Ocean Currents
Ocean Currents
Ocean currents are large-scale, continuous, predictable movements of seawater driven by wind, temperature and salinity differences, the Coriolis effect, and the shape of ocean basins. They act as a global conveyor belt, distributing heat, nutrients, and dissolved gases across the world's oceans, profoundly influencing climate, marine ecosystems, and fisheries.
Key Dates
Ponce de Leon documented the Gulf Stream — one of the first recorded observations of a major ocean current
Benjamin Franklin produced the first chart of the Gulf Stream, improving trans-Atlantic navigation
Matthew Fontaine Maury published ocean current charts that revolutionized maritime navigation
Sir Gilbert Walker served as Director General of Indian Observatories; discovered the Southern Oscillation
Saji et al. identified the Indian Ocean Dipole (IOD) as an independent climate mode
Transports about 30 million cubic metres of water per second — 300 times the flow of the Amazon River
The global thermohaline circulation (ocean conveyor belt) takes about 1,000 years to complete one cycle
ENSO events significantly affect the Indian monsoon — El Nino years often see below-normal monsoon rainfall in India
The Indian Ocean is unique — its currents reverse seasonally due to the monsoon wind reversal
Coastal upwelling zones produce about 50% of the world's fish catch despite covering <1% of ocean area
Indian Ocean Dipole — temperature difference between western and eastern Indian Ocean — affects Indian rainfall
Strongest positive IOD in decades coincided with a late but robust monsoon season over India
Atlantic Meridional Overturning Circulation showing signs of weakening — potential global climate implications
Causes of Ocean Currents
Ocean currents are driven by multiple forces: (1) Wind Stress — the primary driver of surface currents; persistent planetary winds (trades, westerlies) drag the ocean surface, setting it in motion; the friction between wind and water transfers momentum to a depth of about 100-200 m (the Ekman Layer); due to the Coriolis effect, surface water does not flow in the exact direction of the wind but is deflected — to the right in the Northern Hemisphere and left in the Southern Hemisphere; the net transport of water in the Ekman Layer is at 90 degrees to the wind direction (Ekman Transport). (2) Thermohaline Circulation — driven by differences in water density caused by variations in temperature (thermo) and salinity (haline); cold, salty water is denser and sinks (at high latitudes), while warm, less salty water is lighter and stays at the surface; this density-driven circulation creates the global "ocean conveyor belt" — a continuous deep-ocean circulation that redistributes heat globally. (3) Coriolis Effect — deflects currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere; this causes surface currents to form large circular patterns called gyres. (4) Gravity — water flows downhill from higher to lower sea levels; wind piling up water against a coast creates a pressure gradient that drives return flows. (5) Shape of Ocean Basins — continents act as barriers, deflecting and channelling currents; straits and passages concentrate flow. (6) Differences in Water Level — variations in sea level (due to heating, salinity, or wind stress) cause water to flow from higher to lower levels.
Classification of Ocean Currents
Ocean currents can be classified in several ways: By temperature: Warm currents — carry warm water from equatorial regions toward the poles; generally flow on the western side of ocean basins (western boundary currents); they warm adjacent coasts and increase rainfall; examples: Gulf Stream, Kuroshio, Brazil, North Atlantic Drift, Agulhas, East Australian. Cold currents — carry cold water from polar or deep ocean regions toward the equator; generally flow on the eastern side of ocean basins (eastern boundary currents); they cool adjacent coasts, create fog, and cause coastal deserts; examples: Labrador, Canary, Benguela, Peru/Humboldt, California, West Australian. By depth: Surface currents — affect the upper 200-400 m; driven primarily by wind; represent about 10% of ocean water; most studied and charted. Deep currents — below the thermocline; driven by density differences (thermohaline); move slowly; represent about 90% of ocean water; critical for global heat distribution and nutrient cycling. By permanence: Permanent currents — flow in the same direction year-round (e.g., Gulf Stream, Labrador); driven by persistent winds and the thermohaline circulation. Seasonal currents — reverse direction seasonally; the Indian Ocean currents are the prime example, reversing with the monsoon. Western boundary currents (Gulf Stream, Kuroshio, Agulhas) are narrow, deep, fast (up to 2 m/s), and warm. Eastern boundary currents (California, Canary, Benguela, Peru) are broad, shallow, slow (0.1-0.3 m/s), and cold — associated with coastal upwelling.
Major Ocean Current Systems — Atlantic Ocean
The Atlantic Ocean has two major gyre systems: North Atlantic Gyre (clockwise in NH): North Equatorial Current (warm, westward along ~15 degrees N, driven by northeast trades) flows into the Caribbean and the Gulf of Mexico; Gulf Stream (warm, northward along the US east coast — one of the strongest, deepest, and warmest currents on Earth; transports 30 million cubic metres/sec; width 80-160 km; depth ~600 m; speeds up to 2 m/s; discovered by Ponce de Leon in 1513, charted by Benjamin Franklin in 1770s); North Atlantic Drift (warm, extension of Gulf Stream toward Northern Europe — keeps Western Europe 5-10 degrees C warmer than equivalent latitudes in North America; the Norwegian coast is ice-free above the Arctic Circle because of this current); Canary Current (cold, southward along northwest Africa, completing the gyre; causes coastal fog and dry conditions along the Saharan coast; supports rich fisheries off Mauritania and Senegal). South Atlantic Gyre (counterclockwise in SH): South Equatorial Current (warm, westward along the equator); Brazil Current (warm, southward along South America's east coast — weaker than the Gulf Stream); West Wind Drift/Antarctic Circumpolar Current (cold, eastward around Antarctica — the largest ocean current by volume, connecting all three major oceans); Benguela Current (cold, northward along southwest Africa — creates the Namib Desert and its famous fog; supports one of the world's most productive fishing grounds off Namibia and South Africa). The Labrador Current (cold, southward along eastern Canada carrying icebergs from Greenland) meets the Gulf Stream near the Grand Banks of Newfoundland — this convergence creates dense fog and one of the world's richest fishing grounds; the sinking of the Titanic in 1912 was linked to icebergs carried by the Labrador Current. The Sargasso Sea is a region of calm water in the center of the North Atlantic Gyre, known for floating Sargassum seaweed.
Major Ocean Current Systems — Pacific and Southern Oceans
Pacific Ocean: North Pacific Gyre (clockwise): North Equatorial Current (warm, westward); Kuroshio Current (warm, northward along Japan — the Pacific equivalent of the Gulf Stream; Japanese for "black stream" due to its deep blue color; warms Japan and supports rich fisheries); North Pacific Drift (warm, eastward toward North America); California Current (cold, southward along the US west coast — creates cool, foggy conditions in San Francisco; supports anchovy and sardine fisheries). South Pacific Gyre (counterclockwise): South Equatorial Current (warm, westward); East Australian Current (warm, southward along Australia — featured in the movie "Finding Nemo"); West Wind Drift (cold, eastward); Peru/Humboldt Current (cold, northward along South America — one of the most significant currents globally; brings cold, nutrient-rich water to the surface through coastal upwelling; creates the Atacama Desert — the driest non-polar desert on Earth; supports one of the world's most productive fisheries — Peru is the world's largest anchovy producer; the disruption of this current during El Nino events collapses Peruvian fisheries and affects global weather). The Oyashio Current (cold, southward along the Kuril Islands and northeast Japan) meets the Kuroshio at about 40 degrees N, creating one of the richest fishing grounds in the Pacific. Southern Ocean: The Antarctic Circumpolar Current (ACC/West Wind Drift) is the largest ocean current — transporting about 130 million cubic metres/sec; it encircles Antarctica unobstructed by any landmass and connects the Atlantic, Indian, and Pacific Oceans; it isolates Antarctica thermally, maintaining the continental ice sheet. The East Wind Drift flows westward close to the Antarctic coast.
Indian Ocean Currents — Monsoon Reversal System
The Indian Ocean is unique among the world's oceans because its surface current system reverses direction seasonally in response to the monsoon wind reversal. This is because the Indian Ocean is largely landlocked to the north by the Asian continent, preventing the development of a permanent subtropical gyre in the northern part. During the Southwest Monsoon (Summer, June-September): The strong southwest monsoon winds drive the North Indian Ocean currents in a clockwise direction. The Southwest Monsoon Current flows eastward from the Arabian Sea to the Bay of Bengal, south of India and Sri Lanka. The Somali Current — a strong western boundary current — flows northward along the east coast of Africa at speeds reaching 3.7 m/s (one of the fastest currents globally), bringing cold upwelled water from depths of 200+ metres to the surface; this coastal upwelling makes the Somali coast and the Arabian Sea extremely productive for fisheries; the Somali Current is the only western boundary current that reverses seasonally. The West Indian Coastal Current (WICC) flows northward along India's western coast, influenced by the southwest monsoon. During the Northeast Monsoon (Winter, November-February): The winds reverse, and the North Indian Ocean currents flow counterclockwise. The Northeast Monsoon Current flows westward across the northern Indian Ocean from the Bay of Bengal to the Arabian Sea. The Somali Current reverses to flow southward along the East African coast. The East Indian Coastal Current (EICC) flows southward along India's eastern coast. The WICC also reverses to flow southward. The Equatorial Counter Current (flowing eastward between the two equatorial currents) is stronger during the transition periods. The seasonally reversing currents significantly affect marine productivity, fisheries (sardines and mackerel along the Kerala coast depend on summer upwelling), navigation, and coastal erosion/deposition along India's 7,516.6 km coastline.
Thermohaline Circulation — The Global Conveyor Belt
The thermohaline circulation (THC) is a global-scale deep ocean circulation driven by differences in water density determined by temperature and salinity. It operates like a giant conveyor belt, transporting heat from the tropics to the poles and returning cold water toward the equator through the deep ocean. The process: In the North Atlantic near Greenland, Iceland, and the Norwegian Sea, warm salty water carried by the Gulf Stream and North Atlantic Drift cools rapidly in contact with cold polar air. As it cools, its density increases (cold water is denser). Additionally, sea ice formation removes freshwater, increasing salinity and density further. This extremely dense water sinks to the ocean floor — forming North Atlantic Deep Water (NADW) at a rate of about 15-20 million cubic metres/sec. The sinking NADW flows southward through the deep Atlantic, rounds the southern tip of Africa, enters the Indian Ocean as deep bottom water, continues to the Pacific Ocean (where it is the oldest and richest in nutrients), gradually warms and rises through upwelling, and returns to the Atlantic as warm surface water — completing the global conveyor belt. The entire cycle takes approximately 1,000-2,000 years. Antarctic Bottom Water (AABW), formed around Antarctica, is the coldest and densest water mass (about -1.8 degrees C, 34.62 ppt salinity) and creeps along the ocean floor in all major basins. The THC transports about 1 petawatt (10 to the 15 watts) of heat poleward — equivalent to the output of about 1 million nuclear power plants. Climate change threatens the AMOC (Atlantic Meridional Overturning Circulation — the Atlantic component of the THC): increased freshwater from melting Greenland ice and Arctic precipitation could reduce North Atlantic surface salinity, weakening NADW formation. Studies suggest the AMOC has weakened by about 15% since the mid-20th century. A theoretical AMOC shutdown could cool Europe by 5-10 degrees C, disrupt monsoon systems globally, and shift tropical rain belts — though a complete shutdown is considered low probability this century.
Upwelling, Downwelling, and Marine Productivity
Upwelling is the vertical movement of cold, nutrient-rich water from the deep ocean (200-1,000 m) to the surface, replacing surface water moved away by wind. It is one of the most important oceanographic processes for marine productivity and global fisheries. Types: (1) Coastal upwelling — occurs when winds blow parallel to the coast; Ekman Transport moves surface water perpendicular to the wind and away from shore; cold deep water rises to fill the gap; major coastal upwelling zones: Peru (Humboldt Current), California, Canary (northwest Africa), Benguela (southwest Africa), and Somali coast — these cover less than 1% of ocean area but produce about 50% of the world's fish catch. (2) Equatorial upwelling — southeast trade winds cause Ekman Transport of surface water away from the equator in both hemispheres, drawing up deep water along the equatorial belt; this creates a cold tongue of surface water along the eastern equatorial Pacific that is key to the ENSO cycle. (3) Open-ocean upwelling — caused by wind-driven divergence of surface currents in the open ocean; typically occurs where cyclonic (counterclockwise in NH) wind patterns cause surface divergence. (4) Topographic upwelling — seamounts and ridges force deep currents upward. Downwelling: the opposite — surface water convergence forces water downward; occurs where currents converge or winds push water toward a coast; downwelling zones transport oxygen and dissolved organic matter to the deep ocean; they are generally low in surface productivity but important for deep ocean ecosystems. In the Indian context: upwelling along the Kerala coast during the southwest monsoon (June-September) is critical for India's sardine and mackerel fisheries — the southwest monsoon winds blowing parallel to the coast cause Ekman Transport offshore, drawing up cold nutrient-rich water; the Somali upwelling zone in summer is one of the most intense in the world; upwelling also occurs along the eastern coast of India near the Visakhapatnam-Kakinada region. India is the world's third-largest fish producer (~14.2 million tonnes, 2021-22) and the second-largest aquaculture producer.
El Nino, La Nina, and ENSO — Mechanism and Indian Impact
El Nino-Southern Oscillation (ENSO) is a coupled ocean-atmosphere phenomenon in the tropical Pacific that has profound effects on global weather, including the Indian monsoon. Normal conditions (La Nina phase or neutral): Trade winds blow westward across the Pacific, piling up warm surface water (28-29 degrees C) in the western Pacific near Indonesia and Australia (the "warm pool"); sea level in the western Pacific is about 50 cm higher than in the eastern Pacific; this causes upwelling of cold, nutrient-rich water along the South American coast (Peru); the western Pacific has low pressure, heavy rainfall, and convection; the eastern Pacific is cooler with higher pressure and dry conditions; the atmospheric component (the Walker Circulation) has rising air over the western Pacific and sinking air over the eastern Pacific. El Nino ("The Boy Child" — named because it often appears around Christmas off Peru): Trade winds weaken or reverse; warm water flows eastward, suppressing upwelling off Peru; the Pacific warm pool shifts eastward; rainfall follows the warm water — Australia and Indonesia become drier; Peru and Ecuador experience flooding; the Walker Circulation weakens or reverses. Impact on India: El Nino events are statistically associated with below-normal monsoon rainfall — about 60-65% of El Nino years have had deficient monsoons (not deterministic — 2023 was an El Nino year with near-normal monsoon aided by a positive IOD); mechanisms: El Nino shifts the Walker Circulation eastward, reducing the convection over the Indian Ocean and weakening the monsoon's driving mechanism. La Nina: Trade winds strengthen; eastern Pacific cools further; western Pacific warms more; Australia and India generally receive above-normal rainfall. India experienced severe droughts in El Nino years: 2002, 2009, 2015 (partial); good monsoons in La Nina years: 2010, 2011, 2020. Southern Oscillation: pressure seesaw between Tahiti and Darwin measured by SOI. ENSO events recur every 2-7 years with no strict periodicity.
Indian Ocean Dipole (IOD) — India's Own Climate Mode
The Indian Ocean Dipole (IOD) is an oscillation of sea surface temperatures (SST) between the western equatorial Indian Ocean (Arabian Sea off East Africa, 50-70 degrees E, 10S-10N) and the eastern equatorial Indian Ocean (off Sumatra, 90-110 degrees E, 10S-0). First identified by Saji et al. (1999), it is now recognized as an independent climate mode distinct from ENSO, though they interact. Positive IOD: The western Indian Ocean warms anomalously while the eastern Indian Ocean cools; the SST gradient drives enhanced moisture transport toward the western Indian Ocean and the Indian subcontinent; winds over the Indian Ocean change, enhancing convection over India; result: above-normal monsoon rainfall over India. Critically, a positive IOD can partially or fully counteract the negative effects of El Nino on the Indian monsoon — in 2019, a strong positive IOD produced an above-normal monsoon despite a weak El Nino; in 2023, a strong positive IOD helped offset the developing El Nino. Negative IOD: The western Indian Ocean cools while the eastern warms; moisture supply to India is reduced; when combined with El Nino, can cause severe drought — 2006 had a negative IOD and drought in parts of India. IOD dynamics: typically develops in May-June, peaks in October-November, and decays rapidly by December; thus it most strongly affects the latter half (September-October) of the monsoon season. The IOD index = SST anomaly in the western pole minus SST anomaly in the eastern pole; values above +0.4 degrees C = positive IOD; below -0.4 degrees C = negative IOD. Indian meteorological agencies (IMD, IITM Pune) now use both ENSO and IOD indices in their Dynamic and Statistical Ensemble Prediction System (DSEPS) for seasonal monsoon forecasting. Climate change projections suggest that extreme positive IOD events may become more frequent under global warming, potentially increasing the variability of Indian monsoon rainfall. The 2019 positive IOD also triggered severe flooding in East Africa (Kenya, Somalia) while causing catastrophic bushfires in Australia (by suppressing rainfall there).
Climate Effects of Ocean Currents — Global and Indian
Ocean currents profoundly influence the climate of coastal regions and global climate: (1) Warm Currents Warm Adjacent Coasts — the Gulf Stream and North Atlantic Drift keep Western Europe warm; London (51 degrees N) has a mild winter while Labrador (same latitude) has a subarctic climate; the Norwegian port of Tromso (69 degrees N) is ice-free while Hudson Bay (55 degrees N) freezes; the Kuroshio warms Japan. (2) Cold Currents Cool Adjacent Coasts and Create Deserts — the Benguela Current creates the Namib Desert (Namibia); the Peru/Humboldt Current creates the Atacama Desert (Chile/Peru — driest non-polar desert; some weather stations have never recorded rainfall); the California Current keeps San Francisco (average summer temperature 18 degrees C) much cooler than New York at a similar latitude; the West Australian Current contributes to the aridity of western Australia. Mechanism: cold currents create a stable temperature inversion over the coast — warm air above cold surface water prevents convection and cloud formation, suppressing rainfall. (3) Fog Formation — cold currents cause advection fog by cooling moist onshore air below its dew point; San Francisco fog (California Current), Grand Banks fog (Labrador meets Gulf Stream), Namibian coast fog (Benguela). (4) Fisheries — convergence of warm and cold currents creates rich mixing zones: Grand Banks (Labrador + Gulf Stream), seas around Japan (Oyashio + Kuroshio), Peru coast (Humboldt upwelling); cold current upwelling zones are the most productive. (5) Storm Intensity — warm ocean waters above 26.5 degrees C fuel tropical cyclones; the warm Brazil Current fuels South Atlantic hurricanes; the warm Bay of Bengal waters (28-30 degrees C) fuel intense cyclones affecting eastern India; the Gulf Stream corridor influences Atlantic hurricane tracks. In the Indian context: the warm Agulhas Current and the East African Coastal Current contribute to the moisture loading of air masses that eventually reach India during the monsoon; the cold Somali upwelling in summer affects the climate of the Horn of Africa; the seasonal reversal of Indian Ocean currents redistributes heat and affects monsoon timing.
Ocean Currents and Navigation — Historical and Modern
Ocean currents have profoundly shaped maritime history, trade routes, and modern shipping: Historical: Arab and Indian traders used the seasonal reversal of Indian Ocean currents and monsoon winds for millennia — sailing to East Africa and Southeast Asia with the monsoon and returning when the winds reversed; this knowledge predated European maritime exploration by centuries. The Gulf Stream was used by Spanish treasure ships returning from the Americas to Europe. The Roaring Forties (40 degrees S) and Furious Fifties (50 degrees S) — powerful westerly winds and the Antarctic Circumpolar Current — were used by clipper ships on the route from Europe to Australia (the "Great Circle Route"). The Sargasso Sea in the center of the North Atlantic Gyre was feared by sailors — becalmed ships trapped by floating Sargassum weed. Modern: Shipping routes still account for currents — sailing with a current can save 10-15% fuel; the Gulf Stream gives a 2-3 knot boost to eastbound transatlantic ships. The International Maritime Organization (IMO) publishes current charts. Undersea internet cables are routed considering current patterns. India's maritime trade depends on the Indian Ocean current system: about 95% of India's trade by volume and 68% by value moves by sea; the Strait of Malacca and the Strait of Hormuz are critical chokepoints; India's SAGAR (Security and Growth for All in the Region) doctrine acknowledges the geopolitical significance of Indian Ocean currents and trade routes. Pollution and ocean currents: currents transport marine plastic debris — the Great Pacific Garbage Patch (between California and Hawaii, in the North Pacific Gyre) covers about 1.6 million sq km; similar garbage patches exist in all five subtropical gyres; microplastics have been found in the deepest ocean trenches (Mariana Trench — 10,000+ m).
Ocean Currents and Climate Change — Future Concerns
Climate change is altering ocean current systems with potentially far-reaching consequences: (1) AMOC Weakening — the Atlantic Meridional Overturning Circulation (which includes the Gulf Stream system) has weakened by about 15% since the mid-20th century according to multiple studies; increased freshwater input from melting Greenland ice (~280 billion tonnes/year) is reducing North Atlantic surface salinity and potentially slowing NADW formation; a 2021 Nature Geoscience study warned the AMOC may be approaching a tipping point; consequences of significant AMOC weakening: rapid cooling of Western Europe (paradoxically, as the rest of the planet warms), disruption of the West African and Indian monsoons, changes in Amazon rainfall, accelerated sea level rise along the US East Coast, and collapse of Atlantic fisheries. (2) Warming Oceans — global sea surface temperatures have risen about 0.88 degrees C since the pre-industrial era; warmer waters expand (thermal expansion), contributing about 50% of observed sea level rise; warmer waters hold less dissolved oxygen, creating expanding "dead zones"; warmer waters intensify tropical cyclones. (3) Changes in Upwelling — some models predict intensified coastal upwelling due to increased land-ocean temperature contrast (more wind); others predict weakened upwelling as overall circulation slows; either way, changes in upwelling will dramatically affect fisheries that feed hundreds of millions of people. (4) ENSO Changes — some models predict more frequent and intense El Nino events under climate change, which would increase monsoon variability for India; the "El Nino Modoki" (Central Pacific El Nino) appears to be becoming more common, with different impacts on India than the traditional Eastern Pacific El Nino. (5) Arctic Current Changes — melting Arctic sea ice is opening new shipping routes (Northern Sea Route, Northwest Passage) and altering the formation of cold deep water, with cascading effects on global circulation. India's vulnerability: as a monsoon-dependent economy with 140+ million people in coastal zones and a fisheries sector employing 28+ million people, changes in Indian Ocean currents and ENSO/IOD patterns directly threaten food security, water resources, and livelihoods.
Measuring and Monitoring Ocean Currents
Modern oceanography employs sophisticated technologies to measure and monitor ocean currents: (1) Satellite Altimetry — satellites (Jason-3, Sentinel-6) measure sea surface height with millimetre precision; differences in sea level indicate current flow (water flows from higher to lower sea level; the Gulf Stream surface is about 1 m higher than surrounding waters); NASA's GRACE satellites measure ocean bottom pressure changes. (2) Argo Floats — a global network of about 4,000 autonomous profiling floats deployed across the world's oceans; each float sinks to 1,000-2,000 m, drifts with deep currents for 10 days, then rises to the surface measuring temperature and salinity, transmits data via satellite, and repeats; India contributes through INCOIS (Indian National Centre for Ocean Information Services, Hyderabad) and has deployed 500+ Argo floats in the Indian Ocean. (3) Drifting Buoys — surface drifters tracked by satellite to map surface current patterns; the Global Drifter Program maintains ~1,300 buoys. (4) Moored Buoy Arrays — permanent instrumented buoys measuring temperature, salinity, and current at fixed locations; the Indian Ocean Mooring Network includes the RAMA (Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction) array. (5) Acoustic Doppler Current Profilers (ADCP) — use sound waves to measure water velocity at different depths; deployed on ships and moorings. (6) Ocean Models — numerical computer models simulate ocean circulation; India's INCOIS runs the Indian Ocean Forecasting System (INDOFOS) providing ocean state forecasts. Key Indian institutions: INCOIS (Hyderabad) — provides ocean state forecasts, tsunami warnings, and Potential Fishing Zone (PFZ) advisories to fishermen via SMS; National Institute of Oceanography (NIO, Goa) — premier research institute for ocean science; Indian Institute of Tropical Meteorology (IITM, Pune) — leads ENSO/IOD research and monsoon prediction; National Centre for Polar and Ocean Research (NCPOR, Goa) — manages India's Antarctic and Arctic research programmes.
Relevant Exams
Ocean currents is a high-priority topic for UPSC, with questions on warm vs cold currents, ENSO and its impact on the Indian monsoon, the IOD, thermohaline circulation, and upwelling. SSC and banking exams test factual recall — names of major currents (Gulf Stream, Labrador, Kuroshio, Peru), warm vs cold classification, and El Nino/La Nina effects. The discovery of the Southern Oscillation by Gilbert Walker (an India connection) is a popular UPSC fact.