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Weather - The Effects of El Nino and La Nina

Updated on March 3, 2017

What are normal weather conditions like?

During a normal year the trade winds blow westwards towards Australia, and they bring the warm surface water of the Pacific with them. This warm air rises and creates thunderstorms and floods over Australia, and this is one of the main ways Australia acquires its water.

As this warm air cools down it travels along the Pacific to South America, where the dry sinking air causes droughts. There is high pressure, which results in clouds not being able to form, which in turn results in there being no rain. Also, the surface water on the Pacific is warm, but the deep water is still cold, so as the warm water gets blown westwards towards Australia, the cold water moves east towards South America.

This process is called 'up-welling', and the cold water that gets 'up-welled' on the coast of South America is rich in nutrients and these nutrients fertilise surface waters, meaning that these surface waters often have high biological productivity. Therefore, good fishing grounds are found where there is up-welling.

So, to sum up a normal year:

- Australia has low pressure resulting in rain, thunderstorms and floods, this is where they get their water, and the coast has warm surface water.

- South America has high pressure so they experience droughts, and the deep cold water up-wells resulting in perfect breeding grounds for fish, and therefore fish is an important part of the South American diet and they rely a lot of fishing.

Up-welling explained
Up-welling explained

What happens during an El Nino year?

El Nino is a climate phenomenon that takes place every two - seven years.

During an El Nino year, the normal weather patterns are flipped. This has drastic consequences on Australia and South America.

In an El Nino year the trade winds reverse direction, so instead of travelling west towards Australia, they travel east towards South America. This means that the warm air will be above South America instead of Australia, and South America will receive thunder storms, rain and floods, while Australia has dry sinking air, which results in drought.

Australia relies on the rain and floods as a major water source, so when El Nino comes around they are left with none or very little water because they get virtually no rain. This results in widespread drought and they would have to import water.

While Australia are in struggling in drought, South America is getting flooded, and as South America isn't as developed as Australia they wouldn't be able to cope with the flooding as well and would sustain some damages. Additionally, South America wouldn't experience the deep sea water up-welling, so this destroys the fish's habitats and kills off a large proportion of the fish population and this affects South America's food and exports. The South American fishing industry takes a huge hit whenever El Nino occurs.

To sum up, during an El Nino year:

- Australia doesn't receive much rain and is therefore in drought

- South America experiences floods, and a lot of the fish population die because the up-welling ceases and they aren't getting enough nutrients to sustain life


Diagram of a normal year vs an El Nino year
Diagram of a normal year vs an El Nino year

What is La Nina and what effect does it have?

El Nino is the reverse/opposite of normal weather patterns, and La Nina is the opposite of El nino. So essentially La Nina is a much more extreme version of normal weather patterns. During periods of La Nina the normal weather patterns are amplified.

The effects of La Nina on Australia include increased rainfall, cooler maximum temperatures, shift in temperature extremes, decreased frost risk, greater tropical cyclone numbers, earlier monsoon onset and deeper relative snow cover. The increased rainfall increases floods, and this coupled with the increased number of cyclones result in Australia experiencing a lot more hydro-meteorological hazards each time La Nina occurs.

The effects of La Nina on South America result in increased drought in the coastal regions of Peru and Chile. La Nina also causes a higher amount of rainfall on the Andes, and this causes catastrophic flooding in Bolivia.

La Nina
El Nino
 
Trade winds become stronger than normal
Trade winds weaken in the western Pacific
 
Trade winds cause warmer waters in the west
Trade winds cause warmer waters in the East
 
Sea surface temp reduces across Eastern Pacific by 3-5 degrees celcius
El Nino occurs every 2-7 years
 
Enhanced up-welling of nutrient rich waters
Reduced up-welling of deep nutrient rich water leads to reduction in fish population
 

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