The big idea: The ocean and the atmosphere are coupled — they exchange heat, water and momentum, and together they drive the world's climate.
How the ocean moves heat
- Surface currents are driven by winds.
- The deep thermohaline conveyor belt is driven by differences in temperature and salinity.
- Together they move warm and cold water around the planet, redistributing heat and nutrients.
ENSO: The El Nino-Southern Oscillation (ENSO) is a natural see-saw in the tropical Pacific. El Nino and La Nina are its two opposite phases, and they shift weather and ocean conditions across the whole globe.
Key terms for this micro
- Ocean current — a continuous, directed flow of seawater (e.g. the warm Gulf Stream, the cold Humboldt Current).
- Gyre — a large, roughly circular system of surface currents driven by winds and the Coriolis effect (clockwise in the northern hemisphere).
- Thermohaline conveyor belt — the slow, global deep-ocean circulation driven by differences in temperature (thermo) and salinity (haline).
- Upwelling — cold, nutrient-rich deep water rising to the surface, e.g. off the coast of Peru.
- El Nino — the warm ENSO phase: trade winds weaken, warm water spreads east, upwelling off South America is suppressed.
- La Nina — the cold ENSO phase: trade winds strengthen, the eastern Pacific cools, upwelling intensifies.
El Nino vs La Nina in one line: El Nino = the eastern Pacific warms, the rain shifts east, and the cold, fish-feeding upwelling off Peru shuts down.
La Nina = the eastern Pacific cools, the rain stays in the west, and the upwelling strengthens. They are opposite swings of the same see-saw.
The ocean as a store and source of carbon dioxide
- A store (sink) — the ocean has taken up roughly a quarter of the CO2 that people have emitted. Cold water dissolves more gas, and plankton take in carbon that sinks when they die (the biological pump).
- A source — warm water holds less gas and can release CO2; upwelling brings carbon-rich deep water to the surface, as in the eastern equatorial Pacific; decay releases stored carbon.
- Changing role — as the ocean warms, it absorbs less, so more CO2 stays in the air.
- Ocean acidification — dissolved CO2 forms carbonic acid; surface pH has fallen from about 8.2 to about 8.1 since the 1800s, making it harder for corals and shellfish to build skeletons (reef impacts: 8.2.2).
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How this is tested: The stimulus is usually a line graph of sea-surface temperature in the tropical Pacific. Its warm peaks flag each El Nino, and its cooler dips flag each La Nina.
What the data part asks
- Estimate a value or a range.
- Identify the year of an event.
- State the direction a gyre rotates.
Reading the graph
Pick the axis
Match your reading to the correct axis.
Quote the units
Give the units (degrees C, or the year).
Range = subtract
For a range, subtract the lowest reading from the highest.
Right axis, units quoted, highest minus lowest.
Read the key and the axes first. Where is the line highest (strongest El Nino) and lowest (strongest La Nina)?
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Estimate the range of sea-surface temperature shown on the graph.
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El Nino vs La Nina — conditions and global impacts
| Feature | El Nino (warm phase) | La Nina (cold phase) |
|---|---|---|
| Trade winds | Weaken or reverse | Strengthen |
| Eastern Pacific (off Peru) | Warm surface water | Cold surface water |
| Upwelling off South America | Suppressed - few nutrients, fish stocks collapse | Intensified - rich nutrients, large fish catches |
| Rainfall shifts | Rain moves east - floods in Peru/Ecuador | Rain stays west - floods in SE Asia/Australia |
| Western Pacific (Indonesia/Australia) | Drought, wildfires | Heavy rain, flooding |
| Atlantic hurricanes | Fewer (more wind shear) | More active hurricane season |
The global thermohaline conveyor belt (a styled walk-through)
- 1. Sinking near the poles — in the North Atlantic, cold, salty water becomes dense and sinks to the deep ocean.
- 2. Deep return flow — this dense water creeps slowly along the sea floor southward, then into the Indian and Pacific oceans.
- 3. Upwelling — in places the deep water rises back to the surface, carrying nutrients up with it.
- 4. Warm surface return — warm surface currents carry the water back towards the Atlantic, completing the loop over ~1000 years.
- Why it matters — the conveyor moves heat (warming north-west Europe) and nutrients (feeding plankton, fish and food webs).
Range = subtract the two readings: If a question asks for the range of a temperature anomaly, read the highest value and the lowest value off the graph, then subtract. A value of +2.25 and -1.50 gives a range of 3.75.
An ENSO graph shows the ocean temperature anomaly peaking at +2.25 degrees in one event and falling to -1.50 degrees in another. Estimate the range of the temperature anomaly shown.
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ENSO phases reach far beyond the Pacific. The same event brings benefits to some places and harms to others — that contrast is exactly what the exam tests.
What a strong answer does
Name a real place
Anchor the point in a real place.
State the ENSO change
Say what El Nino or La Nina changes there.
Link a specific effect
Link that change to a specific climatic, economic or ecological effect.
Place, ENSO change, specific effect.
Peru and the Humboldt upwelling: Off Peru, the cold Humboldt Current normally drives strong upwelling, feeding one of the world's biggest anchovy fisheries.
During El Nino the warm water shuts the upwelling down — fish stocks collapse and fishing communities lose income. During La Nina the upwelling strengthens, so catches and fishing profits rise.
The Great Barrier Reef and the Galapagos MPA
El Nino warming can trigger mass coral bleaching on Australia's Great Barrier Reef, damaging the reef and the tourism it supports.
In the Galapagos Marine Protected Area, El Nino's warm, nutrient-poor water reduces the plankton that marine iguanas, penguins and sea lions depend on — so populations fall.
Australia, California and the Atlantic
La Nina brings heavy rain and flooding to eastern Australia but worsens drought and wildfire risk in California.
La Nina years also tend to produce more Atlantic hurricanes, while El Nino years bring fewer — a benefit for the US Gulf and Caribbean coasts.
Always pair the change with a place + a mechanism: Don't just say 'El Nino causes problems'. Name the place and the chain: El Nino warms the eastern Pacific -> upwelling off Peru stops -> anchovy stocks collapse -> fishing income falls.
Explain one beneficial and one harmful effect that an El Nino event produces in contrasting places.
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How this is tested - the [10] Examine essay: Paper 1 Option B ends with a 10-mark Examine / Evaluate essay, marked on markbands. A recurring version asks whether La Nina's benefits outweigh its harms across different regions.
What the top band needs
- Accurate ENSO terms.
- A developed benefit AND harm in different regions.
- A weighing of which matters more.
- A balanced, justified conclusion.
Examine the extent to which the benefits of a La Nina event outweigh its harms for people in different regions of the world.
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