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Ch. 9: Circulation of the Ocean
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Ch. 9: Circulation of the Ocean
“Currents are the very heart of physical oceanography. Their global effects, great masses of water, complex flow, and possible influence on human migrations make their study of particular importance.” -- Tom Garrison
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Major Concepts Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. Surface currents affect the uppermost 10% of the world ocean, mostly above the pycnocline. Some surface currents are rapid and riverlike, with well-defined boundaries; others are slow and diffuse. The largest surface currents are organized into huge circuits known as gyres. Water near the ocean surface moves to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect modifies the courses of currents, with currents turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The Coriolis effect is largely responsible for the phenomenon of westward intensification in both hemispheres. Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents; downwelling is often caused by surface current convergence or an increase in the density of surface water.
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Major Concepts Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. Surface currents affect the uppermost 10% of the world ocean, mostly above the pycnocline. Some surface currents are rapid and riverlike, with well-defined boundaries; others are slow and diffuse. The largest surface currents are organized into huge circuits known as gyres. Water near the ocean surface moves to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect modifies the courses of currents, with currents turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The Coriolis effect is largely responsible for the phenomenon of westward intensification in both hemispheres. Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents; downwelling is often caused by surface current convergence or an increase in the density of surface water.
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Major Concepts Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. Surface currents affect the uppermost 10% of the world ocean, mostly above the pycnocline. Some surface currents are rapid and riverlike, with well-defined boundaries; others are slow and diffuse. The largest surface currents are organized into huge circuits known as gyres. Water near the ocean surface moves to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect modifies the courses of currents, with currents turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The Coriolis effect is largely responsible for the phenomenon of westward intensification in both hemispheres. Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents; downwelling is often caused by surface current convergence or an increase in the density of surface water.
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Major Concepts Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. Surface currents affect the uppermost 10% of the world ocean, mostly above the pycnocline. Some surface currents are rapid and riverlike, with well-defined boundaries; others are slow and diffuse. The largest surface currents are organized into huge circuits known as gyres. Water near the ocean surface moves to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect modifies the courses of currents, with currents turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The Coriolis effect is largely responsible for the phenomenon of westward intensification in both hemispheres. Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents; downwelling is often caused by surface current convergence or an increase in the density of surface water.
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Major Concepts Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. Surface currents affect the uppermost 10% of the world ocean, mostly above the pycnocline. Some surface currents are rapid and riverlike, with well-defined boundaries; others are slow and diffuse. The largest surface currents are organized into huge circuits known as gyres. Water near the ocean surface moves to the right of the wind direction in the Northern Hemisphere, and to the left in the Southern Hemisphere. The Coriolis effect modifies the courses of currents, with currents turning clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. The Coriolis effect is largely responsible for the phenomenon of westward intensification in both hemispheres. Upwelling and downwelling describe the vertical movements of water masses. Upwelling is often due to the divergence of surface currents; downwelling is often caused by surface current convergence or an increase in the density of surface water.
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Major Concepts (cont.) El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to build eastward across the Pacific at the equator. Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Water masses almost always form at the ocean surface. The densest (and deepest) masses were formed by surface conditions that caused water to become very cold and salty. Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climate. Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.
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Major Concepts (cont.) El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to build eastward across the Pacific at the equator. Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Water masses almost always form at the ocean surface. The densest (and deepest) masses were formed by surface conditions that caused water to become very cold and salty. Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climate. Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.
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Major Concepts (cont.) El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to build eastward across the Pacific at the equator. Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Water masses almost always form at the ocean surface. The densest (and deepest) masses were formed by surface conditions that caused water to become very cold and salty. Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climate. Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.
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Major Concepts (cont.) El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to build eastward across the Pacific at the equator. Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Water masses almost always form at the ocean surface. The densest (and deepest) masses were formed by surface conditions that caused water to become very cold and salty. Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climate. Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.
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Major Concepts (cont.) El Niño, an anomaly in surface circulation, occurs when the trade winds falter, allowing warm water to build eastward across the Pacific at the equator. Circulation of the 90% of ocean water beneath the surface zone is driven by gravity, as dense water sinks and less dense water rises. Since density is largely a function of temperature and salinity, the movement of deep water due to density differences is called thermohaline circulation. Water masses almost always form at the ocean surface. The densest (and deepest) masses were formed by surface conditions that caused water to become very cold and salty. Because they transfer huge quantities of heat, ocean currents greatly affect world weather and climate. Currents are the very heart of physical oceanography. Their global effects, vast masses of water, complex flow, and possible influence on human migrations make their study of particular importance.
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Palm Trees in Britain? How can this be?
Cities of western Europe and Scandinavia are warmed by winds and ocean currents that carry warmth from the Tropics to these northern latitudes.
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Palm Trees in Britain? Currents: moving masses of water
a. Surface currents: driven by wind; influence the waters above the pycnocline. Some are rapid and riverlike with well-defined boundaries. Others are slow and diffuse. b. Deep-water currents: driven by density. Most move at an almost imperceptible pace, with water rising in some places, falling in others and creeping around the oceans.
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Surface Currents Water flowing in upper 400 m (1,300 ft)
About 10% of ocean’s water. Primary force responsible for surface currents is wind, esp.: Tradewinds Westerlies The force of friction from the air moving over the surface of water drags the water and starts a surface current flowing. Water “piles up” in the direction the wind is blowing. With higher water on the “piled up” side, the force of gravity pulls the water down the slope against the pressure gradient.
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Surface Currents The Coriolis effect causes surface currents to be deflected: Northern Hemisphere: to the right of the wind direction. Southern Hemisphere: to the left of the wind direction. The continents and ocean basin topography, along with the Coriolis effect, cause the moving water to move in gyres, great circular patterns around the peripheries of ocean basins.
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North Atlantic Gyre
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Flow Within a Gyre North Atlantic Gyre:
At 45 N: westerlies blow surface currents northeast. Current turns to right of its path due to Coriolis effect. When the current reaches the coast of western Europe, it deflects clockwise (south to about 15 N latitude). Here the tradewinds drive the current to the west until they hit the Americas and turn again to the right (i.e. to the north). Surface water flows at a velocity no more than 3% of the speed of the driving wind.
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Flow Within a Gyre Ekman spiral: the theoretical flow of water in layers due to surface winds (Active Figure 9.05). Surface water flows at a 45 angle to the right of wind direction in the Northern Hemisphere. The next layer down flows at an angle to the right of the water layer above. This pattern continues down to a depth of about 100 m (330 ft) at mid-latitudes. Due to frictional losses, the “pull” of each layer on the one below is slightly less than the one above. This causes each subsequently lower layer to move a bit slower than the one above. ** At some depth, the friction depth, the path of water will be opposite from the surface current (see Fig. 9.5, p. 203).
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Flow Within a Gyre Ekman transport: the net motion of water down to 100 m, owing to the combined effects of the Ekman spiral. In theory, this is: 90 to the right of wind direction in the Northern Hemisphere 90 to the left of wind direction in the Southern Hemisphere In reality, the angle of Ekman transport barely reaches 45. Due to the interaction of the Coriolis effect and a pressure gradient that is caused by the flow of water. Water flowing to the right as it circulates around the Atlantic, forms a hill of water toward the center of the ocean. Water that would normally flow toward the center of the ocean as it diverts to the right, is not able to climb the hill of water, and so it continues in a relatively straight path (see Fig. 9.4, p. 202 & Fig. 9.7, p. 204).
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Flow Within a Gyre There really is a “hill” of water in the Sargasso Sea (to the east of Florida and the Caribbean). This “hill” is maintained by wind energy. The combined effects of: 1) wind energy, 2) friction, 3) the Coriolis effect and 4) the pressure gradient . . . result in the currents of a gyre flowing along the outside edges of an ocean basin.
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Geostrophic Gyres Geostrophic gyres (Geos = Earth; strophe = turning): gyres in balance between the pressure gradient and the Coriolis effect. Geostrophic currents: currents produced by these gyres. There are six great current circuits in the world ocean (see Fig. 9.8, p. 205): Two in the Northern Hemisphere: North Atlantic gyre; North Pacific gyre Four in the Southern Hemisphere: South Atlantic gyre; South Pacific gyre; Indian Ocean gyre; West Wind Drift (Antarctic Circumpolar Current).
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Geostrophic Gyres Of these, five are geostrophic gyres.
The one that does not flow around the periphery of an ocean basin is the Antarctic Circumpolar Current, the greatest of all ocean surface currents. The junction of the equatorial currents occurs at the meteorological equator, 5 - 8° north of the geographic equator. Ocean circulation is balanced around the meteorological equator, as is atmospheric circulation.
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Currents Within Gyres Currents within gyres are classified by their position within the gyre: western boundary currents eastern boundary currents transverse currents
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Western Boundary Currents
Fastest, deepest geostrophic currents Narrow, fast, deep Move warm water poleward in each gyre Five large western boundary currents: Gulf Stream: North Atlantic Japan (Kuriosho) Current: North Pacific Brazil Current: South Atlantic Agulhas Current: Indian Ocean East Australian Current: South Pacific
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Western Boundary Currents
The Gulf Stream (an example): Largest of western boundary currents Avg. speed: 2 m/s (5 mi/h) Reaches a depth of 450 m (1,500 ft) Can move 160 km (100 mi)/day Avg. width: 70 km (43 mi) Flow: 55 sv (55 million m3/s) Active Figure 9.12a
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Western Boundary Currents
Sverdrup (sv): unit used to express volume transport in ocean currents. Named after Harald Sverdrup, a pioneering oceanographer 1 sv = 1 million m3/s Consider flow rates as listed in Fig. 9.9, p. 206
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Western Boundary Currents
Water in currents, especially a western boundary current, can travel great distances within well-defined boundaries (“current as a river” analogy). Frequently warm, clear, blue, depleted of nutrients, incapable of supporting much life. Without well-defined banks along its edges, currents can form waves along their boundaries from friction with adjacent water.
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Western Boundary Currents
Can also form eddies (see Fig. 9.12, p. 209) Cold-core eddies: trap cold water in their centers; rotate counterclockwise Warm-core eddies: trap warm water in their centers; rotate clockwise Eddies can extend to great depths and be very long-lasting (> 3 yrs). May be up to 1000 km (620 mi) in diameter Responsible for abyssal storms, leaving ripple marks on ocean floor. May bring nutrients to the surface, supporting marine phytoplankton.
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+
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Box 9.1: Streaming Along Read on own
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Eastern Boundary Currents
See Table 9.1, p. 209 for comparison with WBC’s Eastern edge of ocean basins Carry cold water equatorward Shallow, broad, ill-defined boundaries Generally don’t form eddies Flow rates much less than western boundary counterparts (see Fig. 9.9, p. 206)
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Eastern Boundary Currents
Five eastern boundary currents: Canary Current: North Atlantic Benguela Current: South Atlantic California Current: North Pacific Peru (Humboldt) Current: South Pacific West Australian Current: Indian Ocean
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Transverse Currents Transverse currents: currents that flow in an east-west direction, linking western and eastern boundary currents. Equatorial transverse currents are driven by tradewinds. Moderately shallow and broad currents Causes water to “pile up” on western side of oceans, e.g. Caribbean and western Pacific.
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Transverse Currents Mid-latitude transverse currents are driven by westerlies. Wider and slower-flowing than equatorial counterparts In Northern Hemisphere, eastward journey across ocean basin is shorter due to continental land mass size. In Southern Hemisphere Antarctic Circumpolar Current Carries more water than any other current (100 sv at Drake Passage)
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Westward Intensification
Refers to the fact that western boundary currents are faster (up to 10x faster), deeper, and narrower (up to 20x narrower) than eastern boundary currents. Western boundary currents in both hemispheres are more intense than their eastern counterparts. This is due to the fact that the “hill” of water is not centered in the ocean basin, but is asymmetrically positioned closer to the western boundary. Thus, western boundary currents are compressed, narrower, while eastern boundary currents are spread out and more diffuse (see Fig. 9.13, p. 210).
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Countercurrents and Undercurrents
Countercurrent: surface flow in the opposite direction from the main current. At the equator, air is rising; tradewind is relatively weak. Thus, some water flows back to the east at the meteorological equator.
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Countercurrents and Undercurrents
Undercurrent: a countercurrent beneath the surface. Can be very large, e.g. Pacific Equatorial Undercurrent has a volume almost half of that of Gulf Stream Can have significant influence on surface conditions, e.g. ocean around Galapagos is cold due to upwelling of Pacific Equatorial (AKA Cromwell) Undercurrent waters. Once this water reaches surface, it travels west with the South Equatorial Current, passing the Galapagos along its route.
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Humboldt (Peru) Current
Panama Current Cromwell Current Humboldt (Peru) Current
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Countercurrents and Undercurrents
Cause of undercurrents is still unclear, but some water flowing west at equator does not divert poleward; instead it is sinks and flows eastward as an undercurrent.
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Exceptional Surface Currents
Due to local or seasonal variations in wind and geography.
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Monsoon Currents In Indian Ocean northeast trades normally propel water to west (November to March). In summer months, ITCZ moves north and winds change direction, blowing from southwest. Water flows clockwise Usually westward-flowing North Equatorial Current reverses, and becomes the Southwest Monsoon Current (not shown in Fig. 9.8b)
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High-Latitude Currents
Form as split-offs from larger currents. Deflected by collisions with continents. Include: Greenland and Labrador Currents: North Atlantic Kamchatka and Alaska Currents: North Pacific Technically, not considered geostrophic currents.
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High-Latitude Currents
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Gyres: A Final Word Gyres consist of currents that blend into one another. Flow is continuous around periphery of ocean basin. Combined effects of wind, friction, Coriolis effect, and pressure gradient keep gyres flowing around outside of ocean basins. Active Figure 9.08
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Effects of Surface Currents on Climate
Surface currents distribute tropical heat worldwide. Warm water flows to higher latitudes, releases heat to air, cools, returns to equator, absorbs heat and repeats cycle. Effect is most dramatic at mid-latitudes, where 10 million billion (10 quadrillion?) calories are transferred each second (more than a million times the power used by the entire human population in the same period of time!).
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Effects of Surface Currents on Climate
This transfer of heat affects climate and weather, e.g. British Isles and western Europe are warmed by heat carried in Gulf Stream San Francisco is cooled by air in contact with California Current returning south from polar regions. Washington, D.C. is hot and sultry in summer due to warm, moist winds flowing in from southeast.
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Upwelling and Downwelling
Wind-driven horizontal movement of water can induce vertical movement in surface water: wind-induced vertical circulation. Upwelling: brings, deep, cold, usually nutrient-rich water toward surface. Downwelling: sends surface water to depths.
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Equatorial Upwelling Westward-flowing equatorial waters are diverted poleward to north and south (see Fig. 9.15, p. 212). This stimulates an equatorial upwelling to replace these waters. Brings up water from pycnocline and below, which is laden with nutrients. Leads to a band of biological productivity extending westward along equator from S.A. Important to fisheries on western coast of S.A.
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Coastal Upwelling Wind blowing parallel to shore can cause coastal upwelling (see Fig. 9.16, p. 213): Wind blows surface waters parallel to shoreline. Coriolis effect diverts waters to right (in Northern Hemisphere), away from shore. Deep water rises to replace diverted surface waters. New surface water is rich in nutrients. Prolonged wind can increased biological productivity.
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Southern Hemisphere
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Coastal Upwelling Upwellings can influence weather:
Northwind blowing along California coast causes surface waters to move away from shore. Upwelling replaces this water with cold water, which cools air above. Cold air contributes to San Francisco’s fog banks and cool summers. Similar dynamics influence weather in Peru, Palmer Peninsula of Antarctica, parts of Mediterranean, and some large Pacific islands.
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Downwelling Water driven toward coast is forced downward (see Fig. 9.17, p. 214). Helps supply deeper waters with dissolved gases and nutrients. Helps distribute living organisms Has no direct impact on weather conditions.
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Langmuir Circulation See Fig. 9.18, p. 214
Steady winds blowing across ocean surface generate small waves. This leads to the formation of long rows of counterrotational vortices (cells) in the surface water. Foam, seaweed, or debris can accumulate in long rows (windrows) where vortices converge. Windrows and vortices named for Irving Langmuir, who first observed and explained them in 1938. Operate only in surface water. Do not bring nutrients up to surface.
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El Niño and La Niña
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El Niño and La Niña Tradewinds normally drive equatorial waters from an area of high pressure in the eastern Pacific to the normally low pressure zone of the western Pacific (east to west equatorial current; see Fig. 9.19a – b, p. 215).
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El Niño and La Niña Approximately every 3 – 8 years, the pressure zones reverse themselves, and the tradewinds die down, allowing waters to move from west to east (Fig. 9.19c – d). This brings the exceptionally warm water from the western Pacific to the coast of S.A. This reversal of pressure zones (and wind patterns) is called the Southern Oscillation. The eastward current that occurs during a Southern Oscillation usually occurs around Christmas time. Thus the name: corriente del Niño (“current of the Christ Child”) El Niño. Since the phenomena are coupled, the terms describing them are often combined: El Niño Southern Oscillation (ENSO). ENSO conditions usually last about a year.
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El Niño and La Niña The Peru Current normally flows northward toward the equator and causes a coastal upwelling along western S.A. This upwelling brings nutrient-rich waters to the surface and supports a major fishing industry along the S.A. coast. During ENSO conditions, the Peru Current falters and the upwelling ceases. This greatly impacts S.A. economy.
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El Niño and La Niña During an ENSO, the sea level of the eastern Pacific rises by as much as 20 cm (8 in). Water temps. also increase – by as much as 7C (13F). Low pressure zone over eastern Pacific intensifies. Warmer water increased evaporation precipitation in normally dry areas. Increased evaporation more intense coastal storms. Recent El Niño’s have triggered other meteorological events in other areas: 1997 – 98 : series or “wall” of tornadoes in southeast U.S.; heavy rains in Peru; record drought in Hawaii, southwestern Africa, and Papua New Guinea; record rainfall in California
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El Niño and La Niña Also causes a shifting northward of the normal jet stream.
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El Niño and La Niña La Niña: the return to normal, but exaggerated, conditions: Powerful upwellings along western S.A. Colder than normal events in eastern Pacific. Western Pacific warms rapidly, and water piles up, depressing the thermocline to a depth of 100 m (328 ft). Causes of ENSO not completely understood, but sufficient knowledge exists that meteorologists can predict severe El Niño’s up to a year in advance.
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Thermohaline Circulation
Slow circulation of water (horizontal and vertical) occurs below the pycnocline, as well as on the ocean surface. Movement of deep water is a function of the water’s density, and thus, a function of temperature and salinity: thermohaline circulation.
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Water Masses The ocean is density stratified, and the various layers of ocean water may be classified according to differences in temperature and salinity. Density stratification is most pronounced in temperate and tropical waters, as this is where the greatest differences in temperatures between surface and deep waters exist. Water masses do not mix easily; thus, they retain their individual character for extended periods of time and over great distances.
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Water Masses Water masses are named according to their relative position in the water column. In the temperate and tropical latitudes five common water masses exist: Surface water: to about 200 m (660 ft). Central water: to the bottom of the main thermocline (whose depth varies with latitude). Intermediate water: to about 1,500 m (5,000 ft). Deep water: to a depth of about 4,000 m (13,000 ft); water below the intermediate water, but not in contact with the ocean floor. Bottom water: in contact with seafloor.
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Water Masses Surface currents usually move in the relatively warm environment of the surface and central water. Water masses form at the surface, and their characteristics are determined by conditions of heating, cooling, evaporation, and dilution at the surface when they were formed. This is true even of the densest (and deepest) masses. Probably formed near the poles. Low temperatures cold water. Freezing of water in ice concentrates the salt content of the water. Both of these phenomena greater density. Deep water exhibits smaller variations in salinity and temperature than surface waters (which are constantly moving) do.
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The Temperature-Salinity Diagram
Temperature-salinity (T-S) diagram: a graph showing the relationship of temperature and salinity with depth (see Fig. 9.21, p. 218). * Note: different combinations of temperature and salinity can result in the same density.
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The Temperature-Salinity Diagram
** Note: density of water tends to increase with depth.
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The Temperature-Salinity Diagram
*** Note: pts. a and b have the same density, but different temps. and salinities.
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Formation and Downwelling of Deep Water
Freezing of water formation of dense brine (saltwater). This mixes with water from Antarctic Circumpolar Current. Mixture sinks to deep sea bed and moves slowly northward. May take 1000 yrs to reach equator in Pacific! yrs to Aleutians! Flow in Atlantic is somewhat faster.
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Formation and Downwelling of Deep Water
See Fig. 9.22, p. 219 Active Figure 9.27 Antarctic Bottom Water: The most distinctive of all deep-water masses. Densest of all world ocean waters. Produced in great amounts near Antarctic coasts (mostly in Weddell Sea during winter). Migrates north along seafloor.
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Formation and Downwelling of Deep Water
North Atlantic Deep Water: Forms in Arctic Ocean. Relatively warm water of North Atlantic is cooled by polar winds from Canada. Prevented from spreading south by land masses, except in deep channels between Scotland, Iceland, and Greenland, which conduct some of the water to the North Atlantic. Very little enters Pacific. As the water cools, it releases heat to the overlying air, which moderates the European winters.
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Formation and Downwelling of Deep Water
Other deep-water masses: South Atlantic Deep Water and South Pacific Deep Water are both less dense than Antarctic Bottom Water, and, therefore float on top of it, out of contact with the ocean floor. Mediterranean Deep Water: Water in the Mediterranean is highly saline due to the great amount of evaporation that takes place there. When it spills over the Gibraltar Sill and flows into the Atlantic, it sinks, as it is much more dense than the central water mass of the Atlantic (Fig. 9.23, p. 219). Though saltier than Antarctic Bottom Water, Mediterranean Deep Water is less dense because it is much warmer.
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Formation and Downwelling of Deep Water
One can age deep water by analyzing its oxygen content: Ocean water picks up oxygen only at surface from contact with atmosphere, or from photosynthetic organisms. Oxygen is lost from water over time through respiration of aquatic organisms or by chemical reactions. Oxygen content, then, is a rough approximation of the age of water, i.e. the length of time since it left the surface (where it was formed). Over time, a water mass will lose its oxygen, mix with surrounding water, and lose its identity.
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Thermohaline Circulation Patterns and Water Masses
Sinking water at ocean edges must be offset by rising water elsewhere. Dense water sinks rather rapidly in cold, polar regions. It rises much more gradually in temperate and tropical regions. Once back at the surface, the water moves poleward to repeat the cycle (see Figs & 9.25, p. 220). The continual upwelling of deep water maintains the existence of the thermocline which exists at low and mid-latitudes.
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Thermohaline Circulation Patterns and Water Masses
Water masses of varying densities are slowly propelled by gravity. In certain areas (convergence zones), two masses abut one another. Here, the denser water mass will slide below the lighter one (see Fig. 9.26, p. 221).
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Thermohaline Circulation Patterns and Water Masses
When two water masses of equal density, but different temperature and salinity measurements, come together, they will converge to form a new water mass of greater density in a process called caballing (observe pts. a, b, and c in Fig. 9.21, p. 218).
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Thermohaline Circulation Patterns and Water Masses
The combined water mass, now more dense, will sink. North Atlantic Intermediate Water, Antarctic Intermediate Water, and some Antarctic Bottom Water are produced by caballing.
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Thermohaline Circulation Patterns and Water Masses
Water masses may take centuries to complete a circuit, or mix completely with surrounding water to lose their identity: Antarctic Bottom Water: up to 1600 yrs. Pacific Bottom Water: 200 – 300 yrs. For comparison: North Atlantic Gyre Water: just over a year
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Thermohaline Circulation Patterns and Water Masses
Contour currents: relatively strong, localized bottom currents in which dense water flows around (rather than over) seafloor projections. Usually flow equatorward below western boundary surface currents.
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Convergence Zones and the Surface and Central Water Masses
Antarctic Convergence: Most pronounced convergence zone. Encircles Antarctica between 50S and 60 S. To the south is the cold Antarctic Circumpolar Water, flowing in the Antarctic Circumpolar Current.
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Convergence Zones and the Surface and Central Water Masses
Southern Hemisphere Subtropical Convergence: Occurs between 40S and 50S. Separates Subantarctic Surface Water to the south and Central Water to the north. The position of this convergence zone can change with the time of year, the position of the currents, and other factors. Central Water: most abundant, warmest, and saltiest of the surface water masses.
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Convergence Zones and the Surface and Central Water Masses
Northern Hemisphere Subtropical Convergence: Forms boundary between Central Water to the south and Subarctic Surface Water to the north.
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Convergence Zones and the Surface and Central Water Masses
Arctic Convergence: Often poorly defined. Organization of water masses in Northern Hemisphere is more complex due to intervening land masses (continents).
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Thermohaline Flow and Surface Flow: The Global Heat Connection
Swift, narrow currents along the western edges of ocean basins carry warm equatorial water poleward. As the water cools, it loses heat to the atmosphere and sinks, becoming deep water, esp. in the North Atlantic. This cold, dense water moves at depths toward the Southern Hemisphere and eventually wells up to the surface in the Indian and Pacific Oceans. This cycle may take a thousand years. This transport of tropical waters is part of a worldwide conveyor belt for heat. This global oceanic circulation distributes dissolved gases, solids, nutrients, and living organisms (esp. larval stages) among the ocean basins.
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Studying Currents Two broad categories:
Float method: measures movement of a free-floating object, e.g. a drift bottle. Flow method: measures speed and direction of current as it flows past a fixed point.
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Studying Currents 1. Float method: measures movement of a free-floating object, e.g. a drift bottle. Drift bottles or drift cards: are set adrift and later collected at prespecified locations. Do not show path of the current. Drogue arrangement: (Fig. 9.28a) tracked at regular intervals by radio direction finders, or radar. Allow for tracing of current’s course.
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Studying Currents 1. Float method: measures movement of a free-floating object, e.g. a drift bottle. Unintentional events: May, 1990: containership, Hansa Carrier, struck by storm; 30,190 pairs of Nike shoes set adrift; recovered along shores from BC to Oregon. January 1992: freighter with 29,000 rubber toys hit by storm; toys recovered along 500 mi. of Alaskan shoreline (Fig. 9.28b).
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Studying Currents 1. Float method: measures movement of a free-floating object, e.g. a drift bottle. Swallow float: used for detecting flow of intermediate water masses. Emit sonar “pings” which are recorded by tracking vessel. Sofar probes: (Fig. 9.28c) work autonomously in sofar layer to transmit sound to listening stations Have yielded data on current flow in North Atlantic for depths ranging from 700 to 2,000 m (2,300 – 6,600 ft).
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Sofar Float
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Studying Currents 2. Flow method: measures speed and direction of current as it flows past a fixed point. Ekman type current meter: (Fig. 9.28d) use vanes to measure current speed and compass to record direction. Electromagnetic force sensing buoys: sense EM force generated by moving seawater Capable of recording speed and direction of current.
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Ekman Flow Meter
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Studying Currents 2. Flow method: measures speed and direction of current as it flows past a fixed point. Acoustic Doppler current profilers (ADCP’s): (Fig. 9.28e) use reflected sound from tiny particles, organisms, or bubbles in water to sense current strength and direction Can measure current to ~ 200 m (660 ft).
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Studying Currents 2. Flow method: measures speed and direction of current as it flows past a fixed point. Slocums: (Fig. 9.28f) use energy from temperature differences between ocean surface and deep waters, gravity, and buoyancy to move up and down in water column to map global ocean’s thermohaline depth and transmit data to satellites during surfacing events. Chlorofluorocarbon (CFC) analysis: track ocean currents by sensing chemical tracers (e.g. CFC’s) in seawater.
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Joshua Slocum with his Slocum Device
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Hmwk (pp. 226 – 27): “Reviewing What You’ve Learned” 1 – 6
“Thinking Critically” 1 – 4
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