What this lesson teaches
I can explain how monsoons, tropical cyclones and ENSO change rainfall in the tropics.
Syllabus 9173, Topic 2.1, 1. Tropical Climates. How does synoptic scale circulation affect rainfall in the tropics?
Make a guess
During El Nino, the Pacific trade winds weaken. What usually happens to rainfall in Indonesia?
- It rises, because the trade winds push more water west.
- It stays the same, because Indonesia is on the equator.
- It often falls, because the warm water moves east.
- It rises, because El Nino brings rain everywhere.
Show the answer
It often falls, because the warm water moves east.
Weaker trade winds let warm surface water spread east. The heaviest rain follows it, so Indonesia often gets drier.
ENSO links ocean temperatures and winds across the tropical Pacific, shifting where the heaviest rain falls.
ENSO is the El Nino-Southern Oscillation: linked changes in Pacific sea temperature, air pressure and winds. In the neutral phase, easterly trade winds push warm surface water west. Warm water piles up near Indonesia, where moist air rises and brings heavy rain. Cold deep water rises in the east. The thermocline, the layer where water gets cold quickly with depth, is deeper in the west and shallower in the east.
This east-west loop is the Walker circulation. Air flows west at the surface, rises over the warm west, returns east high up and sinks over the cool east. It is different from the north-south Hadley circulation.
In El Nino, the warm phase, trade winds weaken and warm water spreads east. Upwelling of cold water weakens. Heavy rain shifts to the central and eastern Pacific. Indonesia often gets drier, while parts of South America get wetter.
In La Nina, the cool phase, trade winds strengthen. More warm water piles up in the west, and the east gets colder. Rain increases in the western Pacific. ENSO changes the odds of wet or dry conditions, not every storm.
- ENSO
- Linked changes in Pacific sea temperature, pressure and winds.
- El Nino
- Weaker trade winds; warm water and rain shift east.
- La Nina
- Stronger trade winds; more rain in the west.
Step by step
Winds change
Trade wind strength moves warm water.
Ocean responds
Thermocline depth and upwelling change sea temperature.
Rain shifts
Storms follow the warm water.
Real examples to use
El Nino and haze
During the strong El Nino of 2015, drought in Indonesia let peat fires spread, causing severe haze across Southeast Asia, including Singapore.
Worked example: Reading a Pacific comparison
Weaker trade winds, a deeper eastern thermocline and rain moving east together point to El Nino. One warm reading near Indonesia does not, because the western Pacific is also warm in neutral years.
Watch out for this
El Nino means more rain across the whole tropics.
It shifts rain. Some places get wetter while others get drier.
Check your understanding
Which set of changes fits La Nina best?
- Stronger trade winds, a cooler east and more rain in the west.
- Weaker trade winds and warm water moving east across the Pacific.
- Calm winds, no upwelling and equal rain across the Pacific.
Show the answer
Stronger trade winds, a cooler east and more rain in the west.
Yes. These all reinforce each other in La Nina.
Check your understanding
During El Nino, why do parts of Southeast Asia often become drier?
- Trade winds weaken, and warm water and rain shift east, away from the region.
- Sea temperatures near Southeast Asia rise sharply, so less water evaporates.
- The monsoon becomes stronger and blows rain away.
Show the answer
Trade winds weaken, and warm water and rain shift east, away from the region.
This is why El Nino years bring drought and haze risk.