Chapter 9 Circulation of the Ocean

Transcription

1 Chapter 9 Circulation of the Ocean Surface Currents Are Driven by the Winds The westerlies and the trade winds are two of the winds that drive the ocean s surface currents. About 10% of the water in the world ocean is involved in surface currents, water flowing horizontally in the uppermost 400 meters (1,300 feet) of the ocean s surface, driven mainly by wind friction. (left) Winds, driven by uneven solar heating and Earth s spin, drive the movement of the ocean s surface currents. The prime movers are the powerful westerlies and the persistent trade winds (easterlies). Surface Currents What are some effects of ocean currents? Transfer heat from tropical to polar regions Influence weather and climate Distribute nutrients and scatter organisms Surface Currents Are Driven by the Winds A combination of four forces surface winds, the sun s heat, the Coriolis effect, and gravity circulates the ocean surface clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, forming gyres. Surface currents are driven by wind: Most of Earth s surface wind energy is concentrated in the easterlies and westerlies. Due to the forces of gravity, the Coriolis effect, solar energy, and solar winds, water often moves in a circular pattern called a gyre. The North Atlantic gyre, a series of four interconnecting currents with different flow characteristics and temperatures. 1

2 Surface Currents Flow around the Periphery of Ocean Basins Wind Surface water Friction Direction of motion Surface water blown by the winds at point A will veer to the right of its initial path and continue to the east. Water at point B veers right and continues to the west. 4 5 Net Direction of Ekman transport Surface Currents Flow around the Periphery of Ocean Basins 90 to the right of wind direction is up here Trade wind At 15 N The Ekman spiral and the mechanism by which it operates. Stepped Art Fig. 9-6, p

3 Surface Currents Flow around the Periphery of Ocean Basins Surface Currents Flow around the Periphery of Ocean Basins Consider the North Atlantic. The effect of Ekman spiraling and the Coriolis effect cause the water within a gyre to move in a circular pattern. The movement of water away from point B is influenced by the rightward tendency of the Coriolis effect and the gravitypowered movement of water down the pressure gradient. The surface is raised through wind motion and Ekman transport to form a low hill. The westward-moving water at B feels a balanced pull from two forces: the one due to Coriolis effect (which would turn the water to the right) and the one due to the pressure gradient, driven by gravity (which would turn it to the left). The hill is formed by Ekman transport. Water turns clockwise (inward) to form the dome, then descends, depressing the thermocline. Seawater Flows in Six Great Surface Circuits Geostrophic gyres are gyres in balance between the pressure gradient and the Coriolis effect. Of the six great currents in the world s ocean, five are geostrophic gyres. Note the western boundary currents in this map. Ocean Currents 3

4 Boundary Currents Have Different Characteristics Western boundary currents These are narrow, deep, fast currents found at the western boundaries of ocean basins. The Gulf Stream The Japan Current The Brazil Current The Agulhas Current The Eastern Australian Current Eastern boundary currents These currents are cold, shallow and broad, and their boundaries are not well defined. The Canary Current The Benguela Current The California Current The West Australian Current The Peru Current Boundary Currents Have Different Characteristics Unit for measuring flow rates (or volume transported by ocean currents): sverdrups 1 sv = 1 million cubic meters of water per second The general surface circulation of the North Atlantic. Boundary Currents Have Different Characteristics N Steep Gulf slope Stream Top of hill Sargasso Sea Gentle slope Canary Current Eddy formation The western boundary of the Gulf Stream is usually distinct, marked by abrupt changes in water temperature, speed, and direction. (a) Meanders (eddies) form at this boundary as the Gulf Stream leaves the U.S. coast at Cape Hatteras. The meanders can pinch off (b) and eventually become isolated cells of warm water between the Gulf Stream and the coast (c). Likewise, cold cells can pinch off and become entrained in the Gulf Stream itself (d). (C = cold water, W = warm water; blue = cold, red = warm.) Narrow, deep, warm, strong currents Broad, shallow, cold, weak currents 4

5 Boundary Currents Have Different Characteristics Surface Currents Affect Weather and Climate Water flow in the Gulf Stream and the Canary Current, parts of the North Atlantic gyre. 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. Surface Currents Affect Weather and Climate Nutrient-Rich Water Rises near the Equator Wind induced vertical circulation is vertical movement induced by wind-driven horizontal movement of water. Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface. Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases. Equatorial upwelling. The South Equatorial Current, especially in the Pacific, 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 meters or less. 5

6 Wind Can Induce Upwelling near Coasts Wind Can Also Induce Upwelling 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 ccontinent. Water moved offshore by Ekman transport is replaced by cold, deep, nutriend-laden water. In this diagram, temperature of the ocean surface is shown in degrees Celsius. Coastal downwelling. Wind blowing from the south along a Northern Hemisphere west coast for a prolonged period can result in downwelling. Areas of downwelling are often low in nutrients and therefore relatively low in biological productivity. Sea surface Windrows (convergences where seaweed, debris, and foam accumulate) Divergence Wind Helical vortices El Niño and La Niña Are Exceptions to Normal Wind and Current Flow An El Niño Year A Non-El Niño Year ~ 50 m Downwelling ~165 ft (2 6 cm/sec) Upwelling (1 2 cm/sec) ~6 m ~20 ft Level of no motion Fig. 9-18b, p. 248 In an El Niño year, when the Southern Oscillation develops, the trade winds diminish and then reverse, leading to an eastward movement of warm water along the equator. The surface waters of the central and eastern Pacific become warmer, and storms over land may increase. In a non-el Niño year, normally the air and surface water flow westward, the thermocline rises, and upwelling of cold water occurs along the west coast of Central and South America. 6

7 Moist air rises L Rainfall H 180º Surfa ce Warm -wate r pool Thermo cl wind s ine Upw elling 0 Tropical Pacific Normal Conditions 200 m El Nino/Southern Oscillation 7

8 Effects of El Nino from: La Niña impact on the global climate In the U.S., winter temperatures are warmer than normal in the Southeast, and cooler than normal in the Northwest. Global climate La Niña impacts tend to be opposite those of El Niño impacts. In the tropics, ocean temperature variations in La Niña tend to be opposite those of El Niño. At higher latitudes, El Niño and La Niña are among a number of factors that influence climate. However, the impacts of El Niño and La Niña at these latitudes are most clearly seen in wintertime. In the continental US, during El Niño years, temperatures in the winter are warmer than normal in the North Central States, and cooler than normal in the Southeast and the Southwest. During a La Niña year, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest. An anomaly is the value observed during El Niño or La Niña subtracted from the value in a normal year. 8

9 Thermohaline Circulation Affects All the Ocean s Water The movement of water due to different densities is thermohaline circulation. Remember that the ocean is density stratified, with the densest water at the bottom. There are five common water masses: Surface water Central water Intermediate water Deep water Bottom water Water Masses May Converge, Fall, Travel across the Seabed, and Slowly Rise A model of thermocline circulation caused by heating in lower latitudes and cooling in higher latitudes. The thermocline at middle and low latitudes is held up by the slow upward movement of cold water. The water layers and deep circulation of the Atlantic Ocean. Arrows indicate the direction of water movement. Convergence zones are areas where water masses approach one another. 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 back again. Begin with the formation of North Atlantic Deep Water north of Iceland, which 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. Chapter 9 - Summary Ocean water circulates in currents caused mainly by wind friction at the surface and by differences in water mass density beneath the surface zone. 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. 9

10 Chapter 9 - Summary 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. 10