Chapter 8 Atmosphere-Ocean Interactions
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- Surface currents
- Coastal Upwelling
- Deep Water currents
Ocean CurrentsAn ocean current is a massive ordered pattern of water flow. There are two basic types of ocean currents, surface currents and deep water currents. Surface currents occur in the uppermost 10% of the world oceans, 90% of the ocean waters circulate beneath the surface currents in the deep-water currents. Like the atmosphere, the oceans are subjected to forces that determine the direction and intensity of the flow of water. First we will list these forces and then discuss how they affect ocean currents. The primary forces that determine the ocean currents are:
Surface currentsThe wind and absorption of solar energy drive the ocean surface currents. Air blowing over the oceans transfers some of the energy from the moving air to the water. This causes the water to flow. The surface current is comprised of the water flowing beneath the surface winds and extends to a depth of about 100 meters. As you learned in Chapter 6, the global wind patters are a function of latitude and at the surface can be broken down into three main patterns: the trade winds, the mid-latitude westerlies and the polar easterlies. The trade winds and the midlatitude westerlies have a strong influence on the ocean surface currents because they are steady and flow over large areas of open water.
In the tropics, the trade winds push water from one side of an ocean basin to the other, causing the water to pile up on one end of the basin. Since the water is higher on one side, gravity will act to move the water down to lower heights, but it is opposed by the trade winds. As with the winds, the Coriolis effect acts over large distances. In the Northern Hemisphere it acts to the right and to the left in the Southern Hemisphere. This effect causes the to water to flow within the basin in a circular pattern called a gyre. Details of the North Atlantic gyre are shown in Figure 8.7, along with the fundamental surface wind direction. Solar heating also influences surface ocean currents. Tropical regions receive more solar radiation than midlatitude and polar regions. So, just like the atmosphere, on average tropical waters are warmer than polar waters. As water warms it expands. As a result, the sea level near the equator is about 8 centimeters higher than the sea level of mid-latitude oceans. Gravity causes water to flow edownhill, and so the water moves from tropics toward the poles. However, the Earth is rotating faster than the water can flow, so the water tends to flow toward the west, and the Coriolis effect moves it to the right contributing to the formation of the gyres. As we learned in Chapter 6, friction and the Coriolis effect caused the wind to turn to right in the Northern Hemisphere. We see a similar effect in surface ocean currents. The surface water currents shown in Figure 8.7 are flowing at an angle of approximately 45 to the right of the surface winds. This results from a combination of friction and the Coriolis effect. These surface currents pull water below them so that the flow extends down approximately 100 meters (330 feet). The direction of the currents changes with depth. This is illustrated in Figure 8.8 for a Northern Hemisphere current. The direction of the water flow decreases with depth and rotates counterclockwise. This spiraling motion is called the Ekman spiral. Ekman Spiral is an explanation of how wind blowing over the ocean surface can generate water flow below the surface. To explain the Ekman spiral, let's divide the ocean into slabs of water. Starting with the top layer and working down into the ocean. Friction pushes the surface water and the Coriolis force moves the water to the right, and the water moves at a 45 angle to the right of the wind. This shallow surface layer of water exerts a frictional drag force on the layer below. This moves the water below, but at a lower speed and the Coriolis force again bends it to the right. Each layer exerts a frictional drag on the layer below, but the flow decreases with depth as friction slows the water, explaining the decrease in water speed with depth. Each underlying layer of water turns to the right due to the drag exerted by the layer above and the Coriolis effect-explaining the clockwise rotation of the water movement. The warm currents on the western side of the major Ocean basins (east coast of the continents) are called the Western Boundary Currents. The western boundary currents move over large distances. The Gulf Stream is the largest of these currents. Figure 8.9 is a detailed infrared satellite image of the Gulf Stream. Note the looping meanders and eddies. Eddies form when the current loops and cuts-off an eddy (Figure 8.10) from the main flow. Warm eddies that form in the Gulf Stream rotate counterclockwise, while cold eddies rotate clockwise. Similar eddies can form in the upper regions of the atmosphere! Coastal UpwellingUpwelling is a circulation pattern where cold nutrient-rich deep waters move to the surface. Upwelling can be generated along coastlines by the winds. The wind-driven surface water can induce vertical motions of the upper layer of the ocean. This process of inducing vertical motions is called upwelling. Coastal upwelling occurs along coastlines and brings cold, nutrient-rich water to the surface, an important source of nutrients for other marine organisms. Consider a wind blowing parallel to the shoreline. This is a typical situation off the west coast of continents due to the presence of the subtropical highs. Friction causes the surfaces waters to move and the Coriolis effect deflects the motion of the water. The resulting Ekman spiral causes the surfaces waters to move away from the shoreline. As the surface water moves away from the coast it is replaced by water rising from below (Figure 8.11), which is nutrient rich and cold. The coastal upwelling is observed off the west coast of North and South America images of sea surface temperatures (Figure 8.5). Deep Water currentsAbout 90% of the world ocean currents occur in the ocean deep waters. The slow circulation throughout the great depths of the oceans are driven by density differences in water. Cold water is denser than warm water and saline water is denser than fresh water. Because the deep-water currents are driven by density difference it is also called a thermohaline circulation (therme meaning heat and halos salt). An ideal thermohaline circulation is shown if Figure 8.12. As warm tropical surface water flow poleward in the surface layer, the water cools as it transfers heats to the atmosphere. As it cools, the density increases. When the surface waters reaches the polar regions, the water is cold and it sinks. This sinking occurs over a relatively small area near the poles. It moves toward the equator below the surface water and slowly rises to the surface to replace the surface water moving poleward. The upward motions are very slow, approximately 1 centimeter (about 0.5 inch) per day. Download 249.69 Kb. Share with your friends:
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