Ocean Dynamics and Climate Practice Quiz (20 questions)

1. Approximately what fraction of Earth's surface is covered by the oceans?

A. 61%

B. 29%

C. 71%

D. 81%

Answer: (C)

Explanation

The oceans cover about 71% of Earth's surface. The alternatives confuse this with the land share or misstate the ocean fraction.

2. What is the ocean's average salinity?

A. About 34.5 g/kg

B. About 34.5%

C. About 3.45 g/kg

D. About 45.3 g/kg

Answer: (A)

Explanation

The ocean's average salinity is about 34.5 grams of dissolved salts per kilogram of seawater. The other values misstate the concentration or use a different unit.

3. A warm, dry spell increases evaporation over a patch of ocean, with no change in freshwater inputs. What happens to its surface salinity?

A. It falls as dissolved salts become concentrated.

B. It stays fixed because evaporation removes salt and water together.

C. It falls because evaporation adds freshwater to the surface.

D. It rises as dissolved salts become concentrated.

Answer: (D)

Explanation

Evaporation removes freshwater while leaving dissolved salts behind, so surface salinity rises. The alternatives reverse evaporation's effect, incorrectly infer a change from unchanged inputs, or confuse evaporation with freshwater addition.

4. Which description correctly identifies sources of dissolved elements in the ocean and the origins of sodium and chlorine?

A. Continental waters, volcanoes, and hydrothermalism supply elements; sodium and chlorine have different origins.

B. Biology, sedimentation, and evaporites supply elements; sodium and chlorine have different origins.

C. Continental waters and volcanoes supply elements; sodium and chlorine have different origins.

D. Continental waters, volcanoes, and hydrothermalism supply elements; sodium and chlorine share one origin.

Answer: (A)

Explanation

Dissolved elements enter the ocean through continental waters, volcanoes, and hydrothermalism, while sodium and chlorine do not share the same origin. The alternatives confuse sources with sinks, incorrectly give sodium and chlorine a common origin, or omit hydrothermalism.

5. Measurements show that the ocean's surface layer has uniform density, salinity, and temperature. Which feature do these observations describe?

A. A seasonal temperature transition

B. The mixed layer

C. The thermocline

D. The deep-water layer

Answer: (B)

Explanation

The mixed layer is the homogeneous surface layer, uniform in density and therefore in salinity and temperature. The thermocline is instead a zone of rapid temperature change; the other alternatives do not describe this defining surface-layer property.

6. A vertical temperature profile shows a rapid transition between warm surface water and colder deep water. What feature does the profile identify?

A. A mixed layer

B. A thermocline

C. A deep-water layer

D. A surface salinity gradient

Answer: (B)

Explanation

A thermocline is the zone of rapid temperature transition between surface and deep waters, and it may be permanent or seasonal. The alternatives confuse it with the homogeneous mixed layer, identify a water layer rather than the transition, or incorrectly rule out a permanent thermocline.

7. A scientist compares vertical water mixing in a low-latitude ocean region with mixing near the poles. Which pattern follows from the temperature contrasts in these regions?

A. Low-latitude stratification limits vertical mixing, while weak polar temperature contrasts permit it.

B. Low-latitude stratification limits vertical mixing, while weak polar temperature contrasts also limit it.

C. Low-latitude stratification permits vertical mixing, while weak polar temperature contrasts also permit it.

D. Low-latitude stratification permits vertical mixing, while weak polar temperature contrasts limit it.

Answer: (A)

Explanation

Strong stratification at low latitudes prevents vertical mixing, whereas weak temperature contrasts at the poles permit it. The alternatives reverse the relationship or incorrectly assign the same mixing effect to both settings.

8. What drives the large-scale sinking and return of ocean waters in thermohaline, or meridional overturning, circulation?

A. Density variations caused by temperature and salinity

B. Tidal forces generated by the Moon and Sun

C. Wind stress acting directly on the ocean surface

D. Sea-level pressure gradients caused by Ekman transport

Answer: (A)

Explanation

Temperature and salinity create density differences that drive the sinking and return of water in thermohaline circulation. Wind stress is the key confusion with surface circulation; tides and sea-level pressure gradients are different mechanisms.

9. In the North Atlantic, surface water cools and becomes saltier until it is dense enough to sink. Which depth range and water mass should result?

A. Below 4,000 m, forming Antarctic Bottom Water

B. About 2,000–4,000 m, forming Antarctic Bottom Water

C. About 2,000–4,000 m, forming North Atlantic Deep Water

D. About 500–1,000 m, forming intermediate water

Answer: (C)

Explanation

Cooling and/or increasing salinity makes North Atlantic water dense enough to sink to 2,000–4,000 m, where it forms North Atlantic Deep Water. The deeper range belongs to Antarctic Bottom Water; shallower intermediate depths and the reversed density effect do not fit this process.

10. Researchers measure the sea surface relative to the geoid and want to infer current speed. What should they use?

A. Absolute sea level measured without a geoid reference

B. Geoid height measured without sea-level information

C. Wind speed measured above the ocean surface

D. Dynamic topography from the sea-level–geoid difference

Answer: (D)

Explanation

The difference between sea level and the geoid defines dynamic topography, which can be used to calculate current speed. Absolute sea level or geoid height alone omits the required comparison, and wind speed is not derived from that difference.

11. At the equator, converging trade winds drive surface currents away from one another. What vertical motion and surface pattern follow?

A. Upwelling beneath a surface mound

B. Downwelling beneath a surface depression

C. Upwelling beneath a surface depression

D. Downwelling beneath a surface mound

Answer: (C)

Explanation

Equatorial surface-current divergence creates a surface depression and induces upwelling. Downwelling is associated with subtropical convergence, while a surface mound reverses the stated surface pattern.

12. Ekman transport carries water into the center of a subtropical gyre beneath an atmospheric high-pressure region. What vertical motion occurs there?

A. Upwelling in the gyre center

B. Upwelling at the gyre boundary

C. Downwelling in the gyre center

D. Downwelling at the gyre boundary

Answer: (C)

Explanation

Ekman transport makes water converge in subtropical gyre centers beneath atmospheric highs, producing downwelling. Equatorial divergence produces upwelling, and placing the motion at the gyre boundary confuses the location.

13. Which description correctly characterizes the main pattern of surface ocean gyres?

A. Five anticyclonic subtropical and four cyclonic subpolar gyres; subtropical gyres have intense western boundary currents

B. Five anticyclonic subtropical and four cyclonic subpolar gyres; subpolar gyres have intense western boundary currents

C. Five anticyclonic subtropical and four cyclonic subpolar gyres; all four subpolar gyres form dense deep water

D. Five cyclonic subtropical and four anticyclonic subpolar gyres; subpolar gyres have intense western boundary currents

Answer: (A)

Explanation

Surface circulation includes five anticyclonic subtropical gyres and four cyclonic subpolar gyres, with intense western boundary currents in the subtropical gyres; three subpolar gyres are dense-water formation sites. The distractors swap rotation types, misplace the intense currents, or incorrectly extend dense-water formation to all four subpolar gyres.

14. A researcher measures biological activity in surface waters and at 500 meters depth. Which pattern should they expect for photosynthesis and respiration?

A. Photosynthesis at the surface; respiration at the surface and at depth

B. Photosynthesis at the surface and at depth; respiration at the surface

C. Photosynthesis and respiration at the surface; neither at depth

D. Photosynthesis and respiration at the surface and at depth

Answer: (A)

Explanation

Photosynthesis is confined to the ocean surface, while respiration occurs both there and at depth. The second choice extends photosynthesis downward, the third restricts respiration to the surface, and the fourth extends photosynthesis throughout the water column.

15. A marine region covers about 0.02% of the ocean surface yet supports a disproportionately large share of global fisheries. Which contribution best matches eastern-boundary upwelling zones?

A. About 17% of ocean surface area

B. About 0.02% of world fisheries

C. About 17% of world fisheries

D. About 50% of world fisheries

Answer: (C)

Explanation

Eastern-boundary upwelling zones support approximately 17% of world fisheries while covering approximately 0.02% of ocean surface area. The second and third choices swap these proportions, while the fourth substitutes an unrelated larger estimate.

16. When accounting for excess energy trapped by greenhouse gases and unable to escape at the top of the atmosphere, what share has the ocean absorbed since 1970?

A. About 17%

B. About 91%

C. About 25%

D. About 50%

Answer: (B)

Explanation

Since 1970, the ocean has absorbed 91% of this excess energy. The 25% figure applies to anthropogenic CO2 emissions, the 50% figure to sea-level rise from thermal expansion, and 17% to fisheries supported by upwelling zones.

17. In an assessment attributing the observed rise in sea level to ocean warming, what approximate share is contributed by thermal expansion of seawater?

A. About 17%

B. About 25%

C. About 50%

D. About 91%

Answer: (C)

Explanation

Thermal expansion contributes approximately 50% of sea-level rise. The other percentages refer to the ocean's absorption of excess atmospheric energy, its uptake of anthropogenic CO2, and the fisheries share associated with upwelling zones.

18. At a coastal site, evaporation is 8 units, precipitation is 3 units, and river input is 2 units. What is the net surface freshwater loss, using $F = E - (P + R)$?

A. 13 units

B. 7 units

C. 5 units

D. 3 units

Answer: (D)

Explanation

The formula gives $F = 8 - (3 + 2) = 3$ units. Omitting river input gives 5; adding freshwater inputs gives 13; omitting precipitation gives 6, so the distractors should reflect distinct calculation errors.

19. In the classical Ekman model, wind blows in a new direction over the ocean surface. Approximately what angle should the surface current direction make with the wind direction?

A. About 180°

B. About 0°

C. About 45°

D. About 90°

Answer: (C)

Explanation

Friction and the Coriolis effect produce an approximately 45° angle between the wind and the surface current in the classical Ekman model. A 90° angle confuses this surface-current direction with a simplified description, while 0° and 180° imply alignment or opposition rather than Ekman deflection.

20. Approximately what share of anthropogenic CO2 emissions did the ocean absorb during 2011–2020?

A. About 91%

B. About 25%

C. About 29%

D. About 50%

Answer: (B)

Explanation

The ocean absorbed approximately 25% of anthropogenic CO2 emissions during 2011–2020, equivalent to 2.9 GtC per year. The 91% figure concerns excess energy, while the other distractors confuse this measure with different reported climate-related figures.