Atmospheric Systems

Global Atmospheric Circulation

The Three-Cell Model

The global atmospheric circulation is driven by differential heating of Earth's surface. The equator receives more solar radiation per unit area than the poles (because solar radiation strikes the equator more directly, spreading over a smaller area). This energy imbalance drives a system of atmospheric convection that redistributes heat from the equator toward the poles.

The simplified three-cell model of atmospheric circulation identifies three circulation cells in each hemisphere:

The Hadley Cell (0$^\circ$--30$^\circ$). Intense solar heating at the equator causes air to warm, expand, and rise (convection). The rising air cools adiabatically, producing condensation, clouds, and heavy rainfall. At the tropopause (approximately 12--15 km altitude), the air diverges poleward. Coriolis deflection causes this poleward-moving air to be deflected eastward, creating the upper-level westerlies. By approximately 30$^\circ$ latitude, the air has cooled sufficiently to descend, creating zones of subsidence and high atmospheric pressure. The descending air warms adiabatically, suppressing cloud formation and producing the world's major desert belts (the Sahara, Arabian, Thar, Kalahari, and Australian deserts lie within these subtropical high-pressure belts). At the surface, air flows back toward the equator, completing the cell. Coriolis deflection turns this equatorward surface flow to the west, creating the north-east and south-east trade winds.

The Ferrel Cell (30$^\circ$--60$^\circ$). The Ferrel cell is a thermally indirect cell (driven not by local heating but by the motions of the Hadley and Polar cells that flank it). Surface winds at 30$^\circ$ flow poleward, deflected eastward by Coriolis to become the prevailing westerlies. At approximately 60$^\circ$ latitude, the westerlies encounter cold polar air flowing equatorward, creating the polar front zone of convergence, uplift, and cyclonic activity.

The Polar Cell (60$^\circ$--90$^\circ$). Cold, dense air descends at the poles and flows equatorward at the surface, deflected westward by Coriolis to become the polar easterlies. At approximately 60$^\circ$ latitude, this cold equatorward-flowing air meets the warm poleward-flowing air of the Ferrel cell, rising at the polar front.

Pressure and Wind Belts

The three-cell model produces a characteristic pattern of pressure belts and wind systems:

Latitude	Pressure System	Surface Winds	Climate Characteristics
0$^\circ$	Equatorial low (ITCZ)	Variable, light (doldrums)	Hot, wet, convective rainfall
0$^\circ$--30$^\circ$	Subtropical high (descending air at 30$^\circ$)	Trade winds (NE in NH, SE in SH)	Dry at 30$^\circ$; wet at equatorward margins
30$^\circ$--60$^\circ$	Variable (mid-latitude cyclones)	Prevailing westerlies	Variable; seasonal; cyclonic rainfall
60$^\circ$	Subpolar low (polar front)	Convergence zone	Cool, wet, stormy
60$^\circ$--90$^\circ$	Polar high	Polar easterlies	Cold, dry

Limitations of the Three-Cell Model

1. Seasonal variation. The three-cell model is based on annual averages. In reality, the circulation shifts seasonally: the ITCZ migrates northward in the northern summer (reaching approximately 15$^\circ$--20$^\circ$N over South Asia, producing the monsoon) and southward in the southern summer.
2. Continental effects. Land surfaces heat and cool more rapidly than oceans, creating continental pressure systems that modify the zonal pattern. In winter, large continents develop high-pressure anticyclones (the Siberian High, which drives cold, dry air southward over East Asia). In summer, continents develop low-pressure thermal lows (the Asian Low, which draws warm, moist air northward from the Indian Ocean).
3. Ocean-continent contrasts. The zonal pattern is significantly modified by the distribution of land and sea, particularly in the Northern Hemisphere where continents are large.
4. It is a simplification. The real atmosphere is turbulent, with complex wave patterns, jet streams, and mesoscale systems that the three-cell model does not capture.

The Intertropical Convergence Zone (ITCZ)

The ITCZ is a narrow band of low pressure near the equator where the north-east and south-east trade winds converge. The convergence of warm, moist air masses produces vigorous convection, towering cumulonimbus clouds, and heavy rainfall.

Seasonal migration. The ITCZ migrates northward during the northern summer and southward during the southern summer, following the apparent movement of the sun. The migration is more pronounced over land than over ocean, because continental heating amplifies the thermal low. Over South Asia, the ITCZ migrates to approximately 20$^\circ$--25$^\circ$N during July--August, drawing in the south-west monsoon and producing extremely heavy rainfall (Cherrapunji in Meghalaya, India, receives approximately 11 000 mm of rain annually, most of it during the monsoon season).

Rossby Waves and Jet Streams

Rossby Waves

Rossby waves are large-scale meanders in the mid-latitude westerly flow, caused by the latitudinal variation of the Coriolis parameter (the beta effect). Rossby waves typically have wavelengths of 4000--8000 km and propagate westward relative to the mean westerly flow.

Wave types. Rossby waves can be classified by their amplitude:

• Low-amplitude (zonal) flow: relatively straight west-to-east flow with weak meridional (north-south) excursion. Associated with relatively stable, moderate weather.
• High-amplitude (meridional) flow: large north-south meanders, with ridges (northward bulges of warm air) and troughs (southward excursions of cold air). High-amplitude patterns can produce extreme weather: ridges bring warm, dry conditions (heat waves); troughs bring cold, wet conditions (cold spells, storms).

Blocking patterns. Occasionally, Rossby waves become quasi-stationary, with large-amplitude ridges and troughs that persist for days or weeks. These "blocks" can cause prolonged extreme weather: the 2003 European heat wave was associated with a persistent blocking ridge over western Europe; the 2010 Russian heat wave was associated with a blocking anticyclone.

Jet Streams

Jet streams are narrow bands of fast-moving air (wind speeds typically exceeding 60 knots, approximately 110 km/h) in the upper troposphere. They form at the boundaries between air masses of different temperatures, where the temperature gradient creates a strong pressure gradient that accelerates the air.

The polar front jet stream flows at approximately 9--12 km altitude, along the boundary between the cold polar air and warm subtropical air, typically at 50$^\circ$--60$^\circ$ latitude. It is closely associated with Rossby waves and the development of mid-latitude cyclones.

The subtropical jet stream flows at approximately 12--15 km altitude, near the tropopause, at approximately 25$^\circ$--35$^\circ$ latitude. It is associated with the poleward edge of the Hadley cell.

Relationship to weather. The polar front jet stream steers mid-latitude weather systems (depressions and anticyclones) from west to east. Regions north of the jet stream tend to be under the influence of cold polar air masses; regions south of the jet stream tend to be under the influence of warmer subtropical air. The position and strength of the jet stream determine the track and intensity of storm systems.

Climate change and the jet stream. Arctic amplification (the Arctic is warming 2--4 times faster than the global average) is reducing the temperature gradient between the Arctic and mid-latitudes. Since the strength of the polar front jet stream is proportional to this temperature gradient, some research suggests that the jet stream is weakening and becoming more wavy, increasing the frequency and persistence of extreme weather events (heat waves, cold spells, prolonged rainfall). However, this hypothesis remains an area of active research, and confidence in the specific mechanisms is not yet high.

Ocean Currents and Climate

Surface Currents

Surface ocean currents are driven primarily by global wind patterns (the trade winds, westerlies, and polar easterlies), Coriolis deflection, and the configuration of continents. They form large circular systems called gyres, which rotate clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

Major surface currents and their climatic effects:

Current	Type	Region	Climatic Effect
Gulf Stream	Warm	North Atlantic (Florida to north-west Europe)	Raises winter temperatures in western Europe by 5--10$^\circ$C relative to comparable latitudes (e.g., the UK is at similar latitudes to Labrador, but is much warmer)
North Atlantic Drift	Warm	North Atlantic (continuation of Gulf Stream)	Keeps north-west European ports ice-free in winter (e.g., Murmansk in Russia at 69$^\circ$N is ice-free)
Kuroshio Current	Warm	North Pacific (Japan)	Moderates the climate of Japan; increases rainfall on windward coasts
Humboldt (Peru) Current	Cold	South Pacific (west coast of South America)	Produces the arid Atacama Desert; supports one of the world's most productive fisheries
Benguela Current	Cold	South Atlantic (west coast of southern Africa)	Contributes to the aridity of the Namib Desert; supports productive fisheries
Canary Current	Cold	North Atlantic (west coast of north-west Africa)	Contributes to the aridity of the Sahara's western margin

Thermohaline Circulation (THC)

Thermohaline circulation is the large-scale ocean circulation driven by differences in water density, which is determined by temperature (thermo-) and salinity (-haline). Cold, salty water is denser and sinks; warm, fresh water is less dense and rises.

The Atlantic Meridional Overturning Circulation (AMOC) is the component of the THC most relevant to climate. Warm, salty surface water flows northward from the tropics in the Gulf Stream and North Atlantic Drift. As it reaches high latitudes (particularly the Nordic Seas between Greenland, Iceland, and Norway), it cools and its salinity increases (due to sea ice formation, which rejects salt). The cold, dense water sinks to the deep ocean (North Atlantic Deep Water, NADW) and flows southward at depth, eventually reaching the Southern Ocean and beyond.

The AMOC transports approximately 1.3 petawatts (1.3 x 10$^{15}$ watts) of heat northward, equivalent to approximately 25 times the total energy consumption of human civilisation. It is a major contributor to the relatively mild climate of north-western Europe.

AMOC and climate change. Climate change is weakening the AMOC by reducing the density of North Atlantic surface water through warming (less dense) and freshwater input (from melting Greenland ice, increased precipitation, and river discharge). Observations from the RAPID array (a system of moored instruments across the Atlantic at 26$^\circ$N) indicate that the AMOC has weakened by approximately 15% since the mid-20th century. The IPCC projects further weakening under all emission scenarios, with a risk of collapse under high-emission scenarios, though the threshold for collapse is uncertain.

An AMOC collapse would have profound climatic consequences: cooling of north-western Europe by several degrees; shifting of tropical rainfall belts southward (affecting the Sahel, the Amazon, and Southeast Asian monsoon); reduced carbon uptake by the Southern Ocean; and accelerated sea level rise along the Atlantic coast of North America.

ENSO (El Nino and La Nina)

The Normal State

Under normal conditions, the trade winds blow westward across the tropical Pacific, pushing warm surface water toward the western Pacific (Indonesia, Philippines). This creates a warm pool in the west (sea surface temperatures exceeding 28$^\circ$C) and a cool tongue in the east (off the coast of Peru and Ecuador, where cold water upwells from depth). The temperature gradient drives atmospheric convection over the warm pool, producing heavy rainfall in the western Pacific and dry conditions in the eastern Pacific.

El Nino

During El Nino events (typically occurring every 2--7 years, lasting 9--12 months), the trade winds weaken or reverse, allowing warm water to flow eastward across the Pacific. The warm pool shifts eastward, suppressing upwelling off South America and raising sea surface temperatures in the central and eastern Pacific by 1--3$^\circ$C.

Global impacts of El Nino:

Region	Impact
South America (west coast)	Heavy rainfall and flooding in Peru and Ecuador; reduced upwelling devastates anchovy fisheries
South America (north-east Brazil)	Drought; crop failures in the Amazon
Southeast Asia and Australia	Drought; increased risk of bushfires (the 2019--2020 Australian bushfires coincided with a strong El Nino)
East Africa	Enhanced rainfall; increased risk of flooding and waterborne disease
Southern Africa	Drought; reduced crop yields
North America (southern USA)	Increased rainfall and storminess in southern states; warmer and drier conditions in the Pacific Northwest
Global temperature	El Nino years tend to be warmer globally (the warmest year on record, 2023, was amplified by El Nino conditions)

La Nina

La Nina is the opposite phase: the trade winds strengthen, intensifying the normal pattern. Warm water is pushed further west, upwelling in the eastern Pacific intensifies, and sea surface temperatures in the central and eastern Pacific fall below average.

Global impacts of La Nina: broadly opposite to El Nino: increased rainfall and flooding in Southeast Asia and Australia; drought in South America (particularly north-east Brazil); drought in the southern USA; and enhanced Atlantic hurricane activity (La Nina reduces wind shear over the tropical Atlantic, allowing more hurricanes to form).

The 2015--2016 El Nino

The 2015--2016 El Nino was one of the strongest on record (comparable to the events of 1982--83 and 1997--98). Global temperatures in 2016 were approximately 1.1$^\circ$C above pre-industrial levels, the warmest year on record at the time. Impacts included: severe drought in southern Africa (approximately 36 million people required food assistance); drought in Ethiopia (10 million people food-insecure); severe flooding in Paraguay, Argentina, and Uruguay; and coral bleaching across the Great Barrier Reef and tropical Pacific (the longest global coral bleaching event on record).

Common Pitfalls: Confusing Weather and Climate When Discussing ENSO

El Nino and La Nina are climate phenomena (they represent persistent shifts in the state of the coupled ocean-atmosphere system over months to years), not individual weather events. However, they modify the probability of specific weather events occurring. El Nino increases the probability of drought in Australia but does not guarantee it; La Nina increases the probability of flooding in Southeast Asia but does not guarantee it. When discussing ENSO impacts, use probabilistic language ("increases the likelihood of," "is associated with") rather than deterministic language ("causes"), and provide specific case studies with data.

For related topics, see ./carbon-cycle-and-sequestration and ./climate-adaptation-and-mitigation. The parent topic page is at ../climate-change.