EVSC10001 · The Global Environment
Modern Atmosphere & Ocean Circulation
This chapter follows the energy bookkeeping that drives Earth's climate: because the tropics run a radiation surplus and the poles a deficit, heat must move poleward — fast by winds and slowly by ocean currents. You learn the three-cell general circulation (Hadley, Ferrel, Polar), how the Coriolis force bends the surface return flow into the trade winds, westerlies and polar easterlies, and how density drives the deep thermohaline conveyor. It is a reliable Part-A topic in the closed-book exam, where every short-answer demands a correctly labelled diagram, so mastering these sketches converts directly into marks.
What this chapter covers
- 01Latitudinal energy imbalance & poleward heat transport
- 02Hadley cell: ITCZ ascent (rain) vs ~30° descent (deserts)
- 03Three-cell model: Hadley, Ferrel, Polar
- 04Coriolis deflection (right NH, left SH) & the surface wind belts
- 05Naming winds by where they come FROM (trades, westerlies, easterlies)
- 06Wind-driven surface gyres & warm western boundary currents
- 07Ekman transport & coastal upwelling
- 08Thermohaline conveyor: density, NADW, AABW and the ~1000-year loop
Sketch and explain the three-cell circulation of one hemisphere
- +2Draw the diagram first: a quarter-circle Earth (equator at bottom, pole at top) with three stacked rotating cells labelled Hadley (0–30°), Ferrel (30–60°) and Polar (60–90°). Mark rising limbs at the equator and ~60°, and sinking limbs at ~30° and the pole.
- +1Label the ITCZ at the equatorial rising limb: intense solar heating → moist air ascends, cools and rains → low pressure, wet (rainforests).
- +1Label the subtropical high at the ~30° sinking limb: dried-out air descends and warms, suppressing cloud → high pressure, dry → the desert belt.
- +1Add the surface return flow with a Coriolis arrow (deflect right in NH / left in SH); the equatorward branch of the Hadley cell becomes the trade winds (easterlies), and remember a wind is named for the direction it comes from.
- +1State the link in one line: deserts cluster at ~30° because that is where Hadley-cell air descends; the equator is wet because that is where it ascends.
Key terms
- Hadley cell
- The low-latitude (0–30°) overturning cell in which moist air rises at the equatorial ITCZ, flows poleward aloft, sinks at ~30° as a subtropical high, and returns equatorward at the surface as the trade winds.
- ITCZ (Intertropical Convergence Zone)
- The equatorial belt where the trade winds of both hemispheres converge and air rises, producing persistent low pressure, deep convection and heavy rainfall.
- Coriolis force
- The apparent deflection of any object moving over the rotating Earth — to the right in the Northern Hemisphere and to the left in the Southern — which bends the surface return flow into the trade, westerly and polar-easterly wind belts.
- Western boundary current
- A fast, narrow, warm current on the western side of an ocean basin (e.g. the East Australian Current, Gulf Stream, Kuroshio) that carries tropical heat poleward and warms eastern coasts of continents.
- Thermohaline circulation
- The slow, density-driven deep-ocean circulation (the global conveyor) in which water that becomes cold and salty enough sinks at high latitudes, spreads along the sea floor and returns over roughly a thousand years.
- Antarctic Bottom Water (AABW)
- The densest water mass in the world ocean, formed near Antarctica when sea-ice growth in polynyas expels salt (brine rejection), making surface water cold and very salty so it cascades down the continental slope and spreads north along the sea floor.
Modern Atmosphere & Ocean Circulation FAQ
Why are most large deserts located around 30° latitude?
Because ~30° sits under the descending limb of the Hadley cell. Air that rose and rained out at the equator returns dry, sinks and warms, forming a persistent subtropical high that suppresses cloud and rainfall — the recipe for the world's great deserts. Cold west-coast ocean currents reinforce the aridity.
What is the difference between wind-driven surface currents and the thermohaline conveyor?
Surface gyres are wind-driven: fast, shallow and set by wind stress plus Coriolis. The thermohaline conveyor is density-driven: slow, deep and set by temperature and salinity. They are separate mechanisms that connect where warm surface water is delivered to the polar sinking sites (NADW, AABW).
Why are winds named for the direction they come from?
Meteorological convention names a wind by its source. A 'westerly' blows from the west towards the east, and the trade winds are easterlies (from the east). On an exam diagram, draw the arrow pointing away from the named direction — reversing this is the most common labelling error in Part A.
How does the Coriolis force create the three-cell pattern?
Surface air returning from regions of descent does not flow straight along a meridian; the Coriolis force deflects it (right in the NH, left in the SH). This deflection breaks the flow into three latitudinal cells per hemisphere — Hadley, Ferrel and Polar — with the trade winds, westerlies and polar easterlies at the surface.
Why does temperature rise in the stratosphere but fall in the troposphere?
In the troposphere temperature falls with height because it is heated from the surface below. In the stratosphere temperature rises with height because ozone absorbs solar UV and heats the air directly. A correct atmosphere sketch must show the kink at the tropopause where the gradient reverses.
Exam move
Treat this chapter as a set of four reflexes tied to four diagrams you can draw from memory in under a minute each. (1) For any climate question, locate the latitude on the three-cell diagram — ascending limb = wet, descending limb = dry. (2) Apply Coriolis (right NH, left SH) to name the wind belt, and recall that winds are named for where they come from. (3) For surface ocean, reach for wind-driven gyres plus warm western boundary currents. (4) For the deep ocean, say density: cold + salty sinks → AABW/NADW conveyor. Because Part A scores the labelled sketch directly, rehearse the four must-draw diagrams — the atmosphere T-vs-height profile with the tropopause kink, the three-cell circulation, a gyre with its western boundary current, and the AABW shelf cross-section — until labelling them is automatic, then write the one-line mechanism beneath each.
