Ocean-atmosphere interactions practice questions.

During an El Niño event, the trade winds across the tropical Pacific weaken. What happens off the coast of Peru?

• Upwelling of cold nutrient-rich water is suppressed and anchovy catches fall
• Upwelling intensifies, raising nutrient supply and fish catches
• Sea surface temperatures fall sharply and rainfall increases inland
• The Humboldt Current reverses and flows towards Antarctica

Answer: Upwelling of cold nutrient-rich water is suppressed and anchovy catches fall

The trades normally drive surface water offshore, which lets cold nutrient-rich water rise to replace it. Weaker winds and a deeper thermocline shut that pump down, so the food chain that supports the anchoveta fishery loses its nutrient supply.

Tropical cyclones do not form within about five degrees of the Equator even where sea surface temperatures are well above 27 °C. Why?

• The Coriolis effect is too weak there to start the system rotating
• Sea water is too saline at the Equator for evaporation to occur
• There is never enough atmospheric moisture at the Equator
• Upper-level winds at the Equator are always too weak

Answer: The Coriolis effect is too weak there to start the system rotating

Warm water supplies the energy, but a cyclone also needs spin, and the Coriolis effect falls to zero at the Equator. Listing the temperature threshold without the rotation condition is the most common incomplete answer on this topic.

Which change would most directly weaken the sinking limb of the thermohaline circulation in the North Atlantic?

• A large input of freshwater from melting Greenland ice reducing surface density
• An increase in the salinity of surface water through faster evaporation
• Stronger cooling of surface water during a colder than average winter
• A rise in the rate at which deep water returns to the surface elsewhere

Answer: A large input of freshwater from melting Greenland ice reducing surface density

Deep water forms where surface water becomes dense enough to sink, and density rises with cold and with salt. Freshwater dilutes the surface layer and makes sinking harder; the other three options all increase density or draw the circulation on.

Ocean acidification is caused by carbon dioxide dissolving in seawater. Which organisms are most directly affected, and why?

• Calcifying organisms, because carbonate ions become less available for shells
• Deep-sea fish, because dissolved carbon dioxide is toxic to their gills
• Seagrasses, because they cannot photosynthesise in more acidic water
• Marine mammals, because sound travels more slowly in acidified water

Answer: Calcifying organisms, because carbonate ions become less available for shells

Dissolved carbon dioxide forms carbonic acid, which releases hydrogen ions; these combine with carbonate, leaving less of it for corals, molluscs and plankton to build calcium carbonate structures. Acidification is a chemistry problem for skeletons, not a toxicity problem.

Sea surface temperature in the eastern tropical Pacific is unusually low, the trade winds are unusually strong, and rainfall in eastern Australia is above average. Which phase is this?

• La Niña
• El Niño
• A neutral phase of the Southern Oscillation
• The North Atlantic Oscillation in its positive phase

Answer: La Niña

La Niña is the intensified normal state: stronger trades pile warm water in the west, so convection and rainfall shift towards Australia and Indonesia while the eastern Pacific cools. El Niño is the reversal of exactly these signs.

The Namib Desert reaches the Atlantic shore of Namibia, where the cold Benguela current flows northwards along the coast and coastal fog is frequent. Which mechanism most directly explains the aridity of this coastline?

• The cold current lowers evaporation from the sea so completely that no water vapour ever reaches the coast.
• Winds blowing offshore from the interior strip away every cloud that forms above the current.
• Air chilled over the cold current becomes stable, so moist air cannot rise far enough to give rain.
• The dense, salty water of the current raises air pressure over the land throughout the year.

Answer: Air chilled over the cold current becomes stable, so moist air cannot rise far enough to give rain.

Cooling from below creates a temperature inversion, which suppresses convection: moisture condenses into low fog but the air cannot rise to produce rainfall. Evaporation still occurs over the current, so the coast is foggy and humid yet almost rainless.