Oceanography Chapter 7 Review Questions

Discuss the advantages and disadvantages of building an offshore current power system.

Advantages are the swell is always there to some degree or another and can be harnessed. The disadvantage of harnessing the wave energy on a large scale is that it changes the coastal environment; i.e. destruction of marine habitat in the coastal regions, cost to maintain such a power plant at sea. Also, biofouling is unavoidable and the power system will have to be cleaned much like keeping a boat hull clean of sea life.

Describe how the distribution of life in the ocean would be different if there was very little dissolved oxygen in deep-water currents.

As the oxygen content in the deep ocean decreases so does the life in the ocean. This has happened in Earth's past during the carboniferous period. Warm anoxic oceans meant less life.

Explain why no matter where you are in the ocean, if you go deep enough, you will encounter Oceanic Common Water.

As you go deeper in the ocean below the surface currents, the deep currents dominate and move about the world's oceans very slowly through the global conveyer system.

Describe the different ways in which deep currents are measured.

Deep currents can be measured by using various drift devices, by detecting the presence of a telltale chemical tracer, or by measuring the distinctive temperature and salinity of a deep-water mass.

Explain why the subtropical gyres in the Northern Hemisphere move in a clockwise fashion while the subpolar gyres rotate in a counterclockwise pattern.

In the Northern Hemisphere, differences in pressure cause a clockwise flow of air around high-pressure cells (anticyclonic flow; associated with subtropical gyres). Conversely, a counter-clockwise flow of air occurs around low-pressure cells (cyclonic flow; associated with subpolar gyres). This movement of air drives the surface currents. As mentioned in Chapter 6, the screwdriver analogy applies here: to tighten a screw (equivalent to high pressure), the screwdriver is turned clockwise; to loosen a screw (equivalent to low pressure), the screwdriver is turned counter-clockwise. Try experimenting with this using the surface currents map (Figure 7.5).

Describe changes in atmospheric and oceanographic phenomena that occur during El Niño/La Niña events.

During El Niño events, the Walker Circulation between the southeastern Pacific high-pressure cell and the Indonesian low-pressure call is weakened. Since this circulation reinforces the equatorial trade winds, they weaken and may even reverse themselves. With no trade winds pushing equatorial water into the dome of warm water (the Pacific Warm Pool) that forms near Indonesia, this water pushes east along the equator aided by a strengthened equatorial countercurrent. The Pacific Warm Pool spreads along the equator and even as far as the coasts of North and South America. Its arrival is manifest as high sea surface temperature anomalies in eastern Pacific waters. This wedge of warm water causes the thermocline to be abnormally deep and suppresses upwelling. In some cases, the sloshing of warm water against the western coast of South America causes downwelling. The lack of upwelling limits primary productivity in surface waters, causing marine life to be much less abundant. In severe El Niños, worldwide weather patterns are disrupted that could cause local weather to be wetter, drier, hotter, or colder than normal, depending upon the areas. Typically, severe El Niño events cause flooding, coastal erosion, and impacts on marine life along the eastern Pacific, and droughts and wildfires on the western Pacific and Indian Oceans. Refer to Figure 7.22 (a-b) for other global effects.

How often do El Niño events occur? Using Figure 7.24, determine how many years since 1950 have been El Niño years. Has the pattern of El Niño events occurred at regular intervals?

El Niño events have occurred irregularly at intervals of 2 to 10 years. Since 1950, there have been 13 El Niño events of varying strength, the strongest of which occurred in 1982-1983. Figure 7.24 shows that the pattern of El Niño events does not occur at regular intervals.

Compare the forces that are directly responsible for creating horizontal and deep vertical circulation in the oceans. What is the ultimate source of energy that drives both circulation systems?

Horizontal ocean currents derive their energy directly from the wind. Vertical circulation is driven by density changes in surface water. This circulation is initiated by the creation of dense surface water in the high latitudes of the Atlantic Ocean where water temperatures are low and salinity is increased during the winter months due to ice formation. The Sun is the ultimate source of energy for both circulation systems.

Discuss the origin of thermohaline vertical circulation. Why do deep currents form only in high-latitude regions?

In low latitudes the temperature of the water is too high to become dense enough to sink, even when high salinity values develop, as they do in the subtropics. As a result, deep currents only form in high latitudes. Even in high latitude regions where the surface water temperatures are always low, sinking may only take place during the winter when sea ice formation increases surface water salinity.

Sketch and discuss how Ekman transport produces the "hill" of water within subtropical gyres that causes geostrophic current flow. As a starting place on the diagram, use the wind belts (the trade winds and the prevailing westerlies). Upload your well-labeled sketch.

In the Northern Hemisphere, the trade winds and the prevailing westerlies (dashed lines in the figure above) create a westerly and easterly current, respectively, flowing at 45° to the right of the winds. The Coriolis effect creates a clockwise flow as continents interrupt the currents. Ekman transport (solid lines with closed arrows in the figure above) shows a net transport of water at an angle of 45° to the right of the surface current (in the Northern Hemisphere), pushing water toward the center of the rotating gyre.

How is La Niña different from El Niño? Describe the pattern of La Niña events in relation to El Niños since 1950 (see Figure 7.24).

La Niña is a cooling of surface ocean temperatures in the eastern Pacific that commonly occurs following an El Niño. La Niña is characterized by trade winds that are very strong and surface waters in the equatorial South Pacific that are cooler than usual. Figure 7.24 shows that since 1950, conditions in the tropical Pacific have oscillated between La Niña and El Niño events, and that "normal conditions" have occurred rarely.

Name the two major deep-water masses and give the locations of their formation at the ocean's surface.

North Atlantic Deep Water forms in the Norwegian Sea and off the tip of Greenland in the North Atlantic Ocean. Antarctic Bottom Water forms primarily as a result of the mixing of water in the Weddell Sea with water from the West Wind Drift.

Describe the general global and local (MD) effects (meteorological, biological and socioeconomic) of severe El Niño episodes. Why does this phenomenon in the Pacific impact weather in other areas on Earth?

Severe El Niño events are associated with significant changes in global and local weather patterns due to the displacement of the polar and subtropical jet streams in the atmosphere. Severe drought and brush fires in Australia, cold wet winters in the southeastern United States, a heightened risk of coastal snowstorms along the eastern U.S. coast, drought in the inter-mountain western United States, an inactive hurricane season in the Atlantic and an active typhoon season in the Pacific Ocean are all effects of a strong El Niño event. Corals in the western portion of the south Pacific Ocean are stressed due to unusually high water temperatures, while primary productivity in the eastern Pacific Ocean significantly decreases, negatively impacting commercial fishing and marine wildlife.

Explain why Gulf Stream eddies that develop northeast of the Gulf Stream rotate clockwise and have warm-water cores, whereas those that develop to the southwest rotate counterclockwise and have cold-water cores.

Since eddies are formed by the pinching off of meanders, the water is following a clockwise path in the meanders on the northwest side of the Gulf Stream and a counterclockwise path on the southeast side. This pattern of movement is maintained in the pinched-off meanders.

Describe the different ways in which surface currents are measured.

Surface currents can be measured either directly or indirectly: • Direct current measurement is accomplished by releasing a device that is transported by the current and is tracked through time or by lowering into the water from a stationary position a current-measuring device capable of measuring flow rates (usually with a propeller). • Indirect current measurement is completed in one of three ways: by determining the internal distribution of density and the corresponding pressure gradient across an area of the ocean; by using radar altimeters mounted on satellites to determine the shape of the ocean surface (dynamic topography) from which current flow is inferred; or by using low-frequency sound signals sent through the ocean to determine differences in pressure, which indicate current movement.

Why do ocean currents have the potential to generate even more power than wind farms do?

Swell before it breaks has a lot of untapped energy. The whistle buoy is one example of continuously harnessing wave energy.

Explain why upwelling areas are associated with an abundance of marine life.

The areas of upwelling are rich in dissolved oxygen and nutrients that were hoisted up from the deeper ocean into the photic zone where primary producers and microorganisms, the base of the food web, can take advantage of all the necessities for life: light, nutrients, and water.

Describe the main differences between surface and deep currents.

The main differences are how they are generated and speed. Surface currents are generated by surface wind belts and are much faster than deep currents. Deep currents are generated by density changes in surface water and move extremely slowly.

Describe the relationship between major wind belts of the world and major currents involved in surface circulation gyres of the ocean.

The map should resemble Figure 7.5, with the westerly boundary currents of all subtropical gyres marked as western intensified. The overlay should show that there is a correlation between ocean surface currents and the major wind belts of the world (refer to Figure 6.12); this makes sense because the surface currents are wind-induced.

What causes the apex of the geostrophic "hills" to be offset to the west of the center of the ocean gyre systems?

The offset of the geostrophic "hills" to the west of center in ocean gyres is a result of an equilibrium reached among a number of forces, but the primary factor is the increasing strength of the Coriolis effect with latitude.

Describe western intensification, including the characteristics of western and eastern boundary currents of subtropical gyres.

The western intensification of the western boundary current is partly the result of the change in strength of the Coriolis effect with latitude and wind stress curl in combination with land blocking the equatorial current from just following the wind around the Earth. The Coriolis effect and Ekman transport deflects some of the water following the trade wind belt to flow slightly to the right in the north and slightly to the left in the Southern Hemisphere. This wind belt is close to the equator and the Coriolis effect is very small here. On the other hand, the westerlies between 30o and 60o grad surface waters along with it and the Coriolis effect is stronger at this location, deflecting more water away from the intended path. Thus in the Northern Hemisphere subtropical gyres, more water is deflected south from higher latitudes and less deflected to the north from lower latitudes. The result is the rotational centers are shifted to the west along the 30o line.

How many subtropical gyres exist worldwide? How many main currents exist within each subtropical gyre?

There are five subtropical gyres: North Pacific, South Pacific, North Atlantic, South Atlantic, and the Indian Ocean below the equator. There is land above the equator at 30o North so no gyres form at this location, rather this area experiences seasonal reversing monsoons. Each subtropical gyre has four currents: two that follow the wind belts traveling roughly East or West, and two that are boundary currents that are deflected by land which travel either toward or away from the polar region. Of the latter two, one western intensified or western boundary current and one eastern boundary current.

Describe several different oceanographic conditions that produce upwelling.

• Current divergence (equatorial upwelling) is caused when surface waters move away from an area on the ocean's surface. • Ekman transport (coastal upwelling) occurs where surface water is moved away from shore. • Offshore winds move water away from shore. • Sea floor obstructions restrict the flow of water and cause it to rise to the surface. • Coastline geometry produces upwelling where a sharp bend in the coastline causes deep water to surface. • High latitude regions have upwelling because there is no pycnocline to present vertical water movement.