La Niña events and their global consequences

Overview

What intensified trade winds do to the Pacific, and the weather a La Niña year brings to Australasia, South America and North America.

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 a La Niña event?

   Model answer

   A La Niña event is a phase of the Pacific cycle in which the trade winds blow more strongly than usual from South America towards Indonesia. They pile even more warm water into the western Pacific and drive stronger upwelling of cold water in the east, so sea surface temperatures in the central and eastern Pacific fall well below average, often by 1 to 4 degrees Celsius off Peru, while the western Pacific becomes even warmer than normal. Rising air, low pressure and rainfall over Indonesia and eastern Australia become more intense than usual, and the eastern Pacific becomes even drier.

2. 2

   What happens to the trade winds during a La Niña event?

   Model answer

   They strengthen well beyond their usual force and blow persistently from east to west along the equator, from South America towards Indonesia. The stronger drag on the ocean surface pushes more warm water westwards, so the warm pool in the west grows larger and deeper and sea level there rises further above that in the east. The intensified circulation is essentially the normal pattern taken to an extreme.

3. 3

   What happens to the thermocline and to upwelling off South America during a La Niña event?

   Model answer

   The stronger winds strip away more surface water from the eastern Pacific, so the thermocline rises even closer to the surface than usual, to perhaps 30 metres or less. Upwelling therefore intensifies and draws up a greater volume of very cold, nutrient-rich water from depth, and the cold tongue of surface water along the equator extends further west across the ocean.

4. 4

   What are the consequences of a La Niña event for the fishing industry and marine ecosystems of Peru?

   Model answer

   The intensified upwelling delivers an abundant supply of nitrates and phosphates to the sunlit layer, so phytoplankton blooms are exceptionally large, the food chain thrives and anchoveta stocks are plentiful and close to the surface within easy reach of the fleet. Catches are high, fishmeal exports rise and coastal incomes are good, and seabird and sea lion colonies breed successfully. The main risk is that the fishery is overexploited during these good years.

5. 5

   What weather does a La Niña event bring to Indonesia, Papua New Guinea and eastern Australia, and why?

   Model answer

   The warm pool here is larger and warmer than usual, so evaporation is greater and the air above it is more buoyant. Pressure falls further, convection is more vigorous and rainfall is well above average. Eastern Australia and Indonesia experience prolonged heavy rain, widespread river flooding, landslides and damage to homes, crops and infrastructure, while tropical cyclone activity in the Australian region increases. Very wet years can also bring plagues of mosquitoes and locusts.

6. 6

   What weather does a La Niña event bring to Peru, Ecuador and northern Chile, and why?

   Model answer

   The sea off the coast is colder than usual, so the air above it is chilled, is very stable and subsides, and high pressure is stronger than normal. The already arid coastal desert becomes even drier, so drought conditions intensify, river flows fed from the coastal zone fall and water supply becomes difficult. Coastal fog may be more persistent, and the cold water can bring unusually cool conditions to the coastal strip.

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