1.5 - Geography Notes 32

Elaborate Notes

Currents of the South Atlantic Ocean

The circulation in the South Atlantic Ocean is dominated by a large anti-cyclonic gyre. This system is influenced by the trade winds, westerlies, and the Coriolis force.

• South Equatorial Current: This is a broad, westward-flowing current driven by the Southeast Trade Winds. As it approaches the coast of South America, near Cape de São Roque in Brazil, it bifurcates. The northern branch flows into the Caribbean, while the southern branch turns south.
• Brazilian Current: The southern branch of the South Equatorial Current flows southward along the eastern coast of Brazil. It is a warm current, transporting tropical warm water into the temperate latitudes. Its character is analogous to the Gulf Stream in the North Atlantic, though it is considerably weaker and shallower. According to oceanographer Talley (2011) in “Descriptive Physical Oceanography”, its flow is less intense due to the wider continental shelf off Brazil compared to North America.
• West Wind Drift (Antarctic Circumpolar Current - ACC): In the southern latitudes (approximately 40°S to 60°S), the powerful Westerlies drive a massive eastward-flowing cold current, the West Wind Drift. This is the only current that flows unimpeded around the globe, connecting the Atlantic, Pacific, and Indian Oceans. It plays a critical role in global heat distribution.
• South Atlantic Drift: The warm Brazilian Current, upon meeting the cold Falkland Current and being deflected eastward by the Coriolis force, merges with the northern part of the West Wind Drift. This eastward-flowing mixture of warm and cold water is known as the South Atlantic Drift.
• Benguela Current: As the South Atlantic Drift approaches the western coast of Southern Africa, a branch is deflected northward by the coastline and the Coriolis effect. This forms the Benguela Current. It is a broad, slow-moving cold current, augmented by significant upwelling of cold, nutrient-rich deep water. This upwelling is driven by the prevailing offshore winds.
  • Impact on Climate: The presence of this cold water mass has a profound desiccating effect on the air above it, leading to atmospheric stability and inhibiting rainfall. This is a primary factor in the formation and hyper-aridity of the Namib Desert and the aridity of the Kalahari Desert. Historical climate reconstructions based on sediment cores confirm the long-term presence of this current and its influence on regional climate for millennia.
• Falkland Current: This is a cold current that branches off the West Wind Drift and flows northward along the Patagonian coast of Argentina. It carries cold, sub-Antarctic water and is rich in nutrients. The confluence of the cold Falkland Current and the warm Brazil Current creates a zone of high biological productivity and frequent fogs.
• South Atlantic Gyre: The combination of the South Equatorial Current, Brazilian Current, South Atlantic Drift, and Benguela Current forms a large, counter-clockwise rotating gyre that dominates the surface circulation of the South Atlantic.

Currents of the Pacific Ocean

The Pacific Ocean, being the largest ocean basin, features two major gyres in the Northern and Southern Hemispheres.

• North Pacific Ocean:

  • North Equatorial Current: Driven by the Northeast Trade Winds, this current flows westward across the Pacific. Upon reaching the Philippines, the archipelago forces it to bifurcate.
  • Kuroshio Current (or Japan Current): The northward-flowing branch is known as the Kuroshio Current. It is a powerful, warm, and saline current, often called the “Black Stream” due to the deep blue colour of its water. It flows along the coasts of Taiwan, the Ryukyu Islands, and Japan, bringing significant warmth to these regions, moderating their climate. It is the North Pacific’s analogue to the Gulf Stream.
  • North Pacific Current (or North Pacific Drift): The Kuroshio Current is deflected eastward by the Westerlies and the Coriolis force around 30-50°N latitude, forming the North Pacific Current. This is a slow, wide drift that transports warm water across the Pacific towards North America.
  • Alaskan Current: As the North Pacific Current approaches the North American continent, it splits. The northern branch flows into the Gulf of Alaska in a counter-clockwise gyre, forming the warm Alaskan Current. This current keeps the coast of British Columbia and southern Alaska relatively mild and ice-free in winter.
  • California Current: The southern branch flows southward along the coast of California. It is a cold current, characterized by significant coastal upwelling, especially during spring and summer, which brings nutrient-rich deep water to the surface. This creates a highly productive marine ecosystem but also contributes to the coastal fogs and the aridity of regions like the Mojave Desert. The California Current eventually turns westward to join the North Equatorial Current, completing the North Pacific Gyre.
  • Oyashio Current (or Kurile Current): This is a cold, subarctic current that flows southwestward from the Bering Sea and along the east coast of Kamchatka Peninsula and the Kuril Islands. It carries cold, nutrient-rich water.
  • Okhotsk Current: A similar cold current originating in the Sea of Okhotsk. The Oyashio and Okhotsk currents converge with the warm Kuroshio Current off the eastern coast of Japan, creating one of the world’s most productive fishing grounds and an area of dense fog.

• South Pacific Ocean:

  • South Equatorial Current: Flowing westward, driven by the Southeast Trade Winds.
  • East Australian Current (EAC): Upon reaching the coast of Australia, the South Equatorial Current turns south, forming the warm East Australian Current. It is the largest ocean current close to the Australian coast and transports warm tropical water southward, influencing the climate of the eastern seaboard.
  • South Pacific Drift: The EAC flows into the West Wind Drift (Antarctic Circumpolar Current), contributing its warmer waters to this massive eastward flow.
  • Humboldt Current (or Peru Current): A branch of the West Wind Drift is deflected northward along the western coast of South America, forming the cold Humboldt Current. It is characterized by exceptionally strong upwelling, making the waters off Peru and Chile one of the most productive marine ecosystems on Earth, historically supporting the world’s largest fishery (anchoveta). The cold current is a major factor in the extreme aridity of the Atacama Desert.
  • The Humboldt Current flows westward to join the South Equatorial Current, completing the counter-clockwise South Pacific Gyre.

Indian Ocean Currents

The circulation in the Indian Ocean is unique, particularly in its northern part, due to the seasonal reversal of monsoon winds and its landlocked nature to the north.

• Southern Indian Ocean: The circulation is relatively stable and forms a standard anti-cyclonic gyre.

  • South Equatorial Current: Flows westward, driven by the Southeast Trade Winds.
  • Upon reaching the coast of Africa, it is split by the island of Madagascar into two branches: the Mozambique Current (flowing through the Mozambique Channel) and the Madagascar Current (flowing east of the island). Both are warm currents.
  • Agulhas Current: The Mozambique and Madagascar currents combine south of Madagascar to form the powerful, warm, and swift Agulhas Current, which flows down the east coast of South Africa. It is one of the strongest western boundary currents in the world.
  • The Agulhas Current eventually retroflects (turns back on itself) and joins the eastward-flowing West Wind Drift.
  • Western Australian Current: A branch of the West Wind Drift turns northward along the western coast of Australia, forming the cold Western Australian Current. This current is weaker than its counterparts (Benguela, Humboldt) but still has a cooling and drying effect on the western Australian coast. It then flows westward to complete the gyre by joining the South Equatorial Current.

• Northern Indian Ocean: The surface circulation completely reverses with the seasons.

  • Winter (Northeast Monsoon - November to March): The Northeast Monsoon winds blow from the continent towards the ocean. This drives the North Equatorial Current westward. A weak Counter-Equatorial Current flows eastward between the North and South Equatorial Currents. The overall circulation in the Arabian Sea and the Bay of Bengal is anti-clockwise, forming the Northeast Monsoon Drift.
  • Summer (Southwest Monsoon - May to September): The Southwest Monsoon winds blow from the ocean towards the continent. This strong wind system reverses the entire surface circulation. The North Equatorial and Counter-Equatorial currents disappear. The winds drive a strong clockwise circulation known as the Southwest Monsoon Drift. A notable feature is the powerful, northward-flowing Somali Current along the Horn of Africa, which is a rare example of a major eastern boundary current that is strong and driven by upwelling.

Significance of Ocean Currents

Ocean currents are a fundamental component of the Earth’s climate system and have profound impacts on marine and terrestrial life.

• (a, b, g) Global Heat Distribution: Currents act as a global heat conveyor belt. Warm currents like the Gulf Stream and Kuroshio transport vast amounts of heat from the tropics towards the poles, while cold currents like the Labrador and Oyashio transport cold water towards the equator. This redistribution of thermal energy prevents the tropics from becoming excessively hot and the poles from becoming excessively cold, thus moderating global temperatures.
• (d, e) Influence on Climate and Weather:
  • Desert Formation: Cold currents flowing along the western margins of continents in sub-tropical regions (e.g., Benguela, Humboldt, California, Western Australian currents) cool the air above them, reducing its moisture-holding capacity and creating atmospheric stability (temperature inversion). This suppresses convection and precipitation, leading to the formation of some of the world’s driest coastal deserts: Namib/Kalahari, Atacama, Sonoran/Mojave.
  • Precipitation and Mild Winters: Warm currents bring warmth and moisture to adjacent coastlands. For instance, the North Atlantic Drift provides Western Europe with mild, wet winters, keeping ports like Murmansk (Russia) and those in Norway ice-free year-round, despite their high latitudes.
• (f) Marine Ecosystems and Fishing Grounds:
  • Upwelling: Offshore winds push surface water away from the coast, causing cold, deep, nutrient-rich water to rise to the surface. This process, known as upwelling, supports vast blooms of phytoplankton, which form the base of the marine food web. The world’s most productive fishing grounds are found in upwelling zones, such as along the coast of Peru (Humboldt Current) and off the coast of Southern Africa (Benguela Current).
  • Mixing Zones: The convergence of warm and cold currents leads to mixing of water masses and stirs up nutrients. These zones, like the Grand Banks off Newfoundland (where the cold Labrador Current meets the warm Gulf Stream) and the seas off Japan (where the cold Oyashio meets the warm Kuroshio), are exceptionally rich fishing grounds.
• (c) Cyclone Formation: The accumulation of warm water (above 26.5°C) along the western parts of oceans in tropical regions provides the necessary thermal energy for the formation and intensification of tropical cyclones (hurricanes, typhoons).
• (j) Navigation and Fog:
  • Historically, mariners have used currents to their advantage to speed up voyages. Even today, shipping routes are planned to leverage currents to save fuel.
  • The meeting of warm and cold currents causes the warm, moist air over the warm current to cool to its dew point, resulting in the formation of dense advection fog. This is a significant navigational hazard in areas like the Grand Banks and off the coast of Japan.

Upwelling and Downwelling

These are vertical movements of water that are crucial for ocean circulation and marine biology.

• Upwelling: The process where deep, cold, and typically nutrient-rich water rises towards the surface.
  • Causes: Primarily driven by wind (coastal upwelling, where winds parallel to the coast push surface water away due to Ekman transport) and also by ocean floor topography (seamounts) that can deflect deep currents upwards.
  • Significance: It is the primary mechanism that fertilizes the sunlit surface waters of the ocean, supporting a majority of global fishery production.
• Downwelling: The process where surface water is forced downwards.
  • Causes: Primarily driven by wind (onshore winds piling up water against a coastline) and by changes in water density.
  • Significance: Downwelling transports dissolved oxygen from the surface to the deep ocean, which is essential for deep-sea organisms. It is also a key component of the thermohaline circulation.

Thermohaline Circulation (THC)

This refers to the deep ocean circulation driven by global density gradients created by surface heat and freshwater fluxes.

• Mechanism: The term ‘thermohaline’ comes from thermo (heat) and haline (salt), the two factors that determine seawater density. In polar regions, particularly the North Atlantic (near Greenland) and the Antarctic coast, surface water becomes very cold and, as sea ice forms, the remaining water becomes saltier. This cold, salty water is extremely dense and sinks to the deep ocean.
• The Great Ocean Conveyor Belt: This newly formed deep water then flows slowly in a vast, continuous current across the ocean basins. This deep water eventually rises back to the surface in other parts of the world, primarily in the Pacific and Indian Oceans, through upwelling. This entire global circuit, connecting surface and deep currents, is often called the “Great Ocean Conveyor Belt”. As described by Wallace Broecker (1987), who coined the term, this circulation takes about 1,000 years to complete one cycle and plays a fundamental role in long-term climate regulation.

Atlantic Meridional Overturning Circulation (AMOC)

AMOC is the Atlantic component of the thermohaline circulation and is a critical regulator of climate in the Northern Hemisphere.

• Function: It involves the northward flow of warm, salty surface water in the upper Atlantic and the southward flow of cold, deep water in the lower Atlantic. The sinking of dense water in the high-latitude North Atlantic is the primary engine of this circulation.
• Weakening of AMOC: The Intergovernmental Panel on Climate Change (IPCC) in its recent Assessment Reports (AR6) has stated with high confidence that the AMOC has weakened over the 20th century and is very likely to weaken further over the 21st century due to climate change.
• Factors Responsible for Weakening:
  1. Arctic Warming: The Arctic is warming at more than twice the global rate. This reduces the temperature difference between the tropics and the poles, which weakens the atmospheric and oceanic drivers (like the North Atlantic Drift).
  2. Freshwater Influx: The accelerated melting of the Greenland ice sheet and Arctic sea ice is releasing large volumes of fresh water into the North Atlantic. Fresh water is less dense than salt water, so it reduces the surface salinity and density, inhibiting the sinking process that drives the AMOC.
• Potential Impacts of Weakening/Collapse:
  • Climate Shift: A significant slowdown could lead to much harsher winters in Western Europe as the heat transport via the North Atlantic Drift is reduced.
  • Sea Level Rise: It would cause a significant rise in sea levels along the eastern coast of North America.
  • Precipitation Patterns: It could shift tropical rain belts southward, potentially affecting rainfall patterns in regions like the Sahel in Africa and South Asia.
  • Ecosystem Disruption: The changes in temperature, salinity, and nutrient distribution would severely disrupt marine ecosystems and major fisheries.

Water Mass

A water mass is an identifiable body of water with a specific range of temperature and salinity, and thus a characteristic density, acquired at its formation region at the ocean surface.

• Classification and Examples:
  • Surface Water Mass: Extends to a depth of about 200 meters. Its characteristics are determined by local climatic conditions. Example: Antarctic Surface Water (AASW).
  • Intermediate Water Mass: Found between surface and deep water. Example: Antarctic Intermediate Water (AAIW), which forms at the Antarctic Convergence and sinks to about 1000m, flowing northward.
  • Deep Water Mass: Forms in high latitudes where surface water becomes dense enough to sink to great depths. Example: North Atlantic Deep Water (NADW), which is the primary driver of the AMOC.
  • Bottom Water Mass: The densest water mass, found at the very bottom of the ocean basins. Example: Antarctic Bottom Water (AABW), which forms off the coast of Antarctica and is the coldest, saltiest, and densest water mass in the world ocean.
• Significance:
  • The formation and movement of water masses are the fundamental components of the thermohaline circulation.
  • They are responsible for upwelling and downwelling phenomena.
  • Their movement facilitates the global distribution of heat, salinity, dissolved oxygen, and nutrients.
  • The uniform conditions within a water mass support specific biological communities, influencing the distribution of plankton and other marine life.

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Prelims Pointers

• Warm Currents: Brazilian, Kuroshio, Alaskan, East Australian, Agulhas, Mozambique, Madagascar.
• Cold Currents: West Wind Drift, Benguela, Falkland, California, Oyashio, Okhotsk, Humboldt (Peru), Western Australian.
• South Atlantic Gyre: South Equatorial Current → Brazilian Current → South Atlantic Drift → Benguela Current. Rotates counter-clockwise.
• North Pacific Gyre: North Equatorial Current → Kuroshio Current → North Pacific Current → California Current. Rotates clockwise.
• South Pacific Gyre: South Equatorial Current → East Australian Current → South Pacific Drift → Humboldt Current. Rotates counter-clockwise.
• Desert Formation & Cold Currents:
  • Benguela Current: Namib and Kalahari Deserts.
  • Humboldt (Peru) Current: Atacama Desert.
  • California Current: Mojave/Sonoran Deserts.
• Major Fishing Grounds:
  • Grand Banks (Newfoundland): Formed by the meeting of the warm Gulf Stream and cold Labrador Current.
  • Off the coast of Japan: Formed by the meeting of the warm Kuroshio and cold Oyashio currents.
  • Peruvian Coast: Rich due to upwelling associated with the Humboldt Current.
• Indian Ocean Currents: Circulation in the Northern Indian Ocean reverses seasonally due to Monsoons.
  • Winter (NE Monsoon): Anti-clockwise circulation (NE Monsoon Drift).
  • Summer (SW Monsoon): Clockwise circulation (SW Monsoon Drift).
• Upwelling: Vertical movement of cold, nutrient-rich deep water to the surface, often driven by offshore winds.
• Downwelling: Sinking of surface water, often due to onshore winds or density increase.
• Thermohaline Circulation (THC): Deep ocean circulation driven by differences in water density, which is controlled by temperature (thermo) and salinity (haline). Also known as the Great Ocean Conveyor Belt.
• Atlantic Meridional Overturning Circulation (AMOC): The Atlantic component of the THC.
• AMOC Weakening: Caused by global warming, specifically Arctic warming and melting of Greenland ice. This is confirmed by recent IPCC reports.
• Water Mass: A body of water with uniform temperature and salinity.
• Key Water Masses: North Atlantic Deep Water (NADW), Antarctic Bottom Water (AABW).

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Mains Insights

The Role of Ocean Currents in Global Climate Regulation and Socio-Economic Systems

1. Climate Regulation and Link to Climate Change (GS-I, GS-III):

   • Cause-Effect: Ocean currents are a primary mechanism for the poleward transfer of heat, making them a crucial regulator of global climate. The North Atlantic Drift’s warming effect on Europe is a classic example. Any disruption to this system has far-reaching consequences.
   • Contemporary Issue (AMOC): The weakening of the AMOC due to anthropogenic climate change is a critical concern. As per the IPCC, this could lead to abrupt climate shifts, not just gradual warming. This illustrates a climatic ‘tipping point’ with potentially irreversible consequences, such as severe cooling in Europe, altered monsoon patterns affecting food security in Asia and Africa, and accelerated sea-level rise on the US East Coast.
   • Analytical Perspective: The ocean is not just a victim of climate change but an active participant in its evolution. The weakening AMOC is a feedback mechanism where warming leads to changes that could further alter the climate system in unpredictable ways.

2. Economic Significance and Vulnerability (GS-III):

   • Fisheries: The world’s most productive fisheries are not randomly located; they are directly sustained by nutrient upwelling driven by currents like the Humboldt and Benguela. These fisheries are vital for the food security and economies of countries like Peru, Chile, and Namibia.
   • Vulnerability: This dependence creates economic vulnerability. The El Niño-Southern Oscillation (ENSO), a phenomenon involving the weakening of trade winds and ocean currents in the Pacific, leads to the collapse of upwelling off the Peruvian coast, devastating the anchovy fishery and causing global economic ripples. The potential weakening of currents due to climate change poses a long-term threat to these resource bases.
   • Navigation and Trade: Ocean currents have shaped maritime trade routes for centuries. Modern shipping still uses them to optimize fuel consumption. Changes in current patterns could impact global supply chains.

3. Geopolitical and Strategic Dimensions (GS-II):

   • Resource Management: Transboundary fisheries sustained by ocean currents require international cooperation for sustainable management. Disputes can arise over fishing rights in these productive zones.
   • Arctic Routes: The warming of the Arctic, linked to changes in oceanic circulation, is opening up new shipping routes like the Northwest Passage and the Northern Sea Route. This has significant geopolitical implications, involving competition for resource access and strategic control among Arctic nations.
   • Climate Justice: The impacts of a weakening AMOC would be felt globally but disproportionately. Developing nations in the tropics (e.g., in the Sahel) that rely on predictable rainfall patterns could face severe droughts, raising questions of climate justice and the responsibility of developed nations.

4. Historiographical and Scientific Viewpoint:

   • Evolution of Understanding: Early maritime explorers like Benjamin Franklin first mapped currents like the Gulf Stream for navigational purposes. The scientific understanding has evolved from simple surface mapping to a complex three-dimensional view incorporating deep ocean circulation (THC).
   • Role of Technology: Our understanding of these systems is heavily reliant on modern technology like satellite altimetry (to measure sea surface height, which indicates current flow), Argo floats (to measure temperature and salinity profiles), and sophisticated climate models. The debate on the future of the AMOC is driven by paleoclimatic data (from ice cores and sediment) and projections from these complex models.
   • Debate: While the scientific consensus points to a weakening AMOC, the exact timing and probability of a complete collapse remain subjects of intense research and debate. This uncertainty complicates policy-making, highlighting the need for precautionary principles in climate action.