Optional theme: Oceans and coastal margins practice questions.

During a strong El Nino event the trade winds over the tropical Pacific weaken and warm surface water spreads eastwards towards the coast of Peru. Which change to the coastal ocean off Peru follows most directly?

• The thermocline deepens and nutrient-rich upwelling is suppressed
• The thermocline rises towards the surface and upwelling strengthens
• Offshore Ekman transport intensifies and surface salinity rises
• Cold water sinks along the shelf and phytoplankton multiply

Answer: The thermocline deepens and nutrient-rich upwelling is suppressed

Upwelling only delivers nutrients if the cold, nutrient-rich layer lies close to the surface; with the thermocline pushed down, the weakened trades drive less upwelling and what still rises is warm, nutrient-poor water from the deepened mixed layer.

A tropical depression over the Bay of Bengal intensifies into a tropical cyclone. Which set of ocean and atmospheric conditions is essential for that intensification?

• Sea surface temperature below 20 C with strong vertical wind shear aloft
• Persistent high surface pressure with subsiding, drying air in the core
• Sea surface temperature above 26.5 C with weak vertical wind shear
• A position within 2 degrees of the equator, where Coriolis is strongest

Answer: Sea surface temperature above 26.5 C with weak vertical wind shear

Warm water through the upper tens of metres supplies the latent heat, and low shear lets the released heat stay stacked in one vertical column instead of being sheared away.

The ocean has absorbed roughly a quarter of human carbon dioxide emissions since 1850, and the mean pH of surface seawater has fallen by about 0.1 unit over that period. Which chemical change explains that fall?

• Dissolved carbon dioxide raises the carbonate ion concentration
• Warmer water holds more dissolved oxygen, and oxygen lowers the pH
• Rising salinity increases the buffering capacity of surface seawater
• Dissolved carbon dioxide forms carbonic acid, which releases hydrogen ions

Answer: Dissolved carbon dioxide forms carbonic acid, which releases hydrogen ions

More hydrogen ions mean a lower pH, and they also combine with carbonate ions, which is why acidification makes it harder for corals and shellfish to build calcium carbonate.

At Swanage on the Dorset coast of England a resistant chalk headland at Ballard Point separates Swanage Bay, cut into weaker clays and sands, from Studland Bay to the north. Waves slow over the shallow water off the headland and their crests bend. What is the effect on the distribution of wave energy?

• Energy converges on the headland and diverges into the bays
• Energy converges into the bays and diverges around the headland
• Energy is spread evenly along both the headland and the bays
• Energy is transferred entirely into a longshore current in the bays

Answer: Energy converges on the headland and diverges into the bays

Wave crests slow first in the shallow water off the headland, so the orthogonals bunch there and spread apart in the deeper bays, which is why the chalk at Ballard Point is being cut back into stacks while sand accumulates in both bays.

A beach is reached by waves breaking about seven times per minute, with a strong swash and a comparatively weak backwash, and a series of berms has built up at the top of the beach. How should these waves be classified, and why?

• Destructive waves, because the high frequency drags sediment offshore
• Constructive waves, because swash exceeds backwash and adds sediment
• Destructive waves, because the long wavelength produces a strong backwash
• Constructive waves, because the steep wave form increases hydraulic action

Answer: Constructive waves, because swash exceeds backwash and adds sediment

Whether a wave builds or removes a beach depends on the balance of swash and backwash, not on wave height: low-frequency, long-wavelength waves lose water to percolation, so more sediment is left behind than is taken back.

During the marine heatwave of 2016, corals across the northern Great Barrier Reef, Australia, expelled the zooxanthellae living in their tissues. What is the immediate consequence for the coral polyps?

• The polyps gain extra nutrients from the water and grow more quickly
• The calcium carbonate skeleton dissolves within a few days of expulsion
• The polyps lose most of their food supply and may starve
• The polyps reproduce faster to replace the algae they have lost

Answer: The polyps lose most of their food supply and may starve

Zooxanthellae supply most of the polyp's energy through photosynthesis, so bleaching is a starvation risk rather than instant death; corals can recover if the water cools before their reserves run out.