NATARAJAThe global circulation, monsoons & currents
Learning outcomes & permitted supports
- Draw the three-cell circulation and name the surface wind belt at any latitude
- Explain why the trades come from the EAST — Coriolis acting on an equatorward flow — and why the horse latitudes are calm and dry
- Track the ITCZ with the season and say why it moves further over land than over sea
- Explain the monsoon reversal, including why the south-east trades become a south-westerly on crossing the equator
- Relate the wind belts to the ocean gyres, and say why western boundary currents are warm and eastern ones cold
Supports in this lab: No instruments. The three-cell model and the migrating ITCZ are the standard teaching picture; the monsoon months are the Indian Ocean's climatological ones. Golden 1031/1031.
Glossary: Hadley cell · trade winds · ITCZ · monsoon · ocean gyre · Coriolis force
One heat engine, three cells
The sun heats the equator hardest, so air rises there and must come down somewhere. On a non-rotating Earth it would come down at the poles and the whole planet would have one enormous convection cell each side — every surface wind blowing straight towards the equator. Rotation breaks that flow into three, and turns each of them sideways.
| Polar cell · 60–90° | polar easterlies — cold, dry, sinking air over the pole |
|---|---|
| Ferrel cell · 30–60° | the WESTERLIES — and the roaring forties in the south — the depression belt: fronts, gales, changeable |
| Hadley cell · 0–30° | the TRADE winds — NE in the north, SE in the south — steady, reliable, the sailing-ship highway |
Read the trades as the proof: air flowing from 30° towards the equator is turned to the RIGHT in the north, so it arrives from the north-EAST; turned left in the south, it arrives from the south-EAST. The names of the world's most reliable winds are a direct statement of the Coriolis force.
The ITCZ follows the sun
Where the two trade-wind systems meet is the Intertropical Convergence Zone — the doldrums: light baffling airs, heavy convective showers, and the birthplace of tropical disturbances. It migrates with the overhead sun, lagging about six weeks, and it moves much further over land than over sea because a continent heats faster than an ocean.
The monsoon — the same physics, with India added
Drag the ITCZ far enough north over a heated continent and the whole system reverses. In the southwest monsoon (June–September) the Asian landmass becomes a vast low; the south-east trades of the southern hemisphere cross the equator, are turned RIGHT by Coriolis as soon as they do, and arrive on the Malabar coast as a strong, wet south-westerly. In the northeast monsoon (December–March) the continent cools, becomes a high, and the flow reverses into a dry north-easterly.
| This month | SOUTH-WEST MONSOON — wet, strong, and the Arabian Sea at force 6–8 for weeks at a time |
|---|---|
| Why it crosses the equator at all | the southern trades are drawn north by the continental low; crossing the line their deflection flips from left-hand to right-hand, which is exactly what turns a south-easterly into a south-westerly |
| What it means for the passage | a fair wind and a foul sea up the Arabian Sea; a head sea and heavy swell going the other way — and reduced visibility in the rain |
The winds drive the sea
The same belts push the surface of the ocean, and Coriolis turns THAT too — which is why every ocean has a great gyre, clockwise in the northern hemisphere and anticlockwise in the southern, warm on its western side and cool on its eastern. It is the reason the Gulf Stream and the Kuroshio run north on the western edges, why the Benguela and California currents are cold, and why a passage plan reads the current chart as carefully as the wind chart.
| Western boundary currents | narrow, deep, fast and WARM — the Gulf Stream, the Kuroshio, the Agulhas. Up to 4 knots: worth a deviation to ride, and dangerous to fight in a heavy sea |
|---|---|
| Eastern boundary currents | broad, shallow, slow and COLD — Canary, Benguela, California. Cold water under warm air is where the great advection fogs live |
| Equatorial counter-current | a narrow eastward river between the two westward drifts, sitting under the doldrums |
| The Southern Ocean | no continent in the way: the Antarctic Circumpolar Current runs right round the world, driven by the roaring forties above it |
| Month | July |
|---|---|
| ITCZ (the doldrums) sits at | 9.7°N — it follows the sun, lagging about six weeks |
| Trade winds | NE in the north, SE in the south, force 4–5 and famously steady |
| Horse latitudes ≈ 30° | the sinking branch of the Hadley cell: light airs, clear skies, and the world's deserts under them |