08 - Circulation sum up Energy from the Sun doesn't fall...

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Unformatted text preview: Circulation sum up Energy from the Sun doesn't fall equally all over the Earth. Most of the Sun's energy enters the Earth at the equator. This leads to large temperature gradients between the equator and the Poles. Movement of both the air and the oceans is controlled by these temperature differences and the result is a transfer of heat from the equator to the poles. About half the heat transport around the planet is by the oceans so the oceans are an extremely important part of the Earth's climate control system. Earth image by Apollo 17 Dec.7, 1972 Solar Heating of Earth Varies with Latitude How solar energy input varies with latitude. Equal amounts of sunlight are spread over a greater surface area near the poles than in the tropics. Ice near the poles reflects much of the energy that reaches the surface there. "Convection current" Sea Breezes and Land Breezes Arise from Uneven Surface Heating The flow of air in coastal regions during stable weather conditions.... conditions... (a) In the afternoon, the land is warmer than the afternoon, ocean surface, and the warm air rising from the land is replaced by an onshore sea breeze. breeze. (b) At night, as the land cools, the air over the ocean is now warmer than the air over the land. The ocean air rises. Air flows offshore to replace it, generating an offshore flow (land breeze). (land breeze). Afternoon SEA BREEZE Night LAND BREEZE 1 Coriolis effect To observers on the surface, Earth's rotation causes moving air (or any moving mass) in the Northern Hemisphere to curve to the right of its initial path, and in the Southern Hemisphere to the left. The Coriolis Effect Deflects the Path of Moving Objects Sketch of the thought experiment in the text, showing that Buffalo travels a shorter path on the rotating Earth each day then Quito does. A continuation of the thought experiment. A look at Earth from above the North Pole shows that Buffalo and Quito move at different velocities. large circuits of circulating air driven by: 1) uneven solar heating 2) coriolis effect Hadley cells- tropical cells on each side of equator cells Ferrel cells- mid-latitudes cells- mid Polar cells- near the poles cellsWind patterns found between and within cells? Doldrums- calm equatorial areas where Hadley cells converge DoldrumsHorse latitudes- areas between Hadley and Ferrel cells latitudesTrade winds- surface winds of Hadley cells windsWesterlies- surface winds of Ferrel cells Westerlies- Atmospheric circulation cellcell- In general... In reality... 2 Intertropical convergence zone (ITCZ) ascent of warm, moist air from the latitudes above and below the equator drawn into the ITCZ by the action of the Hadly cell; regions in the ITCZ receive precipitation > 200 days /year. satellite image of thunderstorms over ITCZ Seasonal changes in the position of the intertropical convergence zone (ITCZ). The zone reaches its most northerly location in July and its most southerly location in January. Because of the thermostatic effect of water, the seasonal northsouth movement is generally less over the ocean than over land. This image is a combination of cloud data from NOAA's newest Geostationary Operational Environmental Satellite (GOES-11) and color land cover classification data (earthobservatory.nasa.gov). Wind-driven surface currents Ocean water circulates at the surface due to wind friction affect the uppermost 10% of the world ocean powered by the warmth of the sun and by wind tend to flow horizontally (also upwelling and downwelling) downwelling) mostly above the pycnocline Some surface currents are rapid and riverlike, with wellriverlike, welldefined boundaries; others are slow and diffuse. flow path effected by Coriolis force The largest surface currents are organized into huge circuits known as gyres transfer heat from the tropics to the poles influence weather and climate distribute nutrients and scatter organisms in ocean have contributed to the spread of humanity around the globe and are important in maritime commerce Major winds influencing surface currents The westerlies and the trade winds are two of the winds that drive the ocean's surface ocean' currents. About 10% of the water in the world ocean is involved in flowing horizontally in the uppermost 400 meters (1,300 feet) of the ocean's surface, ocean' driven mainly by wind friction. Winds, driven by uneven solar heating and Earth's spin, drive Earth' the movement of the ocean's ocean' surface currents. The prime movers are the powerful westerlies and the persistent trade winds (easterlies). surface currents, water Gyres A combination of 4 forces surface winds, the sun's heat, sun' the Coriolis effect, & gravity Example: The North Atlantic gyre, a series of 4 interconnecting currents with different flow characteristics & temperatures circulates the ocean surface clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, Hemisphere, forming gyres. 3 Coriolis influence on surface currents Surface water blown by the winds at point A will veer to the right of its initial path and continue eastward. Water at point B veers right and continues westward. Ekman spiral The Ekman spiral and the mechanism by which it operates. The length of the arrows in the the diagrams is proportional to the speed of the current in each layer. (a) The Ekman spiral model. layer. (b) A body of water can be thought of as a set of layers. The top layer is driven forward by the top wind, and each layer below is moved by friction. Each succeeding layer moves with a slower speed and at an angle to the layer immediately above it to the right in the Northern Hemisphere, to the left in the Southern Hemisphere until water motion becomes negligible. (c) Though the direction of movement varies for each layer in the stack, the theoretical net flow of stack, water in the Northern Hemisphere is 90 to the right of the prevailing wind force. 90 Pressure gradients- "hills" The hill of water in the North Atlantic. (a) The surface of the North Atlantic is raised through wind motion and Ekman transport to form a low hill. Water from point B turns westward and flows along the side of this hill. The westward-moving water is balanced between the Coriolis westwardeffect (which would turn the water to the right) and flow down the the pressure gradient, driven by gravity (which would turn it to the left). Thus, water in a gyre moves along the outside edge of an ocean basin. basin. (b) The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline. thermocline. Avg height of North Atlantic surface (1992 TOPEX/Poseidon satellite). Red=highest surface; green and blue=lowest. The gradually sloping hill is only 2 m high. 6 major surface currents 5 Geostrophic gyres (in balance between the pressure gradient and the Coriolis effect) 1 boundary current 2 in northern hemisphere, 4 in southern hemisphere Sea-surface temperatures measured by a radiometer aboard NOAA-7 July 1984. Red=2528Celsius (7782F); Yellow=2025C (6877F); Green=1520C (5968F); Blue=015C (3259F); Purple=<0C 4 Eastern vs Western boundary currents Western boundary current: ex- Gulf Stream, eastern US Eastern boundary current: ex- California current Western boundary currents narrow, deep, fast, warm The The The The The Gulf Stream Japan Current Brazil Current Agulhas Current Eastern Australian Current Eastern boundary currents cold, shallow, broad, slow The The The The The Canary Current Benguela Current California Current West Australian Current Peru Current Boundary Currents: ex- Gulf Stream exThe general surface circulation of the North Atlantic. Numbers indicate flow rates in sverdrups (1 sv = 1 million m3 of water per second). The unit used to express volume in transport in ocean currents is the sverdrup (sv), named in sv), honor of Harald Sverdrup, one of this century's pioneering century' oceanographers. temperature data from NOAA polar orbiting meteorological satellites (April 1984). Red & orange = 2428C; yellow & green = 1723C; blue= 1016C; purple = 2 9C. The Gulf Stream appears as a red (warm) river as it moves from the southern tip of Florida (1) north along the east coast. Moving offshore at Cape Hatteras (2), it begins to meander, with some meanders pinching off to form warm-core (3) and cold-core (4) eddies. As it moves northeastward, the water cools dramatically, releasing heat to the atmosphere and mixing with the cooler surrounding waters. Surface currents affect weather and climate General summer air circulation patterns of the east and west coasts of the United States. Warm ocean currents are shown in red; cold currents, in blue. Air is chilled as it approaches the west coast and warmed as it approaches the east coast. "Tropical" gardens on Britain's Scilly Isles. Only 48 km off the coast of Cornwall at 50 N, these scenic islands lie in the path of the warm waters of the Gulf Stream. "The coldest winter I ever spent was a summer in San Francisco..." -Mark Twain 5 Wind can induce vertical water movement Coastal downwelling Coastal upwelling In the Northern Hemisphere, coastal upwelling can be caused by winds from the north blowing along the west coast of a continent. Water moved offshore by Ekman transport is replaced by cold, deep, nutrient-laden water. nutrientwater. Wind blowing from the south along a Northern Hemisphere west coast for a prolonged period can result in downwelling. downwelling. Areas of downwelling are often low in nutrients and therefore relatively low in biological productivity. productivity. Nutrient-rich water rises near the equator Nutrient- Equatorial upwelling The South Equatorial Current, especially in the Pacific, straddles straddles the geographical equator. Water north of the equator veers to the right (northward), and water to the south veers to the left (southward). Surface water therefore diverges, causing upwelling. Most of the upwelled water comes from the area above the equatorial undercurrent, at depths of 100 m or less. Thermohaline circulation thermo = heat Circulation which is driven by the differences in the haline = salt density of seawater at different locations. The density of seawater depends on its temperature and salinity. Affects all of the ocean's water. ocean' The ocean is density stratified, with the densest water at the bottom. There are 5 common water masses: Surface water Central water Intermediate water Deep water Bottom water 6 Thermohaline Flow and Surface Flow: The Global Heat Connection The global pattern of deep circulation resembles a vast "conveyor belt" that carries surface water to the depths and belt" back again. Begin with the formation of North Atlantic Deep Water north of Iceland. This water mass flows south through the Atlantic and then flows over (and mixes with) deep water formed near Antarctica. The combined mass circumnavigates Antarctica and then moves north into the Indian and Pacific ocean basins. Diffuse upwelling in all of the ocean returns some of this water to the surface. Water in the conveyor gradually warms and mixes upward to be returned to the North Atlantic by surface circulation. The whole slow-moving system (~1000 years for one loop) slowis important in transporting water and heat. Ocean circulation "conveyer belt" How do we know? float methods (drift bottles, buoys, accidents) flow meters electromagnetic studies gliders chemical tracers It's not easy... this lecture was brought to us by many decades of hard work! Summary The atmosphere and the ocean water circulates in currents. Surface currents affect the uppermost 10% of the world ocean. The movement of surface currents is powered by the warmth of the sun and by winds. Water in surface currents tends to flow horizontally, but it can also flow vertically in response to wind blowing near coasts or along the equator. Surface currents transfer heat from tropical to polar regions, influence influence weather and climate, distribute nutrients, and scatter organisms. organisms. They have contributed to the spread of humanity to remote islands, and islands, they are important factors in maritime commerce. Circulation of the 90% of ocean water beneath the surface zone is driven is by the force of gravity, as dense water sinks and less dense water rises. water Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Currents near the seafloor flow as slow, riverlike masses in a few places, but the greatest volumes of deep water creep through the ocean at an at almost imperceptible pace. The Coriolis effect, gravity, and friction shape the direction and volume of surface currents and thermohaline circulation. Greenpeace | Pacific trash vortex showing drift of ocean pollution. pollution. 7 ...
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This note was uploaded on 04/29/2008 for the course EAS 104 taught by Professor Brown during the Spring '08 term at Purdue University-West Lafayette.

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