Oceanography: UPSC Previous Year Questions (Geography)

Explanation

Isotherms are lines connecting places with equal temperatures. The horizontal distribution of temperature is depicted through isothermal maps, with the temperature reduced to sea level to eliminate altitude effects. Isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere.

Statement 1 is correct: In January (winter in the Northern Hemisphere), continents lose heat quickly because land has a lower heat capacity than water. On the other hand, oceans retain heat longer due to their high heat capacity. As a result over continents (which are colder), isotherms shift southward, following the drop in temperature. While over oceans (which are warmer), isotherms shift northward, reflecting the retained warmth. This pattern is clearly visible on isothermal maps of the North Atlantic and Eurasian regions.

Statement 2 is incorrect: The Gulf Stream and the North Atlantic Drift are warm ocean currents, not cold. The Gulf Stream carries warm water from the Gulf of Mexico along the eastern coast of North America, while the North Atlantic Drift, an extension of the Gulf Stream, warms the western coasts of Europe. These warm currents increase temperatures in the North Atlantic region, making the area warmer, not colder. As a result, isotherms bend northward because the ocean is warmer than the surrounding continents, not due to cold currents. Fig.: Isotherms in January

Explanation

Ocean Mean Temperature (OMT) represents the thermal energy in the upper layer of the ocean. It is measured up to the 26°C isotherm, which represents the depth where water temperature is consistently 26°C. OMT is a more reliable indicator than Sea Surface Temperature (SST) for predicting the Indian monsoon rainfall, as it reflects the ocean’s thermal energy more comprehensively.

Statement 1 is incorrect: OMT is measured up to the 26°C isotherm. In the southwestern Indian Ocean during January-March, the depth of the 26°C isotherm is approximately 59 meters, not 129 meters.

Statement 2 is correct: OMT, especially during the January-March period (pre-monsoon season) has been found to be a useful indicator for forecasting the Indian Summer Monsoon rainfall. Warmer OMT in this period can suggest a stronger monsoon and potentially higher rainfall, while cooler OMT may indicate a weaker monsoon. This is because the ocean’s temperature influences the amount of moisture available for the monsoon winds. Studies by the Indian Institute of Tropical Meteorology (IITM) and other scientific bodies have shown a strong correlation between OMT and monsoon performance.

Explanation

Statement 1 is correct: Coral reefs predominantly thrive in warm, shallow waters, typically between the Tropics of Cancer and Capricorn (30°N to 30°S latitude). These tropical regions provide the ideal conditions--such as appropriate temperature, light, and salinity--for coral growth. The warm water temperatures are essential for the symbiotic relationship between coral polyps and the algae (zooxanthellae) that live within their tissues.

Statement 2 is correct: The Great Barrier Reef in Australia, along with coral reef systems in Indonesia and the Philippines (part of the Coral Triangle), collectively account for more than one-third of the world’s coral reefs. These regions are globally renowned for their extensive and diverse coral systems.

Statement 3 is correct: Coral reefs are among the most bio-diverse ecosystems on Earth, hosting over 25,000 described species from 32 of the 33 known animal phyla, which is more than the number of animal phyla found in tropical rainforests.

Explanation

Statement 1 is incorrect: The Indian Ocean Dipole(IOD) is defined by the difference in sea surface temperatures (SSTs) between the western and eastern parts of the Indian Ocean, not the Pacific Ocean. Specifically, it involves the SST gradient between the western equatorial Indian Ocean (50°E-70°E and 10°S-10°N) and the southeastern equatorial Indian Ocean (90°E-110°E and 10°S-0°N).

Statement 2 is correct: A positive IOD (warmer SST in the western Indian Ocean) can help mitigate the adverse effects of El Niño on the Indian monsoon by enhancing monsoonal rainfall. Conversely, a negative IOD (warmer SST in the eastern Indian Ocean) can exacerbate El Niño’s drought-like conditions over India.

Explanation

The Equatorial Counter Current (CEC) is an eastward-flowing current located between the North Equatorial Current (NEC) and the South Equatorial Current (SEC) in the oceans near the equator. It is most prominent in the Pacific and Indian Oceans and helps redistribute heat across the equator, contributing to global climate regulation.

Option (a) is incorrect: While the Earth’s rotation influences large-scale oceanic circulation through the Coriolis force, it does not directly explain the eastward flow of the Equatorial Counter Current. The Coriolis effect is minimal near the equator.

Option (b) is correct: The North Equatorial Current (NEC) and the South Equatorial Current (SEC) flow westward due to trade winds. As these currents converge in the western parts of the oceans, water piles up. Under the influence of gravity, this piled-up water flows eastward, forming the Equatorial Counter Current.

Option (c) is incorrect: While salinity influences vertical water movement, its impact on large-scale horizontal currents like the Equatorial Counter Current is negligible.

Option (d) is incorrect: The Doldrums (regions of calm winds near the equator) do not actively cause the eastward flow. Instead, they create areas of weak wind, which are more relevant for localized weather conditions than large-scale currents. Fig.: Equatorial counter current in Pacific Ocean

Explanation

Tides are the periodic rise and fall of sea levels, driven by various forces acting upon Earth’s water bodies. These forces include:

The Sun’s gravitational pull plays a secondary role in creating tides, as it is much farther away from Earth compared to the Moon. However, its gravitational influence still contributes significantly to spring tides (higher-than-normal tides) when the Sun, Moon, and Earth are aligned (new moon and full moon phases). The Moon’s gravitational force is the primary driver of tides. Due to its proximity to Earth, the Moon exerts a stronger pull on Earth’s oceans, causing the bulge of water that forms high tides. The gravitational pull of the Moon is nearly 2.5 times more effective than that of the Sun in causing tides due to its closer proximity. The Earth-Moon system rotates around a common center of mass, creating a centrifugal force that acts opposite to the Moon’s gravitational pull. This force also contributes to the formation of tides on the side of Earth opposite the Moon, leading to high tides there. It helps explain the formation of high tides on the side of Earth opposite the Moon.

Explanation

Option (c) is correct: Regions where warm and cold ocean currents meet are highly productive fishing grounds due to:

Plankton Growth: Cold currents bring nutrient-rich deep water, while warm currents stabilize conditions, fostering plankton blooms, the base of the marine food chain. Fish Abundance: These regions attract fish feeding on plankton, creating rich fishing zones like Newfoundland (Labrador Current meets Gulf Stream), North Sea (Gulf Stream meets Arctic currents), and seas near Japan (Kuroshio meets Oyashio).

Explanation

The Earth’s rotation affects ocean currents through the Coriolis effect, which causes moving fluids like air and water to be deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This deflection influences the direction of ocean currents.

Option 2 is correct: Wind patterns which are driven by differences in air pressure, are primary drivers of surface ocean currents. The friction between the wind and the ocean surface pushes water, creating currents.

Option 3 is correct: Differences in water density, which are caused by variations in salinity and temperature, influence vertical ocean currents. Cold water is denser and tends to sink, while warm water is lighter and tends to rise. This process contributes to the thermohaline circulation.

Option 4 is incorrect: The revolution of the Earth does not directly influence ocean currents. While the revolution of Earth impacts seasons and sunlight distribution, it is not a direct cause of ocean current movement.

Explanation

Statement 1 is correct: Ocean acidification reduces the ability of calcareous phytoplankton (such as coccolithophores) to form their calcium carbonate shells. As the ocean becomes more acidic, the process of calcification is impaired, adversely affecting their growth and survival.

Statement 2 is correct: Coral reefs rely on the ability of coral polyps to produce calcium carbonate skeletons. As ocean acidity increases, it becomes more difficult for corals to build and maintain their skeletons, making them more vulnerable. This not only threatens corals themselves but also the ecosystems that depend on them.

Statement 3 is correct: The larvae of some marine organisms, particularly those with phytoplanktonic larvae, rely on calcium carbonate for protection and growth. As the ocean becomes more acidic, these organisms may face difficulties in developing their shells, which could disrupt marine food webs.

Statement 4 is incorrect: While phytoplankton contribute to the release of dimethyl sulfide (DMS), which is linked to cloud formation, ocean acidification does not have a direct and large-scale impact on cloud formation. The reduction in phytoplankton due to ocean acidification may lead to lower DMS emissions, but this effect is relatively minor and unlikely to significantly affect cloud seeding and formation.

Explanation

Statement 1 is incorrect: La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific (not the Indian Ocean). In contrast, El Niño is associated with unusually warm ocean temperatures in the Equatorial Pacific Ocean.

Statement 2 is incorrect: El Niño events are associated with a weakening of the Indian monsoon leading to reduced rainfall and drought conditions in India. This occurs because the warming of the central and eastern Pacific Ocean alters atmospheric circulation patterns, which can suppress the monsoon rains over the Indian subcontinent. La Niña events typically have the opposite effect, strengthening the Indian monsoon and often resulting in above-average rainfall. The cooling of the central and eastern Pacific Ocean during La Niña enhances the monsoon circulation, leading to increased precipitation over India.