range. As wind approaches a mountain, it brings moisture with it, which condense s as rain and other precipitation before coming over the crest of the mountain. Winds also help drive ocean surface currents around the world.
Geophysical Research Letters , doi Yan, W. Published on 06 June The authors. Any reuse without express permission from the copyright owner is prohibited. Iddris et al.
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