Oceans and Ocean Currents: Tides, Salinity and PYQs

The correct answer is C, the Agulhas current. It is a warm current of the Indian Ocean that flows southward along the eastern coast of southern Africa, carrying tropical water towards the Cape before most of it turns back into the Indian Ocean. Along with the Mozambique and West Australian currents it makes up the southern Indian Ocean gyre.

Option A, the Florida current, belongs to the Atlantic; it carries water out of the Gulf of Mexico past Florida and feeds the Gulf Stream. Option B, the Canary current, is also Atlantic, a cold current flowing south along the north-western coast of Africa past the Canary Islands. Option D, the Kurile or Oyashio current, is a cold current of the northern Pacific that flows south past the Kurile Islands towards Japan, where it meets the warm Kuroshio.

The correct answer is B, Peru. El Nino, Spanish for the child, is the warm surface water that appears off the coast of Peru and Ecuador around Christmas in some years, replacing the normally cold Peru or Humboldt current. When it develops strongly the trade winds weaken, the anchovy catch collapses because nutrient-rich cold water stops rising, and Indian monsoon rainfall tends to be below normal.

Option A, Colombia, faces both the Pacific and the Caribbean but lies too close to the equator and is not the coast the phenomenon is named for. Option C, Chile, is washed by the same cold Peru current further south, yet the warming is described and measured off Peru and Ecuador. Option D, Bolivia, is a landlocked country in South America with no sea coast at all, so no ocean current can run along it.

The correct answer is A, the Pacific. The Ring of Fire is a horseshoe-shaped belt running around the rim of the Pacific Ocean, from the Andes up the west coast of the Americas, across the Aleutians and down through Japan, the Philippines and Indonesia to New Zealand. It holds the majority of the world's active volcanoes and is the source of most large earthquakes, because it follows the lines along which oceanic plates are being subducted.

Option B, the Indian Ocean, does have the seismic Sunda arc on its eastern edge, but that arc is counted as part of the Pacific ring and the ocean as a whole has no such belt. Option C, the Atlantic, is widening rather than closing, so its activity is concentrated along the Mid-Atlantic Ridge and is mostly gentle. Option D, the Arctic, is tectonically quiet apart from the slow spreading ridge that continues from the Atlantic.

The correct answer is B, 97.2 percent. Textbooks describing the distribution of water on earth give about 97.2 per cent as ocean water, roughly 2 per cent locked in ice caps and glaciers, and well under 1 per cent as groundwater, lakes, rivers, soil moisture and water vapour together. This is why fresh water is described as a scarce resource even though the planet looks blue from space.

Option A, 94 per cent, option C, 90.2 per cent, and option D, 92.2 per cent, are all too low and would leave an impossibly large share of the planet's water as fresh water. The safe way to remember the set is that the oceans take a little over 97 per cent, the frozen ice sheets of Antarctica and Greenland take most of what is left, and everything that human beings actually use comes out of the small remainder.

The correct answer is A. Spring tides occur at new moon and at full moon, when the sun, the earth and the moon lie in a straight line, a position called syzygy. The gravitational pulls of the sun and the moon then act along the same line and reinforce each other, so the high tides are unusually high and the low tides unusually low. Spring tides therefore come twice in a lunar month.

Option B describes the position at the first and third quarter, called quadrature, when the two pulls partly cancel and produce the weakest tides of the month, the neap tides. Option C, the moon at apogee, makes tides slightly weaker rather than stronger, since the moon's pull falls off with distance. Option D is the common trap: the word spring here means to leap up, and the tide has nothing to do with the spring season.

The correct answer is B, the Bay of Fundy, which lies between the Canadian provinces of Nova Scotia and New Brunswick. The bay narrows and shallows inland, so the incoming tidal wave is funnelled into a smaller and smaller cross-section and its height is greatly magnified, giving the greatest difference between high and low water anywhere on earth. Shape, not size, is what produces a large tidal range.

Option A, the Gulf of Mexico, is wide and almost enclosed, and much of it has only a very small daily tide. Option C, the Persian Gulf, is shallow and nearly landlocked, so the tidal wave entering through the narrow Strait of Hormuz remains modest. Option D, the Gulf of Aden, opens into the Arabian Sea without any funnelling shape. In India the largest ranges are found in the funnel-shaped Gulf of Khambhat and Gulf of Kachchh.

The correct answer is C, the Benguela current. It flows northward along the western coast of southern Africa, carrying cool water from the Southern Ocean towards the equator, and the upwelling along it makes the Namibian coast a rich fishing zone while helping to keep the Namib desert dry. The simple rule is that a current moving from the poles towards the equator is cold.

Option A, the Gulf Stream, carries warm water from the Gulf of Mexico north-east across the Atlantic and is the classic warm current. Option B, the Kuroshio, is the warm current of the western Pacific that flows north past Japan and is sometimes called the Japan current. Option D, the Brazil current, is a warm current flowing south along the eastern coast of South America. All three move away from the equator and so carry warm water.

The correct answer is B, the Gulf Stream and the Labrador current. The warm Gulf Stream moving north-east meets the cold Labrador current coming south past Newfoundland. Nutrients carried by the cold water mix with the warmer surface layer, plankton multiplies, and huge shoals of cod once gathered there. The same meeting cools moist air abruptly and makes the Grand Banks one of the foggiest places at sea.

Option A, the Kuroshio and Oyashio, do meet and create an equally famous fishery, but that is off Hokkaido in Japan, not Newfoundland. Option C, the Canary and Benguela currents, are both cold, flow along African coasts thousands of kilometres apart and never meet. Option D pairs a South American current with an Australian one lying in different oceans, so no meeting is possible.

The correct answer is C, about 35 parts per thousand, which means roughly thirty-five grams of dissolved salt in every kilogram of sea water. Sodium chloride makes up most of it, with magnesium, calcium and potassium salts making up the rest. Salinity varies with evaporation, rainfall, river inflow and the melting or freezing of ice, so the figure is an average rather than a fixed value.

Option A, 15 parts per thousand, is closer to brackish water in an estuary or to the low-salinity Baltic Sea, where many rivers pour in fresh water. Option B, 25 parts per thousand, is still well below the open ocean figure. Option D, 45 parts per thousand, is higher than any open ocean value; even the Red Sea, the most saline of the open seas, stays near the low forties, and only enclosed lakes such as the Dead Sea go far above that.

The correct answer is C, the Mariana Trench, which lies in the western Pacific east of the Mariana Islands. Its southern end holds the Challenger Deep, the deepest surveyed point on earth at roughly eleven kilometres below sea level, deep enough to swallow Mount Everest with water to spare. Trenches like it form where one oceanic plate is being pushed down beneath another, which is why they run beside volcanic island arcs.

Option A, the Puerto Rico Trench, is the deepest point of the Atlantic but is far shallower than the Mariana. Option B, the Java or Sunda Trench, is the deepest part of the Indian Ocean and lies south of Indonesia. Option D, the Tonga Trench, is in the south Pacific and is among the deepest in the world, yet its greatest depth is still less than that of the Challenger Deep.

The correct answer is B. The continental shelf is the gently sloping submerged edge of a continent, usually taken down to about two hundred metres. Because it is shallow, sunlight penetrates to the sea bed and supports abundant plankton, and rivers add nutrients from the land as well. Fish gather where plankton is plentiful, so almost all the world's major fishing grounds, and most offshore petroleum, lie on continental shelves.

Option A is wrong because the deepest parts of the ocean are the trenches, which lie far beyond the shelf and are dark and nearly lifeless by comparison. Option C is wrong because the shelf usually has slightly lower salinity than the open ocean, since rivers dilute it. Option D is wrong as well: currents and upwelling reach shelf waters constantly, and in fact the upwelling of nutrients along some shelves is exactly what makes them so productive.