El Niño events and their global consequences

Overview

What happens to the trade winds, the thermocline and upwelling in an El Niño year, and the weather it brings to South America, Australasia and regions beyond the Pacific.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an El Niño event?

   Model answer

   An El Niño event is a phase of the Pacific cycle in which the trade winds weaken or even reverse and blow from west to east, allowing the warm pool of water normally held in the western Pacific to spread back eastwards across the ocean. Sea surface temperatures in the central and eastern Pacific rise well above average, often by 2 to 5 degrees Celsius off Peru, while the western Pacific cools. The zone of rising air, low pressure and heavy rainfall shifts east with the warm water, so Peru and Ecuador become unusually wet while Indonesia and eastern Australia become unusually dry.

2. 2

   What happens to the trade winds during an El Niño event?

   Model answer

   They weaken substantially and in a strong event they reverse, so that instead of blowing from South America towards Indonesia, westerly winds blow from Indonesia towards South America. Because it is the trade winds that normally hold the warm water piled up in the west, once they weaken there is nothing to keep it there and it flows back east across the Pacific as a series of waves in the ocean.

3. 3

   What happens to the thermocline and to upwelling off South America during an El Niño event?

   Model answer

   The warm water spreading eastwards deepens the thermocline off South America, pushing it down from about 50 metres to 100 metres or more. Any upwelling that the weakened winds can still generate therefore draws water from within the warm surface layer rather than from the cold nutrient-rich water below. The upwelled water is warm and nutrient-poor, so the fertilising effect is lost even though water is still rising.

4. 4

   What are the consequences of an El Niño event for the fishing industry and marine ecosystems of Peru?

   Model answer

   Without the supply of nutrients from below, the phytoplankton blooms fail, the food chain that depends on them collapses and the anchoveta either die or move to deeper, cooler water beyond the reach of small boats. Catches can fall by more than half, fishmeal exports collapse, boats are laid up and coastal communities lose their income and their main source of protein. Seabirds and sea lions that feed on the fish starve in large numbers, and the guano industry suffers.

5. 5

   What weather does an El Niño event bring to Peru, Ecuador and northern Chile, and why?

   Model answer

   The warm water off the coast heats the air above it, so it becomes buoyant and rises, low pressure replaces the usual high, and abundant moisture evaporating from the warm sea condenses into deep convective cloud. Torrential rain falls on a coastal strip that is normally desert, producing flash floods, landslides and mudflows that destroy roads, bridges, homes and crops. Standing water encourages outbreaks of malaria, dengue fever and cholera. Paradoxically the highland Andes and the Amazon basin can suffer drought at the same time.

6. 6

   What weather does an El Niño event bring to Indonesia, Papua New Guinea and eastern Australia, and why?

   Model answer

   With the warm pool displaced eastwards, the sea here is cooler than usual, so the air above it is chilled, becomes dense and subsides, and high pressure replaces the usual low. Without rising air the monsoonal convection fails, so rainfall is far below average and severe drought sets in. Crops such as rice, coffee and wheat fail, water supplies and hydroelectric generation are reduced, livestock die and bushfires and peat and forest fires spread easily, producing a choking regional haze that closes airports and harms health.

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