Climatology & Oceanography: Atmosphere, Winds, Cyclones and the Oceans

The amount of insolation a place receives depends on the angle of the Sun's rays, the length of the day, the transparency of the atmosphere (cloud, dust, water vapour) and the aspect of the land. Distance matters least: the Earth is closest to the Sun at perihelion (about 3 January) and farthest at aphelion (about 4 July), yet the northern winter falls in January — tilt, not distance, makes seasons. One fact surprises most readers: the maximum insolation at the surface is received over the subtropical hot deserts, not at the equator, because the equatorial belt is cloudy almost every afternoon while desert skies are clear.

The Earth's temperature stays steady over the long run because it sends back to space exactly as much energy as it receives — the heat budget. Take the incoming solar radiation as 100 units. In the classic textbook budget, about 35 units are reflected or scattered straight back to space, mostly by clouds, with a little from snow and ice and the air itself; this reflected share is the Earth's albedo (satellite measurements put it at about 30 per cent). Of the remaining 65 units, 14 are absorbed by the atmosphere and 51 by the surface. The surface sends its 51 units upward as long-wave terrestrial radiation: 17 escape directly to space and 34 are absorbed by the atmosphere — partly as radiation, partly by convection and turbulence, and the largest part as latent heat released when water vapour condenses. The atmosphere then radiates its 48 units (14 + 34) to space. 17 + 48 = 65, which balances the 65 absorbed.

• Latitudinal imbalance — between about 40°N and 40°S the Earth receives more radiation than it loses (a surplus); poleward of that it loses more (a deficit). The tropics do not grow ever hotter nor the poles ever colder only because winds and ocean currents carry the surplus poleward. This single fact is the reason for everything in the rest of this chapter.
• How the air is heated — the atmosphere is heated mainly from below, by the ground: by conduction in the lowest layer, by convection (vertical rising of heated air, confined to the troposphere) and by advection (horizontal transfer, which in the middle latitudes brings most day-to-day changes; the loo of north India is advected heat). Short-wave sunlight passes through the air easily; long-wave terrestrial radiation is what the air actually absorbs.
• Isotherms and the thermal equator — the belt of highest temperature lies a little north of the geographical equator, because the Northern Hemisphere has more land. Isotherms bend sharply where land meets sea, and the largest annual range of temperature — over 60 °C — is found in north-eastern Eurasia, far from any moderating ocean.

Normally temperature falls with height. In a temperature inversion it rises instead, so a layer of warm air sits over colder air near the ground. Inversions form on long winter nights with clear skies and calm air, when the ground loses heat fast by radiation and chills the air just above it; snow cover strengthens them. Because cold air is dense, an inversion makes the lower air stable: it suppresses rising currents, so fog, frost and smog are trapped near the ground — the winter smog of north Indian cities is an inversion problem as much as an emissions problem. In hills, cold air slides downslope at night and pools in valley floors (air drainage), so valley bottoms can freeze while the slopes above stay frost-free. That is why orchards and plantations in hill country are planted on the slopes, not in the frost pockets below.

Mean sea-level pressure is about 1013 millibars, and in the lower atmosphere it falls by roughly 1 mb for every 10 m of ascent. Three forces decide how air actually moves between high and low pressure.

Because the Coriolis force vanishes at the equator, a spinning storm cannot organise there: tropical cyclones do not form within about 5° of the equator. Higher up, the general circulation is described by a three-cell model in each hemisphere. In the Hadley cell (0–30°), air rises over the hot equator, flows poleward aloft, sinks around 30° and returns along the surface as the trade winds — a thermally direct cell. In the Ferrel cell (30–60°), surface air moves poleward as the westerlies; it is driven by the cells on either side rather than by heating, so it is called thermally indirect. The polar cell sinks over the poles and flows out as the polar easterlies. Across the tropical Pacific runs a separate east–west loop, the Walker circulation, which is the key to El Niño.

The whole system of belts shifts north and south with the apparent movement of the Sun — about 5–10° with the seasons — and over the continents the shift is much larger. That migration is what draws the ITCZ over the Ganga plain in July and drives the monsoon described in geography-ch02. Local winds add detail to the map; beyond those in geography-ch04, the following are frequently asked.

Jet streams are narrow, meandering bands of very fast wind in the upper troposphere, near the tropopause, commonly blowing at well over 100 km/h and faster in winter, when the temperature contrast between the tropics and the poles is greatest. They form where that contrast is sharpest. Most blow from west to east, which is why an eastbound flight across a jet arrives early and a westbound one late. Their great loops, called Rossby waves, steer surface cyclones and anticyclones; when a loop stalls, a heatwave or a cold spell can sit over one region for days.

An air mass is a huge body of air whose temperature and humidity are nearly uniform horizontally, acquired by resting over a uniform source region — a warm ocean, a cold snowfield, a hot desert. Air masses are named by moisture (c continental, dry; m maritime, moist) and by temperature (T tropical, P polar, A arctic). The five standard types are maritime tropical (mT), continental tropical (cT), maritime polar (mP), continental polar (cP) and continental arctic (cA). Tropical air masses over the oceans are the moisture source of most heavy rain; cP air from Siberia and northern Canada brings winter cold waves.

When two contrasting air masses meet, they do not mix easily; the boundary between them is a front, and the process of its formation is frontogenesis. Fronts are features of the middle latitudes, where cold polar and warm tropical air meet; in the tropics the air masses are too alike in temperature to form them.

A tropical cyclone is a violent storm that forms over warm tropical seas and draws its energy from the latent heat released when moist air condenses. It needs: a large sea surface warmer than about 27 °C; enough Coriolis force to set it spinning (hence none near the equator); little change in wind speed with height (low vertical wind shear), so the rising column is not torn apart; a pre-existing weak low or cyclonic circulation; and divergence in the upper air to remove the rising air. Cut off its supply of moisture — by landfall or by drifting over cooler water — and it weakens quickly.

At the centre is the eye: a calm, often clear zone of sinking air. Around it the eye wall is a ring of towering cloud where air spirals upward most violently — the strongest winds and heaviest rain are here, not in the eye. Spiral rain bands extend outward. On a coast the deadliest effect is usually the storm surge, a dome of sea water pushed ashore by the wind and low pressure. In the North Indian Ocean, the Bay of Bengal produces several times as many cyclones as the Arabian Sea, with peaks in the post-monsoon (October–November) and pre-monsoon (April–May) months. Cyclones are rare in the South Atlantic and the south-eastern Pacific, where cold currents and strong wind shear prevail.

A storm is named once it reaches cyclonic-storm strength. For the North Indian Ocean the names are assigned by the Regional Specialised Meteorological Centre at New Delhi, run by the India Meteorological Department, from a list contributed by the 13 member countries of the WMO/ESCAP Panel on Tropical Cyclones and used in sequence.

Tornadoes are a different scale altogether: small, intensely rotating funnels of extremely low pressure dropping from thunderclouds, most common over the plains of the central United States in spring. Over water the same thing is a waterspout.

Start with a normal year. The south-east trade winds push warm surface water westward across the equatorial Pacific, so a deep warm pool builds up near Indonesia and northern Australia, with rising air and heavy rain there. Off Peru and Ecuador, the water pushed away is replaced by cold, nutrient-rich water upwelling from below — feeding one of the world's richest fisheries. Air rises in the west, flows east aloft and sinks over the cool eastern Pacific: the Walker circulation.

In an El Niño the trades weaken (sometimes reverse), the warm pool sloshes east, upwelling off Peru is suppressed and the coastal fishery collapses. Rain shifts with the warm water: floods strike the normally dry coasts of Peru and Ecuador, while Indonesia and Australia face drought, and the Indian monsoon tends to be weaker. The name, "the Child", comes from Peruvian fishermen who noticed the warm current around Christmas. La Niña is the opposite: stronger trades, an unusually cold eastern Pacific, and a tendency towards a good Indian monsoon. Events recur irregularly, every two to seven years. Agencies such as NOAA declare an El Niño when the sea-surface temperature of the central equatorial Pacific (the Niño 3.4 region) stays at least 0.5 °C above normal for a sustained period.

The ocean change has an atmospheric twin, the Southern Oscillation — a see-saw of surface pressure between the eastern and western tropical Pacific, identified by Sir Gilbert Walker in the 1920s while he was searching for ways to forecast the Indian monsoon. It is measured by the Southern Oscillation Index (SOI), the pressure difference Tahiti minus Darwin. A sustained negative SOI (pressure at Tahiti lower than usual relative to Darwin) accompanies El Niño; a positive SOI accompanies La Niña. Ocean and atmosphere together are called ENSO (El Niño–Southern Oscillation).

The Indian Ocean has its own see-saw, the Indian Ocean Dipole (IOD), described in 1999. It compares the sea-surface temperature of the western Indian Ocean (the Arabian Sea side, off East Africa) with the eastern Indian Ocean south of Indonesia. In a positive IOD the west is warmer than normal and the east cooler: convection shifts west, favouring the Indian monsoon and East African rains while drying Indonesia and Australia. A negative IOD does the reverse. Because the two systems are independent, a positive IOD can offset an El Niño — in 1997 a very strong El Niño was accompanied by a near-normal Indian monsoon. A third influence, the Madden–Julian Oscillation, is an eastward-moving pulse of cloud and rain that circles the tropics every 30–60 days and helps decide the monsoon's active and break spells.

Wladimir Köppen, a climatologist trained as a botanist, built his scheme (first published in 1900 and revised by him until 1936) on the idea that vegetation is the best summary of climate. He drew boundaries using mean monthly temperature and precipitation, labelled with letters. Capital letters give the major group; small letters add the seasonality of rain and the degree of heat or cold.

The small letters: f — no dry season; m — monsoon, heavy rain with a short dry season; w — dry winter; s — dry summer; g — the "Ganges type", where the hottest month comes before the summer solstice and the monsoon rains. (Some modern versions draw the C/D line at 0 °C instead of −3 °C.) The Mediterranean (Cs) type deserves a separate note because it inverts the usual pattern: it lies on the western sides of continents between about 30° and 40° — the Mediterranean basin, California, central Chile, the south-western Cape and south-western Australia — and gets its rain in winter from the westerlies, staying dry in summer under the subtropical high.

The ocean floor has as much relief as the land. Its major divisions run from the coast outward. The continental shelf is the gently sloping, submerged edge of the continent, ending at the shelf break, commonly around 100–200 m deep; its average width is about 80 km, but it is almost absent off the Chilean coast and the west coast of Sumatra and reaches about 1,500 km in the Siberian shelf of the Arctic, the widest in the world. The continental slope beyond drops more steeply (about 2–5°) to depths of roughly 3,000 m and is cut by submarine canyons. The deep-sea (abyssal) plains, between about 3,000 and 6,000 m, are the flattest surfaces on Earth, blanketed by fine clay and silt. The oceanic deeps or trenches are the deepest parts, long narrow troughs at subduction zones, most of them in the Pacific.

The average surface temperature of the oceans is about 27 °C near the equator, falling poleward by roughly 0.5 °C per degree of latitude. The highest surface temperatures lie slightly north of the equator, and the Northern Hemisphere oceans are on the whole warmer, because they are in contact with more land. Enclosed seas in low latitudes such as the Red Sea and the Persian Gulf run warmer than the open ocean. Offshore winds drive surface water away and draw cold water up (upwelling), while onshore winds pile warm water against the coast; currents carry warm or cold water far from its source.

• Surface layer — warm water about 500 m thick, roughly 20–25 °C in the tropics; present all year in the tropics but only in summer in the middle latitudes.
• Thermocline — a layer roughly 500–1,000 m thick in which temperature falls rapidly with depth; it acts as a lid between warm surface water and the cold deep ocean.
• Deep layer — very cold water extending to the ocean floor, close to 0 °C in places. At high latitudes the surface is already cold, so there is little or no thermocline.

Salinity is the total dissolved salt in sea water, expressed in grams per kilogram (‰); the open ocean ranges about 33–37‰ around an average of 35‰, and common salt makes up more than three-quarters of the dissolved salts. It is raised by evaporation and the freezing of sea ice, and lowered by rainfall, river inflow and melting ice. So the open-ocean maximum lies not at the equator, which is rainy, but in the subtropical belts on either side, where skies are clear and evaporation is high; salinity falls again toward the poles. Among regional seas, the Red Sea (about 41‰ in the north) is very saline — hot, dry and enclosed, with no large rivers — while the Baltic is very fresh. Within India's seas, the Arabian Sea is more saline than the Bay of Bengal, which receives the Ganga, Brahmaputra and other large rivers. Where salinity changes sharply with depth, the layer is called a halocline.

Temperature and salinity together set the density of sea water: colder and saltier water is denser and sinks. In the far North Atlantic, near Greenland and in the Norwegian Sea, surface water chilled in winter and made saltier by sea-ice formation sinks to great depth and spreads south along the floor. This slow, density-driven thermohaline circulation — the "global conveyor belt", whose Atlantic limb is the Atlantic Meridional Overturning Circulation (AMOC) — links every ocean and carries heat on a timescale of centuries.

Currents are set in motion mainly by the planetary winds, which drag the surface water, and are shaped by Coriolis deflection, by the outlines of the continents and by differences in temperature and salinity. The result in each ocean basin is a great loop, a gyre, centred on the subtropical high: clockwise in the Northern Hemisphere and anticlockwise in the Southern. Follow a gyre round and a rule emerges: currents on the western sides of oceans (off the east coasts of continents) carry warm water poleward, and are fast, narrow and deep; currents on the eastern sides of oceans (off the west coasts of continents) carry cold water toward the equator, and are slower and broader.

The northern Indian Ocean breaks the gyre rule: it is closed by Asia, so its currents reverse with the monsoon — flowing clockwise in the south-west monsoon of summer and anticlockwise in the north-east monsoon of winter. Elsewhere, currents explain climates. Cold currents on the west coasts of continents in the subtropics chill the air from below, make it stable and dry, and help create coastal deserts: the Atacama (Peru current), the Namib (Benguela), the coast of Baja California (California current), the western Sahara (Canary) and western Australia (West Australian). Where warm and cold currents meet, as the Kuroshio meets the Oyashio off Japan, fog and rich fishing grounds result. Within the North Atlantic gyre lies the Sargasso Sea, a sea with no land coast, named for its floating Sargassum seaweed and known as the spawning ground of Atlantic eels.

Tides are raised by the gravitational pull of the Moon and, less strongly, the Sun, together with the centrifugal effect of the Earth–Moon system turning about their common centre. The result is two bulges of water: one facing the Moon and one on the opposite side. As the Earth rotates beneath them, most coasts pass through two high and two low waters a day. Because the Moon moves forward in its orbit in the same direction as the Earth spins, a place needs about 24 hours 50 minutes to come back under the Moon — so successive high tides are about 12 hours 25 minutes apart, and tides arrive roughly 50 minutes later each day.

• By frequency — semi-diurnal tides (two highs and two lows of nearly equal height each day) are the most common; diurnal tides give one high and one low a day; mixed tides have two highs of unequal height, as along the west coast of North America and many Pacific islands.
• By the Sun–Moon–Earth geometry — spring and neap tides (see geography-ch04); in addition, tidal range is larger when the Moon is nearest the Earth (perigee) and smaller at apogee, and larger when the Earth is nearest the Sun (perihelion, early January).
• Coastline shape — funnel-shaped bays amplify tides. The Bay of Fundy in Canada has the greatest tidal range in the world; in India the Gulf of Khambhat and the Gulf of Kachchh have the largest, and with the Sundarbans they are the sites studied for tidal power. A rising tide forced up a narrowing estuary can form a wall of water, a tidal bore, as on the Hooghly.

The biology of reef-building corals — the polyp, its partner algae (zooxanthellae) and what bleaching does to them — is in geography-ch07. The geography follows from their needs: warm, clear, shallow, sunlit, salty water. Reefs therefore lie mostly between about 30°N and 30°S, and they are largely absent from the western coasts of tropical continents, where cold currents and upwelling keep the water too cool, and from the mouths of large rivers, where fresh water and silt choke them. They thrive off the eastern coasts of continents — the western sides of tropical oceans, washed by warm currents — which is why the world's largest reef system, the Great Barrier Reef, lies off north-eastern Australia.

How does a ring-shaped atoll arise in the open ocean, rising from great depths where corals cannot live? Charles Darwin's subsidence theory (1842) answers with a sequence: a fringing reef grows around a volcanic island; as the island slowly sinks, the coral keeps growing upward toward the light, so the reef becomes a barrier reef separated from the shrinking island by a lagoon; when the island disappears beneath the sea, only the ring remains — an atoll. Later drilling on Pacific atolls found thick coral resting on volcanic rock, supporting Darwin. Rival explanations include Daly's glacial-control theory, which links reef growth to the rise of sea level after the Ice Age, and Murray's standstill theory, which builds reefs on pre-existing submarine platforms.

Bleaching follows marine heatwaves — weeks of sea temperature above the normal summer maximum — and strong El Niño years have repeatedly triggered mass bleaching across ocean basins. NOAA's Coral Reef Watch tracks accumulated heat stress in Degree Heating Weeks: around 4 °C-weeks significant bleaching is likely, and around 8 °C-weeks severe bleaching with widespread mortality. In April 2024 NOAA and the International Coral Reef Initiative confirmed the fourth global coral bleaching event on record, after those of 1998, 2010 and 2014–17.