Lesson: Ocean Circulation

Transcription

1 Lesson: Ocean Circulation By Keith Meldahl Corresponding to Chapter 9: Ocean Circulation As this figure shows, there is a connection between the prevailing easterly and westerly winds (discussed in Chapter 8) and the ocean s surface currents. The prevailing easterlies and westerlies push the surface waters of the oceans around in big circles, called gyres.

2 Summary of Important Concepts Ocean water circulates in currents. Currents occur in both the surface regions and in the deep regions. SURFACE CURRENTS Surface currents are caused mainly by the prevailing winds. Surface currents flow in gyres: circular paths of water traveling around the periphery of each ocean basin. The Coriolis effect changes the direction of surface currents. The Coriolis effect causes Ekman transport, which results in surface currents flowing to the right of the wind direction in the Northern Hemisphere, and to the left of the wind direction in the Southern Hemisphere. The circulation of surface water in current gyres results in movement of heat from the equator toward the poles.

3 Summary of Important Concepts, continued DEEP CURRENTS Upwelling and downwelling describe the vertical movement of water masses. Deep currents are caused mostly by gravity, because dense water sinks downward and less dense water rises upward. Water density changes mainly due to temperature and salinity variations. Cold and/or saltier water is denser than warm and/or less salty water. Therefore cold and/or salty water tends to sink. Because temperature and salinity are the main controls on water density, the circulation of water caused by these differences is called THERMOHALINE CIRCULATION. ( Thermo = temperature; haline = salt.) The main pattern of thermohaline circulation is for cold, dense water to sink in polar regions and eventually rise in lower latitude regions.

4 Surface Currents Surface currents are driven by winds, particularly the prevailing easterlies and westerlies. But the wind direction is not the only force at work controlling the surface currents. The forces of gravity and the Coriolis effect work along with the wind to create in a circular pattern pattern of surface current flow in each ocean basin. These circular patterns of flow called gyres. Water moves clockwise in the Northern Hemisphere gyres and counterclockwise in the Southern Hemisphere gyres.

5 Surface Current Gyres Gyres can be further subdivided into distinct currents. For example, the four interconnected currents in the North Atlantic Gyre shown here are the: - North Equatorial Current - Gulf Stream - North Atlantic Current - Canary Current Each of these currents has different flow characteristics and temperatures.

6 Flow Within Gyres You have learned that the Coriolis effect causes objects to move in rightcurving paths in the Northern Hemisphere and left-curving paths in the Southern Hemisphere. As the wind induces water motion at the surface, the Coriolis effect causes the water to curve off and move at an angle to the wind. Each subsequent layer of water below is driven by the movement of the water above, and so moves off at greater and greater angles to the wind direction. The figure above shows how water in the Northern Hemisphere moves to the right of the wind direction. The opposite direction would apply in the Southern Hemisphere.

7 The change in water flow direction with depth, as described in the last slide, is called the Ekman spiral. What matters for us is the net movement of water from the Ekman Spiral. There is a net flow of surface water to the right of the wind in the Northern Hemisphere, and to the left of the wind in the Southern Hemisphere. This net movement of water at an angle to the wind direction is called EKMAN TRANSPORT. Flow Within Gyres

8 Flow Within Gyres In this figure, notice that the surface water flows to the right of the direction of the prevailing winds in the North Atlantic. The effect of Ekman Transport is that water flows in great circular paths around the outer edges of the ocean basins, forming five great current gyres. (See next slide.)

9 World Surface Current Gyres Notice the pattern of the world s surface current gyres: clockwise flow in the Northern Hemisphere; counterclockwise flow in the Southern Hemisphere.

10 Currents Within Gyres Although a gyre represents a single continuous flow of water, it is useful to divide up a gyre into several currents. Western boundary currents - Flow toward the poles on the west sides of ocean basins. The Gulf Stream in the North Atlantic, and the Kuroshio Current (or Jana Current) in the North Pacific (see last slide), are examples of western boundary currents. Eastern boundary currents - Flow toward the equator on the east sides of ocean basins. The Canary Current in the North Atlantic, and the California Current in the North Pacific (see last slide), are example of eastern boundary currents.

11 Currents Within Gyres Differences between western and eastern boundary currents. Western boundary currents tend to be narrow, deep, and swift. Eastern boundary currents tend to be wide, shallow, and slow.

12 Currents Within Gyres Water flow in the Gulf Stream (a western boundary current) and the Canary Current (an eastern boundary current), both of which are part of the North Atlantic gyre.

13 Surface Currents and Climate Surface currents exert important controls on climate. For example, the cool California current (in blue) controls the weather on the west coast of North America, while the warm Gulf Stream (in red) controls the weather on the east coast.

14 Upwelling and Downwelling We have been discussing the horizontal flow of surface water. But in some circumstances vertical movements of water can result from horizontal flows. Wind induced vertical circulation is vertical movement induced by winddriven horizontal movement of water. Two types of vertical movement occur: Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface, which is important for marine organisms, particularly plants and phytoplankton, which are the base of the marine food chain. Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases, also important for marine life.

15 Upwelling and Downwelling Recall that Ekman Transport causes surface water to move to the right of the wind in the Northern Hemisphere, and to the left of the wind in the Southern Hemisphere. This figure illustrates how upwelling or downwelling would occur along a coastline in the Northern Hemisphere, depending on wind direction. (Explanation continued on the next slide.)

16 Upwelling and Downwelling The upper figure shows that if the wind blows parallel to the coast in one direction, it causes the surface water to move away from shore. (In this case, since it s the Northern Hemisphere, the water is flowing to the right of the wind direction.) The result is upwelling: water flows up from below to replace the water that flows away from shore. The lower figure shows that if the wind blows parallel to the coast in the opposite direction, surface water moves toward the shore. The result is downwelling, as surface water piling up near shore pushes the other water down.

17 El Niño Normal Conditions Normally, strong trade winds push warm equatorial waters across the Pacific, resulting in storms and high rainfall on the west side of the Pacific, and cold upwelling on the east side. El Niño Conditions In an El Niño year, the trade winds diminish and reverse. This allows the warm equatorial water to flow back east, resulting in more storms and rain in the eastern Pacific, and reduced amounts of upwelling.

18 Deep Ocean Circulation The movement of water due to different densities is called thermohaline circulation. The term refers to the two main ways that ocean water density may vary: Temperature ( thermo ) - cold water is denser than warm water. Salinity ( haline ) - saltier water is denser than less salty water. 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

19 The Temperature-Salinity Diagram A general temperaturesalinity (T-S) diagram. The diagram shows how the density of water varies with temperature and salinity. The curving lines represent lines of constant density. Notice that density goes up when: - temperature goes down - salinity goes up

20 Thermohaline Circulation A model of thermohaline circulation caused by water becoming heated near the equator and cooling off near the poles. Cold water near the poles sinks, and warm water near the equator rises, creating a great cycle of movement.

21 Thermohaline Circulation This cross-section through the Atlantic from north (left) to south (right) shows thermohaline flow. Note that the pattern is more complex than the simple model shown in the previous slide.

22 Studying Currents How do scientists study currents? Two main types of methods are used: flow methods and float methods. Flow methods measure the current as it flows past a fixed object.

23 Studying Currents Float methods depend on the movement of a freefloating object.