The big idea: The ocean and the atmosphere are coupled — they exchange heat, water and momentum, and together they drive the world's climate.
Surface currents (driven by winds) and the deep thermohaline conveyor belt (driven by differences in temperature and salinity) move warm and cold water around the planet, redistributing heat and nutrients.
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.
How this is tested: Paper 1 Option B opens with a data-response built on an ENSO graph or an ocean map. You Estimate a value or range, Identify a year, or State the direction a gyre circulates. Read the correct axis carefully, quote the units, and for a range remember to subtract the two values.
| 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.
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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. Strong answers name a real place and link the ENSO change to a specific climatic, economic or ecological 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.
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.
Top band needs: accurate ENSO terms, a developed benefit AND harm in different regions, a weighing of which matters more, and a balanced justified conclusion.