Global circulation through the seasons

How does uneven solar heating organise the atmosphere?

Use the idealised circulation to locate rising and sinking regions before adding real geography.

From equator to pole

The tropics receive more solar energy on average than the polar regions. The atmosphere and oceans redistribute some of that energy. A simple three-cell picture in each hemisphere describes the mean tropospheric overturning: Hadley circulation in low latitudes, Ferrel circulation in mid-latitudes and polar circulation at high latitudes. These are averages, not three uninterrupted conveyor belts visible in every day's chart.

Warm moist air rises in the tropical convergence zone, spreads polewards aloft and subsides in the subtropics, around 30° latitude in the idealised picture. Near the surface, part of that air returns towards the equator as trade winds. In the polar cell, cold air subsides near the pole, flows equatorwards near the surface and is associated with rising motion near the subpolar zone around 60°. The intervening Ferrel mean circulation is strongly shaped by travelling mid-latitude weather systems rather than being a simple thermally direct cell.

The corresponding idealised surface belts are equatorial low pressure, subtropical high pressure, subpolar low pressure and polar high pressure. The atmosphere is not zonally uniform: continents, oceans, mountains and changing weather break the belts into centres and troughs. Keep the model as a framework for organising those features rather than a substitute for the actual pressure chart.

Which way does the low-level return flow turn?

Apply the hemisphere's Coriolis direction to the flow between the mean pressure belts.

The named winds are from-directions

Near the surface, flow from subtropical highs towards the equatorial low is deflected into north-easterly trades in the Northern Hemisphere and south-easterly trades in the Southern Hemisphere. The two trade-wind streams meet in or near the intertropical convergence zone, whose position and convective activity vary with season and region. The trade flow can change character when it crosses the equator or passes over a different surface.

Poleward of the subtropical highs, the mean surface flow towards subpolar low pressure has a westerly component: south-westerly tendencies in the north and north-westerly in the south. Closer to the poles, mean flow from polar high pressure towards the subpolar zone has an easterly component. Individual depressions and anticyclones can produce any local wind direction within these broad belts.

The Southern Hemisphere ocean belt permits a more continuous mid-latitude westerly circulation than the continent-dominated Northern Hemisphere. The idealised bands are useful for climate and route context, but actual low-level wind and hazards still require the regional observation and forecast products. Neither a trade-wind label nor a subtropical high guarantees smooth, cloud-free conditions.

Why do pressure belts shift and break into centres?

Combine the seasonal heating migration with the contrasting response of land and sea.

January

In January the heating maximum and tropical convergence zone tend towards the Southern Hemisphere, with important regional differences. Northern winter continents cool strongly and favour thermal high pressure, notably over Siberia and parts of North America. The North Atlantic Icelandic low and North Pacific Aleutian low are prominent mean winter features. Subtropical ocean highs remain, but their mean positions and strengths differ from summer. The Southern Hemisphere is in summer, so its circulation is shifted relatively polewards. Shallow continental summer heat lows can develop over the Kalahari, the Gran Chaco east of the Andes, and northern or inland Australia. The South Atlantic (St Helena), South Indian (Mascarene) and South Pacific ocean highs remain separate features on either side of those land areas.

July

In July the tropical convergence zone shifts northwards, particularly far over heated continents. Strong heating over Asia supports a broad thermal low and the monsoon circulation. The North Atlantic Azores/Bermuda high and North Pacific subtropical high commonly extend farther polewards than in northern winter. The Southern Hemisphere is in winter: its westerly storm belt tends relatively equatorwards and is stronger than during its summer. High pressure replaces much of the summer low-pressure pattern over southern Africa and the Australian mainland, while the subtropical ocean highs remain. The map selector identifies these representative centres and their near-surface rotation in each hemisphere.

These are schematic monthly-mean tendencies, not a fixed calendar rule for every location. The ITCZ over an ocean often migrates less than over land, and some sectors retain it north of the geographical equator even in January. A seasonal map should show continental highs and lows superimposed on the latitude belts, rather than shifting every point on Earth by one identical angle.

Why are the high-level winds not simply the surface winds repeated?

Connect the upper temperature gradient with westerly jets and seasonal tropical easterlies.

A different balance aloft

Above the friction layer, large-scale winds tend to follow upper pressure-surface height contours. The horizontal temperature contrast between warm low latitudes and cold high latitudes is associated with increasing westerly flow with height across much of the mid-latitude troposphere. Stronger gradients concentrate that flow into jet streams near the tropopause. The polar-front jet is associated with the mid-latitude temperature boundary; the subtropical jet is linked to the poleward upper branch of the Hadley circulation as well as the thermal structure.

In the winter hemisphere, the stronger equator-to-pole temperature contrast generally gives stronger upper westerlies and a more equatorward jet pattern. In the summer hemisphere, the mean westerly belt is weaker and farther polewards. Therefore January favours the northern winter upper-westerly pattern, while July favours the southern winter pattern. These are mean tendencies; a specific jet can meander, split or depart strongly from its mean latitude.

Tropical upper easterlies can occur, especially the Northern Hemisphere summer tropical easterly jet associated with the Asian and African monsoon circulation. The stratospheric circulation is also distinct from the surface trades. Outside the tropics, winter favours stratospheric westerlies around the cold polar vortex, whereas summer easterlies develop higher in the stratosphere; near the tropopause the wind still depends strongly on height and the nearby tropospheric jets. In the tropical lower stratosphere, the quasi-biennial oscillation brings alternating easterly and westerly regimes, so even there a permanent easterly assumption is wrong. Read the product for the actual level: a surface trade-wind map cannot determine a jet-cruise wind.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.